17.2 Oxidation of Fatty Acids in Chapter 17 Fatty Acid Catabolism

17.2 Oxidation of Fatty Acids

As noted earlier, mitochondrial oxidation of fatty acids takes place in three stages (Fig. 17-7). In the first stage — β oxidation — fatty acids undergo oxidative removal of successive two-carbon units in the form of acetyl-CoA, starting from the carboxyl end of the fatty acyl chain. For example, the 16-carbon palmitic acid (palmitate at pH 7) undergoes seven passes through the oxidative sequence, in each pass losing two carbons as acetyl-CoA. At the end of seven cycles, the last two carbons of palmitate (originally C-15 and C-16) remain as acetyl-CoA. The overall result is the conversion of the 16-carbon chain of palmitate to eight two-carbon acetyl groups of acetyl-CoA molecules. Formation of each acetyl-CoA requires removal of four hydrogen atoms (two pairs of electrons and four $H^{+}$ $upper H Superscript plus$ ) from the fatty acyl moiety by dehydrogenases.

A figure shows three stages of fatty acid oxidation. — FIGURE 17-7 Stages of fatty acid oxidation. Stage 1: A long-chain fatty acid is oxidized to yield acetyl residues in the form of acetyl-CoA. This process is called β oxidation. Stage 2: The acetyl groups are oxidized to ${CO}_{2}$ $CO Subscript 2$ via the citric acid cycle. Stage 3: Electrons derived from the oxidations of stages 1 and 2 pass to $O_{2}$ $upper O Subscript 2$ via the mitochondrial respiratory chain, providing the energy for ATP synthesis by oxidative phosphorylation.

Stage 1 shows a vertical sixteen-carbon chain. The bottom carbon is double bonded to O to the right, bonded to O minus below, and bonded to C H 2 above that is connected to another C H 2 above by a bond with a blue line across it. An arrow extends from this line to a vertical line running down along the side of the entire molecule. The molecule has 14 C H 2 groups and ends with C H 3. After every group of two carbons, there is a w line breaking the bond accompanied by a line joining the vertical arrow. In total, seven wavy lines have lines that join the arrow that points down to 28 e minus before curving to a box labeled N A D H, F A D H 2 located where the upper two boxes (stage 1 on the left and stage 2 on the right) intersect with the bottom box for stage 3 below. An arrow accompanied by a box labeled Greek letter beta oxidation points right from stage 1 into stage 2, which it points to a box labeled 8 acetyl Co A. An arrow extends down from this box to the citric acid cycle, which is shown as a series of clockwise arrows. The first arrow begins at the eleven o’clock position, joins with the line down from acetyl Co A, and ends at the one o’clock position. The second arrow runs all the way back to the eleven o’clock position except for a piece that branches away at the five o’clock position to show the loss of 16 C O 2. An arrow from the seven o’clock position is labeled 64 e minus and points down to the box labeled N A D H, F A D H 2. In stage 3, an arrow labeled 3 minus points down from the box labeled N A D H, F A D H 2 to a box labeled respiratory (electron-transfer) chain. A curved arrow on the right side of the box shows the addition of 2 H plus plus one-half O 2 and the removal of H 2 O. A curved arrow on the bottom side of the box shows the addition of an oval labeled A D P plus P i outside of the oval and the removal of an oval labeled A T P. All data are approximate.

In the second stage of fatty acid oxidation, the acetyl groups of acetyl-CoA are oxidized to ${CO}_{2}$ $CO Subscript 2$ in the citric acid cycle, which also takes place in the mitochondrial matrix. Acetyl-CoA derived from fatty acids thus enters a final common pathway of oxidation with the acetyl-CoA derived from glucose via glycolysis and pyruvate oxidation (see Fig. 16-1). The first two stages of fatty acid oxidation produce the reduced electron carriers NADH and $FADH Subscript 2$ $FADH Subscript 2$ , which in the third stage donate electrons to the mitochondrial respiratory chain, through which the electrons pass to oxygen with the concomitant phosphorylation of ADP to ATP (Fig. 17-7). The energy released by fatty acid oxidation is thus conserved as ATP.

We now take a closer look at the first stage of fatty acid oxidation, beginning with the simple case of a saturated fatty acyl chain with an even number of carbons, then turning to the slightly more complicated cases of unsaturated and odd-number chains. We also consider the regulation of fatty acid oxidation, the β-oxidative processes as they occur in organelles other than mitochondria, and, finally, a less-common mode of fatty acid catabolism — α oxidation.

The β Oxidation of Saturated Fatty Acids Has Four Basic Steps

Four enzyme-catalyzed reactions make up the first stage of fatty acid oxidation (Fig. 17-8a). First, dehydrogenation of fatty acyl–CoA produces a double bond between the α and β carbon atoms (C-2 and C-3), yielding a trans- $Δ^{2}$ $bold upper Delta squared$ -enoyl-CoA (the symbol $Δ^{2}$ $upper Delta squared$ designates the position of the double bond; you may want to review fatty acid nomenclature, p. 342). Note that the new double bond has the trans configuration, whereas the double bonds in naturally occurring unsaturated fatty acids are normally in the cis configuration. We consider the significance of this difference later.

A two-part figure shows the Greek letter beta oxidation pathway by showing the steps in part a and the products of repeated passes through the pathway in part b. — FIGURE 17-8 The β-oxidation pathway. (a) In each pass through this four-step sequence, one acetyl residue (shaded in light red) is removed in the form of acetyl-CoA from the carboxyl end of the fatty acyl chain — in this example palmitate $left-parenthesis upper C Subscript 16 Baseline right-parenthesis$ $left-parenthesis upper C Subscript 16 Baseline right-parenthesis$ , which enters as palmitoyl-CoA. Electrons from the first oxidation pass through electron transfer flavoprotein (ETF), and then through a second flavoprotein (ETF:ubiquinone oxidoreductase), into the respiratory chain. Electrons from the second oxidation enter the respiratory chain through NADH dehydrogenase. (b) Six more passes through the β-oxidation pathway yield seven more molecules of acetyl-CoA, the seventh arising from the last two carbon atoms of the 16-carbon chain. Eight molecules of acetyl-CoA are formed in all. The acetyl-CoA may be oxidized in the citric acid cycle, donating more electrons to the respiratory chain.

FIGURE 17-8 The β-oxidation pathway. (a) In each pass through this four-step sequence, one acetyl residue (shaded in light red) is removed in the form of acetyl-CoA from the carboxyl end of the fatty acyl chain — in this example palmitate $left-parenthesis upper C Subscript 16 Baseline right-parenthesis$ $left-parenthesis upper C Subscript 16 Baseline right-parenthesis$ , which enters as palmitoyl-CoA. Electrons from the first oxidation pass through electron transfer flavoprotein (ETF), and then through a second flavoprotein (ETF:ubiquinone oxidoreductase), into the respiratory chain. Electrons from the second oxidation enter the respiratory chain through NADH dehydrogenase. (b) Six more passes through the β-oxidation pathway yield seven more molecules of acetyl-CoA, the seventh arising from the last two carbon atoms of the 16-carbon chain. Eight molecules of acetyl-CoA are formed in all. The acetyl-CoA may be oxidized in the citric acid cycle, donating more electrons to the respiratory chain.

Part a shows palmitoyl-Co A as (C subscript 16) R bonded to C H 2 bonded to C H 2 with the C labeled as Greek letter beta bonded to C H 2 with the C labeled as Greek letter alpha bonded to C double bonded to O below and bonded to S on the right further bonded to Co A. There is a box around the C H 2 with its C labeled as Greek letter alpha bonded to C double bonded to O below. A downward-pointing arrow is labeled acyl-Co A dehydrogenase and is accompanied by a curved arrow to the right showing the addition of F A D and removal of F A D H 2 with E T F written at the inflection point. Another curved arrow extends from F A D H 2 back up to F A D with a red arrow labeled 2 e minus extending away to the right to E T F: ubiquinone oxidoreductase, from which another red arrow points to a blue box labeled respiratory chain. The downward-pointing arrow from palmitoyl-Co A points to italicized trans end italics Greek letter delta superscript 2-Enoyl-Co A. This has R bonded to C H 2 bonded to C bonded to H above and double bonded to C in a red box bonded to H below and bonded to C to the right double bonded to O below, all within the red box, and bonded to S to the right outside of the box further bonded to Co A. An arrow labeled enoyl-Co A hydratase points down and is joined by a line showing the addition of H 2 O. This yields L-Greek letter beta-hydroxy-acyl-Co A. It has R bonded to C H 2 bonded to C bonded to O H above, H below, and C H 2 in a red box on the right further bonded to C double bonded to O below in the same box and bonded to S to the right outside of the box and further bonded to Co A. An arrow labeled Greek letter beta-hydroxyacyl-Co A dehydrogenase. This yields Greek letter beta-ketoacyl-Co A. An accompanying curved arrow shows that N A D plus is added and N A D H plus H plus is produced. Another curved arrow shows that N A D H plus H plus is converted back to N A D plus with a red arrow labeled 2 e minus pointing away to N A D H dehydrogenase, from which a red arrow points to the blue box labeled respiratory chain. Greek letter beta-ketoacyl-Co A has R bonded to C H 2 bonded to C double bonded to O below bonded to C H 2 in a red box bonded to C double bonded to O below, all in a red box, and bonded to S on the right outside of the box that is further bonded to Co A. An arrow labeled acyl-Co A acetyltransferase (thiolase) points down and is joined by a line showing the addition of Co A – S H. This produces acyl-Co A (myristoyl-Co A). It is shown as (C subscript 14). R bonded to C H 2 bonded to C double bonded to O below bonded to S on the right further bonded to Co A plus C H 3 in a red box bonded to C double bonded to O below in the same red box and bonded to S on the right outside of the box further bonded to Co A. Text below shows acetyl in a red box next to hyphe n Co A outside of the box. Part b shows a vertical line of C subscript 14, C subscript 12, C subscript 10, C subscript 8, C subscript 6, and C subscript 4. A coiled arrow forms a loop in front of each of these values and each loop has an arrow pointing right to a red box labeled acetyl next to hyphen Co A outside of the box. At the very bottom, beneath the arrow pointing down, is a red box labeled acetyl next to hyphen Co A outside of the box.

This first step is catalyzed by three isozymes of acyl-CoA dehydrogenase, each specific for a range of fatty-acyl chain lengths: very-long-chain acyl-CoA dehydrogenase (VLCAD), acting on fatty acids of 12 to 18 carbons; medium-chain (MCAD), acting on fatty acids of 4 to 14 carbons; and short-chain (SCAD), acting on fatty acids of 4 to 8 carbons. VLCAD is in the inner mitochondrial membrane; MCAD and SCAD are in the matrix. All three isozymes are flavoproteins with tightly bound FAD (see Fig. 13-27) as a prosthetic group. The electrons removed from the fatty acyl–CoA are transferred to FAD, and the reduced form of the dehydrogenase immediately donates its electrons to an electron carrier, the electron transfer flavoprotein (ETF) (see Fig. 19-15). Electrons move from ETF to a second flavoprotein, ETF:ubiquinone oxidoreductase, and through ubiquinone into the mitochondrial respiratory chain. The oxidation catalyzed by an acyl-CoA dehydrogenase is analogous to succinate dehydrogenation in the citric acid cycle (p. 586); in both reactions the enzyme is bound to the inner membrane, a double bond is introduced into a carboxylic acid between the α and β carbons, FAD is the electron acceptor, and electrons from the reaction ultimately enter the respiratory chain and pass to $upper O Subscript 2$ $upper O Subscript 2$ , with the concomitant synthesis of about 1.5 ATP molecules per electron pair.

In the second step of the β-oxidation cycle (Fig. 17-8a), water is added to the double bond of the trans- $Δ^{2}$ $upper Delta squared$ -enoyl-CoA to form the l stereoisomer of $β$ $bold-italic beta$ -hydroxyacyl-CoA (3-hydroxyacyl-CoA). This reaction, catalyzed by enoyl-CoA hydratase, is formally analogous to the fumarase reaction in the citric acid cycle, in which $H_{2} O$ $upper H Subscript 2 Baseline upper O$ adds across an $α – β$ $alpha en-dash beta$ double bond (p. 587).

In the third step, l-β-hydroxyacyl-CoA is dehydrogenated to form $β$ $bold-italic beta$ -ketoacyl-CoA, by the action of $β$ $bold-italic beta$ -hydroxyacyl-CoA dehydrogenase; ${NAD}^{+}$ $NAD Superscript plus$ is the electron acceptor. This enzyme is absolutely specific for the l stereoisomer of hydroxyacyl-CoA. The NADH formed in the reaction donates its electrons to NADH dehydrogenase (Complex I), an electron carrier of the respiratory chain (see Fig. 19-15), and ATP is formed from ADP as the electrons pass to $O_{2}$ $upper O Subscript 2$ . The reaction catalyzed by β-hydroxyacyl-CoA dehydrogenase is closely analogous to the malate dehydrogenase reaction of the citric acid cycle (p. 587).

The fourth and last step of the β-oxidation cycle is catalyzed by acyl-CoA acetyltransferase, more commonly called thiolase, which promotes reaction of β-ketoacyl-CoA with a molecule of free coenzyme A to split off the carboxyl-terminal two-carbon fragment of the original fatty acid as acetyl-CoA. The other product is the coenzyme A thioester of the fatty acid, now shortened by two carbon atoms (Fig. 17-8a). This reaction is called thiolysis, by analogy with the process of hydrolysis, because the β-ketoacyl-CoA is cleaved by reaction with the thiol group of coenzyme A. The thiolase reaction is a reverse Claisen condensation (see Fig. 13-4).

The last three steps of this four-step sequence are catalyzed by either of two sets of enzymes, with the enzymes employed depending on the length of the fatty acyl chain. For fatty acyl chains of 12 or more carbons, the reactions are catalyzed by a multienzyme complex associated with the inner mitochondrial membrane, the trifunctional protein (TFP). TFP is a heterooctamer of $α_{4} β_{4}$ $alpha 4 beta 4$ subunits. Each α subunit contains two activities, the enoyl-CoA hydratase and the β-hydroxyacyl-CoA dehydrogenase; the β subunits contain the thiolase activity. This tight association of three enzymes allows efficient substrate channeling from one active site to the next, without diffusion of the intermediates away from the enzyme surface. When TFP has shortened the fatty acyl chain to 12 or fewer carbons, further oxidations are catalyzed by a set of four soluble enzymes in the matrix.

As noted earlier, the single bond between methylene $(— {CH}_{2} —)$ $left-parenthesis em-dash CH Subscript 2 Baseline em-dash right-parenthesis$ groups in fatty acids is relatively stable. The β-oxidation sequence is an elegant mechanism for destabilizing and breaking these bonds. The first three reactions of β oxidation create a much less stable $C — C$ $upper C em-dash upper C$ bond, in which the α carbon (C-2) is bonded to two carbonyl carbons (the β-ketoacyl-CoA intermediate). The ketone function on the β carbon (C-3) makes it a good target for nucleophilic attack by the $— SH$ $em-dash SH$ of coenzyme A, catalyzed by thiolase. The acidity of the α hydrogen and the resonance stabilization of the carbanion generated by the departure of this hydrogen make the terminal $— {CH}_{2} — CO — S-CoA$ $em-dash CH Subscript 2 Baseline em-dash CO em-dash upper S hyphen CoA$ a good leaving group, facilitating breakage of the $α – β$ $alpha en-dash beta$ bond.

We have already seen a reaction sequence nearly identical with these four steps of fatty acid oxidation, in the citric acid cycle reaction steps between succinate and oxaloacetate (see Fig. 16-7). A nearly identical reaction sequence occurs again in the pathways by which the branched-chain amino acids (isoleucine, leucine, and valine) are oxidized as fuels (see Fig. 18-28). Figure 17-9 shows the common features of these three sequences, almost certainly an example of the conservation of a mechanism by gene duplication and evolution of a new specificity in the enzyme products of the duplicated genes.

A figure shows a conserved reaction sequence to introduce a carbonyl function on the carbon Greek letter beta to a carboxyl group in Greek letter beta oxidation, the citric acid cycle, and the oxidation of isoleucine. — FIGURE 17-9 A conserved reaction sequence to introduce a carbonyl function on the carbon β to a carboxyl group. The β-oxidation pathway for fatty acyl–CoAs, the pathway from succinate to oxaloacetate in the citric acid cycle, and the pathway by which the deaminated carbon skeletons from isoleucine, leucine, and valine are oxidized as fuels — all use the same reaction sequence.

Greek letter beta oxidation: A molecule has R bonded to C H 2 in a blue box bonded to C H 2 in the same box bonded to C outside of the box double bonded to O to the upper right and bonded to S to the lower right further bonded to Co A. A downward-pointing arrow is accompanied by a curved arrow showing the loss of an orange oval labeled F A D H 2. This yields R bonded to C H in a blue box double bonded to C H in the same box bonded to C outside of the box double bonded to O to the upper right and bonded to S to the lower right further bonded to Co A. An arrow points down accompanied by a line showing the addition of H 2 O. This yields R bonded to C H in a blue box bonded to O H above in the box and to C H 2 to the right in the box further bonded to C outside of the box double bonded to O to the upper right and bonded to S to the lower right further bonded to Co A. An arrow points down accompanied by a curved arrow showing the loss of an oval labeled N A D H. This yields R bonded to C in a blue box double bonded to O above in the box and to C H 2 to the right in the box further bonded to C outside of the box double bonded to O to the upper right and to S to the lower right further bonded to Co A. Citric acid cycle: A molecule has C double bonded to O to the upper left, bonded to O minus to the lower right, and bonded to C H 2 in a blue box to the right bonded to C H 2 in the same blue box bonded to C outside of the box double bonded to O to the upper right and bonded to O minus to the lower right. A downward-pointing arrow is accompanied by a curved arrow showing the loss of an oval labeled F A D H 2. This yields C double bonded to O to the upper right, bonded to O minus to the lower right, and bonded to C H to the right in a blue box double bonded to C H in the same box bonded to C outside of the box double bonded to O to the upper right and bonded to O minus to the lower right. An arrow points down accompanied by a line showing the addition of H 2 O. This yields C double bonded to O to the upper right, bonded to O minus to the lower right, and bonded to C H in a blue box bonded to O H above in the blue box and to C H 2 to the right in the blue box further bonded to C outside of the box double bonded to O to the upper right and bonded to O minus to the lower right. An arrow points down accompanied by a curved arrow showing the loss of an oval labeled N A D H. This yields C double bonded to O to the upper right, bonded to O minus to the lower right, and bonded to C to the right in a blue box double bonded to O above in the blue box and to C H 2 to the right in the box further bonded to C outside of the box double bonded to O to the upper right and bonded to O minus to the lower right. Oxidation of isoleucine (leucine, valine): A molecule has C H 3 bonded to C H 2 in a blue box bonded to C H in the same box bonded to C H 3 above outside of the box and C to the right outside of the box double bonded to O to the upper right and bonded to S to the lower right further bonded to Co A. A downward-pointing arrow is accompanied by a curved arrow showing the loss of an oval labeled F A D H 2. This yields C H 3 bonded to C H in a blue box double bonded to C in the same blue box bonded to C H 3 above outside of the box and bonded to C outside of the box double bonded to O to the upper right and bonded to S to the lower right further bonded to Co A. An arrow points down accompanied by a line showing the addition of H 2 O. This yields C H 3 bonded to C H in a blue box bonded to O H above in the blue box and to C H to the right in the blue box further bonded to C H 3 above outside of the box and to C to the right outside of the box double bonded to O to the upper right and bonded to S to the lower right further bonded to Co A. An arrow points down accompanied by a curved arrow showing the loss of an oval labeled N A D H. This yields C H 3 bonded to C in a blue box double bonded to O above in the blue box and to C H to the right in the blue box further bonded to C H above outside of the box and to C to the right outside of the box double bonded to O to the upper right and to S to the lower right further bonded to Co A.

The Four β-Oxidation Steps Are Repeated to Yield Acetyl-CoA and ATP

In one pass through the β-oxidation sequence, one molecule of acetyl-CoA, two pairs of electrons, and four protons $(H^{+})$ $left-parenthesis upper H Superscript plus Baseline right-parenthesis$ are removed from the long-chain fatty acyl–CoA, shortening it by two carbon atoms. The equation for one pass, beginning with the coenzyme A ester of our example, palmitate, is

\begin{matrix} Palmitoyl-CoA + CoA + FAD + {NAD}^{+} + H_{2} O \to \\ myristoyl-CoA + acetyl-CoA + {FADH}_{2} + NADH + H^{+} \end{matrix}

$StartLayout 1st Row Palmitoyl hyphen CoA plus CoA plus FAD plus NAD Superscript plus Baseline plus upper H Subscript 2 Baseline upper O right-arrow 2nd Row myristoyl hyphen CoA plus acetyl hyphen CoA plus FADH Subscript 2 Baseline plus NADH plus upper H Superscript plus EndLayout$

(17-2)

Following removal of one acetyl-CoA unit from palmitoyl-CoA, the coenzyme A thioester of the shortened fatty acid (now the 14-carbon myristate) remains. The myristoyl-CoA can now go through another set of four β-oxidation reactions, exactly analogous to the first, to yield a second molecule of acetyl-CoA and lauroyl-CoA, the coenzyme A thioester of the 12-carbon laurate. Altogether, seven passes through the β-oxidation sequence are required to oxidize one molecule of palmitoyl-CoA to eight molecules of acetyl-CoA (Fig. 17-8b). The overall equation is

\begin{matrix} Palmitoyl-CoA + 7 CoA + 7 FAD + 7 {NAD}^{+} + 7 H_{2} O \to \\ 8 acetyl-CoA + 7 {FADH}_{2} + 7 NADH + 7 H^{+} \end{matrix}

$StartLayout 1st Row Palmitoyl hyphen CoA plus 7 CoA plus 7 FAD plus 7 NAD Superscript plus Baseline plus 7 upper H Subscript 2 Baseline upper O right-arrow 2nd Row 8 acetyl hyphen CoA plus 7 FADH Subscript 2 Baseline plus 7 NADH plus 7 upper H Superscript plus EndLayout$

(17-3)

Each molecule of ${FADH}_{2}$ $FADH Subscript 2$ formed during oxidation of the fatty acid donates a pair of electrons to ETF of the respiratory chain, and about 1.5 molecules of ATP are generated during the ensuing transfer of each electron pair to $O_{2}$ $upper O Subscript 2$ . Similarly, each molecule of NADH formed delivers a pair of electrons to the mitochondrial NADH dehydrogenase, and the subsequent transfer of each pair of electrons to $O_{2}$ $upper O Subscript 2$ results in formation of about 2.5 molecules of ATP. Thus four molecules of ATP are formed for each two-carbon unit removed in one pass through the sequence.

Note that water is also produced in this process. Each pair of electrons transferred from NADH or ${FADH}_{2}$ $FADH Subscript 2$ to $O_{2}$ $upper O Subscript 2$ yields one $upper H Subscript 2 Baseline upper O$ $upper H Subscript 2 Baseline upper O$ , referred to as “metabolic water.” Reduction of $O_{2}$ $upper O Subscript 2$ by NADH also consumes one $H^{+}$ $upper H Superscript plus$ per NADH molecule: ${NADH + H}^{+} + ½ O_{2} \to {NAD}^{+} + H_{2} O$ $NADH plus upper H Superscript plus Baseline plus one half upper O Subscript 2 Baseline right-arrow NAD Superscript plus Baseline plus upper H Subscript 2 Baseline upper O$ . In hibernating animals, fatty acid oxidation provides metabolic energy, heat, and water — all essential for survival of an animal that neither eats nor drinks for long periods (Box 17-1). Camels obtain water to supplement the meager supply available in their natural environment by oxidation of fats stored in their hump.

Box 17-1

A Long Winter’s Nap: Oxidizing Fats during Hibernation

Many animals depend on fat stores for energy during hibernation, during migratory periods, and in other situations involving radical metabolic adjustments. One of the most pronounced adjustments of fat metabolism occurs in hibernating grizzly bears. These animals remain in a continuous state of dormancy for as long as seven months. Unlike most hibernating species, the bear maintains a body temperature of about 31 °C, close to its normal (nonhibernating) temperature near 40 °C. Although expending about 25,000 kJ/day (6,000 kcal/day) while hibernating, the bear does not eat, drink, urinate, or defecate for months at a time. Its heart rate drops from 90 to 8 beats per minute, and its respiration rate drops from 6 to 10 breaths to approximately 1 breath per minute.

As we shall see in Chapter 19, mitochondrial electron transfer can be uncoupled from ATP production so that all of the energy of fuel oxidation is dissipated as heat, to maintain a body temperature near normal in the face of much lower ambient temperatures. One form of fat tissue, brown adipose tissue, is especially important in thermogenesis; we discuss it in more detail in Chapter 23.

A grizzly bear prepares its hibernation nest near the McNeil River in Canada.

Experimental studies have shown that hibernating grizzly bears use body fat as their sole fuel. Fat oxidation yields sufficient energy to maintain body temperature, synthesize amino acids and proteins, and carry out other energy-requiring activities, such as membrane transport. Fat oxidation also releases large amounts of water, as described in the text, which replenishes water lost in breathing. The glycerol released by degradation of triacylglycerols is converted into blood glucose by gluconeogenesis. Urea formed during breakdown of amino acids is reabsorbed in the kidneys and recycled, with the amino groups reused to make new amino acids for maintaining body proteins.

Bears store an enormous amount of body fat in preparation for their long sleep. An adult grizzly consumes about 38,000 kJ/day during the late spring and summer, but as winter approaches it feeds for 20 hours a day, consuming up to 84,000 kJ daily. This increase in feeding is a response to a seasonal change in hormone secretion. Large amounts of triacylglycerols are formed from the huge intake of carbohydrates during the fattening-up period. The bear will emerge from hibernation having lost 15% to 40% of its maximum body weight.

The winter sleep of bears is sometimes called torpor, and it differs in important ways from the hibernation behavior of a group of smaller animals that undergo alternating periods of high and low body temperature. In these animals, body temperature approaches ambient temperature, close to 0 °C, for much of the time during hibernation, but rises to almost prehibernation level during brief periods of wakefulness. During these periods, the animals eat, drink, and defecate. In the Arctic ground squirrel (Urocitellus parryii), for example, body temperature (37 °C prehibernation) drops to 0 °C during hibernation, and respiration drops to less than 10% of its prehibernation rate.

Studies of hibernation mechanisms may yield insight into several problems in human medicine; for example, slowing the metabolism of organs donated for transplantation might extend the period of their viability. And if humans are to make long trips into space, inducing a torporlike state might relieve the monotony of long missions and conserve onboard resources such as food and oxygen.

The overall equation for the oxidation of palmitoyl-CoA to eight molecules of acetyl-CoA, including the electron transfers and oxidative phosphorylations, is

\begin{matrix} Palmitoyl-CoA + 7 CoA + 7 O_{2} + 28 P_{i} + 28 ADP \to \\ 8 acetyl-CoA + 28 ATP + 7 H_{2} O \end{matrix}

$StartLayout 1st Row Palmitoyl hyphen CoA plus 7 CoA plus 7 upper O Subscript 2 Baseline plus 28 upper P Subscript i Baseline plus 28 ADP right-arrow 2nd Row 8 acetyl hyphen CoA plus 28 ATP plus 7 upper H Subscript 2 Baseline upper O EndLayout$

(17-4)

Acetyl-CoA Can Be Further Oxidized in the Citric Acid Cycle

The acetyl-CoA produced from the oxidation of fatty acids can be oxidized to ${CO}_{2}$ $CO Subscript 2$ and $H_{2} O$ $upper H Subscript 2 Baseline upper O$ by the citric acid cycle. The following equation represents the balance sheet for the second stage in the oxidation of palmitoyl-CoA, together with the coupled phosphorylations of the third stage:

\begin{matrix} 8 Acetyl-CoA + 16 O_{2} + 80 P_{i} + 80 ADP \to \\ 8 CoA + 80 ATP + 16 {CO}_{2} + 16 H_{2} O \end{matrix}

$StartLayout 1st Row 8 Acetyl hyphen CoA plus 16 upper O Subscript 2 Baseline plus 80 upper P Subscript i Baseline plus 80 ADP right-arrow 2nd Row 8 CoA plus 80 ATP plus 16 CO Subscript 2 Baseline plus 16 upper H Subscript 2 Baseline upper O EndLayout$

(17-5)

Combining Equations 17-4 and 17-5, we obtain the overall equation for the complete oxidation of palmitoyl-CoA to carbon dioxide and water:

\begin{matrix} Palmitoyl-CoA + 23 O_{2} + 108 P_{i} + 108 ADP \to \\ CoA + 108 ATP + 16 {CO}_{2} + 23 H_{2} O \end{matrix}

$StartLayout 1st Row Palmitoyl hyphen CoA plus 23 upper O Subscript 2 Baseline plus 108 upper P Subscript i Baseline plus 108 ADP right-arrow 2nd Row CoA plus 108 ATP plus 16 CO Subscript 2 Baseline plus 23 upper H Subscript 2 Baseline upper O EndLayout$

(17-6)

Table 17-1 summarizes the yields of NADH, $FADH Subscript 2$ $FADH Subscript 2$ , and ATP in the successive steps of palmitoyl-CoA oxidation. Note that because the activation of palmitate to palmitoyl-CoA breaks both phosphoanhydride bonds in ATP (Fig. 17-5), the energetic cost of activating a fatty acid is equivalent to two ATP, and the net gain per molecule of palmitate is 106 ATP. The standard free-energy change for the oxidation of palmitate to ${CO}_{2}$ $CO Subscript 2$ and $H_{2} O$ $upper H Subscript 2 Baseline upper O$ is about 9,800 kJ/mol. Under standard conditions, the energy recovered as the phosphate bond energy of ATP is $106 \times 30.5 kJ/mol = 3,230 kJ/mol$ $106 times 30.5 kJ slash mol equals 3,230 kJ slash mol$ , about 33% of the theoretical maximum. However, when the free-energy changes are calculated from actual concentrations of reactants and products under intracellular conditions, the free-energy recovery is more than 60%; the energy conservation is remarkably efficient (see Worked Example 13-2, p. 480).

TABLE 17.1 Yield of ATP during Oxidation of One Molecule of Palmitoyl-CoA to ${CO}_{2}$ $bold CO Subscript bold 2$ and $H_{2} O$ $bold upper H Subscript 2 Baseline bold upper O$
Enzyme catalyzing the oxidation step	Number of NADH or $F A D H_{2}$ $bold upper F bold upper A bold upper D bold upper H bold 2$ formed	Number of ATP ultimately formed^a
β Oxidation
Acyl-CoA dehydrogenase	$7 {FADH}_{2}$ $7 FADH Subscript 2 Baseline$	10.5
β-Hydroxyacyl-CoA dehydrogenase	7 NADH	17.5
Citric acid cycle
Isocitrate dehydrogenase	8 NADH	20
α-Ketoglutarate dehydrogenase	8 NADH	20
Succinyl-CoA synthetase		8^b
Succinate dehydrogenase	$8 {FADH}_{2}$ $8 FADH Subscript 2 Baseline$	12
Malate dehydrogenase	8 NADH	20
Total		108
^aThese calculations assume that mitochondrial oxidative phosphorylation produces 1.5 ATP per ${FADH}_{2}$ $FADH Subscript 2$ oxidized and 2.5 ATP per NADH oxidized. ^bGTP produced directly in this step yields ATP in the reaction catalyzed by nucleoside diphosphate kinase (p. 487).

Oxidation of Unsaturated Fatty Acids Requires Two Additional Reactions

The fatty acid oxidation sequence just described is typical when the incoming fatty acid is saturated (that is, has only single bonds in its carbon chain). However, most of the fatty acids in the triacylglycerols and phospholipids of animals and plants are unsaturated, having one or more double bonds. These bonds are in the cis configuration and cannot be acted upon by enoyl-CoA hydratase, the enzyme catalyzing the addition of $H_{2} O$ $upper H Subscript 2 Baseline upper O$ to the trans double bond of the $Δ^{2}$ $upper Delta squared$ -enoyl-CoA generated during β oxidation. Two auxiliary enzymes — an isomerase and a reductase — must act on the common unsaturated fatty acids to transform them into substrates for the β-oxidation pathway. We illustrate these auxiliary reactions with two examples.

Oleate is an abundant 18-carbon monounsaturated fatty acid with a cis double bond between C-9 and C-10 (denoted $Δ^{9}$ $upper Delta Superscript 9$ ). In the first step of oxidation, oleate is converted to oleoyl-CoA and, like the saturated fatty acids, enters the mitochondrial matrix via the carnitine shuttle (Fig. 17-6). Oleoyl-CoA then undergoes three passes through the fatty acid oxidation cycle to yield three molecules of acetyl-CoA and the coenzyme A ester of a $Δ^{3}$ $upper Delta cubed$ , 12-carbon unsaturated fatty acid, cis- $Δ^{3}$ $upper Delta cubed$ -dodecenoyl-CoA (Fig. 17-10). This product cannot serve as a substrate for enoyl-CoA hydratase, which acts only on trans double bonds. The auxiliary enzyme $Δ^{3}, Δ^{2}$ $bold upper Delta cubed bold comma bold upper Delta squared$ -enoyl-CoA isomerase isomerizes the cis- $Δ^{3}$ $upper Delta cubed$ -enoyl-CoA to the trans- $Δ^{2}$ $upper Delta squared$ -enoyl-CoA, which is converted by enoyl-CoA hydratase into the corresponding l-β hydroxyacyl-CoA (trans- $Δ^{2}$ $upper Delta squared$ -dodecenoyl-CoA). This intermediate is now acted upon by the remaining enzymes of β oxidation to yield acetyl-CoA and the coenzyme A ester of a 10-carbon saturated fatty acid, decanoyl-CoA. The latter undergoes four more passes through the β-oxidation pathway to yield five more molecules of acetyl-CoA. Altogether, nine acetyl-CoAs are produced from one molecule of the 18-carbon oleate.

A figure shows three steps in the oxidation of a monounsaturated fatty acid. — FIGURE 17-10 Oxidation of a monounsaturated fatty acid. Oleic acid, as oleoyl-CoA $left-parenthesis upper Delta Superscript 9 Baseline right-parenthesis$ $left-parenthesis upper Delta Superscript 9 Baseline right-parenthesis$ , is the example used here. Oxidation requires an additional enzyme, enoyl-CoA isomerase, to reposition the double bond, converting the cis isomer to a trans isomer, an intermediate in β oxidation.

FIGURE 17-10 Oxidation of a monounsaturated fatty acid. Oleic acid, as oleoyl-CoA $left-parenthesis upper Delta Superscript 9 Baseline right-parenthesis$ $left-parenthesis upper Delta Superscript 9 Baseline right-parenthesis$ , is the example used here. Oxidation requires an additional enzyme, enoyl-CoA isomerase, to reposition the double bond, converting the cis isomer to a trans isomer, an intermediate in β oxidation.

Oleoyl-Co A is shown as a zigzag chain of 18 carbons. C 1 is on the right, numbered, double bonded to O to the upper right, and bonded to S to the lower right further bonded to Co A. There are wavy lines across the bonds between carbons 2 and 3, 4 and 5, and 6 and 7. There is a red highlighted cis double bond between carbons 9 and 10. Carbon 18 at the left end is numbered. An arrow pointing downward is labeled Greek letter beta oxidation (3 cycles) and has an arrow branching off to show the loss of 3 acetyl-Co A. This yields italicized cis end italics Greek letter delta superscript 3-dodecenoyl-Co A. It is a 12-carbon zigzag chain. C 1 is double bonded to O to the upper right and bonded to S to the lower right that is further bonded to Co A. There is a red cis double bond between C 3 and C 4, and each has a red bond to a red highlighted H. C 12 at the far left end is numbered. Upward- and downward-pointing arrows are labeled blue highlighted Greek letter delta superscript 3, Greek letter delta superscript 2-enoyl-Co A isomerase. This yields italicized trans end italics Greek letter delta superscript 2-dodecenoyl-Co A, a similar 12-carbon molecule with a trans double bond between C 2 and C 3 and with red bonds to red highlighted H atoms on C 2 and C 3. A downward arrow is labeled Greek letter beta oxidation (five cycles) and points to text reading, 6 acetyl-Co A.

The other auxiliary enzyme (a reductase) is required for oxidation of polyunsaturated fatty acids — for example, the 18-carbon linoleate, which has a $c i s hyphen upper Delta Superscript 9$ $c i s hyphen upper Delta Superscript 9$ , $c i s - Δ^{12}$ $c i s hyphen upper Delta Superscript 12$ configuration (Fig. 17-11). Linoleoyl-CoA undergoes three passes through the β-oxidation sequence to yield three molecules of acetyl-CoA and the coenzyme A ester of a 12-carbon unsaturated fatty acid with a $c i s - Δ^{3}$ $c i s hyphen upper Delta cubed$ , $c i s - Δ^{6}$ $c i s hyphen upper Delta Superscript 6$ configuration. This intermediate cannot be used by the enzymes of the β-oxidation pathway: its double bonds are in the wrong position and have the wrong configuration (cis, not trans). However, the combined action of enoyl-CoA isomerase and 2,4-dienoyl-CoA reductase, as shown in Figure 17-11, transforms this intermediate into one that can enter the β-oxidation pathway and be degraded to six acetyl-CoAs. The overall result is conversion of linoleate to nine molecules of acetyl-CoA.

A figure shows six steps in the oxidation of a polyunsaturated fatty acid. — FIGURE 17-11 Oxidation of a polyunsaturated fatty acid. The example here is linoleic acid, as linoleoyl-CoA $(Δ^{9,12})$ $left-parenthesis upper Delta Superscript 9 comma 12 Baseline right-parenthesis$ . Oxidation requires a second auxiliary enzyme in addition to enoyl-CoA isomerase: NADPH-dependent 2,4-dienoyl-CoA reductase. The combined action of these two enzymes converts a $t r a n s - Δ^{2}$ $t r a n s hyphen upper Delta squared$ , $c i s - Δ^{4}$ $c i s hyphen upper Delta Superscript 4$ -dienoyl-CoA intermediate to the $t r a n s - Δ^{2}$ $t r a n s hyphen upper Delta squared$ -enoyl-CoA substrate necessary for β oxidation.

FIGURE 17-11 Oxidation of a polyunsaturated fatty acid. The example here is linoleic acid, as linoleoyl-CoA $(Δ^{9,12})$ $left-parenthesis upper Delta Superscript 9 comma 12 Baseline right-parenthesis$ . Oxidation requires a second auxiliary enzyme in addition to enoyl-CoA isomerase: NADPH-dependent 2,4-dienoyl-CoA reductase. The combined action of these two enzymes converts a $t r a n s - Δ^{2}$ $t r a n s hyphen upper Delta squared$ , $c i s - Δ^{4}$ $c i s hyphen upper Delta Superscript 4$ -dienoyl-CoA intermediate to the $t r a n s - Δ^{2}$ $t r a n s hyphen upper Delta squared$ -enoyl-CoA substrate necessary for β oxidation.

Linoleoyl-Co A is shown as a zigzag chain of 18 carbons. C 1 is on the right, numbered, double bonded to O to the upper right, and bonded to S to the lower right further bonded to Co A. There are wavy lines across the bonds between carbons 2 and 3, 4 and 5, and 6 and 7. There are red cis double bonds between carbons 9 and 10 and between carbons 12 and 13. Carbon 18 at the left end is numbered. An arrow pointing downward is labeled Greek letter beta oxidation (3 cycles) and has an arrow branching off to show the loss of 3 acetyl-Co A. This yields italicized cis end italics Greek letter delta superscript 3, italicized cis end italics Greek letter delta superscript 6. It is a twelve-carbon zigzag chain. C 1 is double bonded to O to the upper right and bonded to S to the lower right that is further bonded to Co A. There are red cis double bonds between C 3 and C 4 and between C 6 and C 7. C 5 is numbered with a red 5. C 12 at the far left end is numbered with a red 12. C 3 is labeled red highlighted 3 (Greek letter beta). Upward- and downward-pointing arrows are labeled blue highlighted Greek letter delta superscript 3, Greek letter delta superscript 2-enoyl-Co A isomerase. This yields italicized trans end italics Greek letter delta superscript 2, italicized cis Greek letter delta superscript 6. It is a similar molecule except that C 1 is bonded to S to the upper right further bonded to Co A and double bonded to O to the lower right, there is a red trans double bond between red highlighted 2 (Greek letter alpha) and 3 (Greek letter beta), there is no double bond between C 3 and C 4, and there is a red cis double bond between C 6 and C 7. A downward arrow is accompanied by text reading, Greek letter beta oxidation (one cycle, and first oxidation of second cycle). An arrow curves away to show the loss of acetyl Co A. This yields italicized trans end italics Greek letter delta superscript 2, italicized cis end italics Greek letter delta superscript 4. It is a 10 carbon zigzag chain with C 1 unchanged, a red trans double bond between C 2 and C 3, and a red cis double bond between C 4 and C 5. It ends at C 10, numbered in red. An arrow points downward accompanied by blue highlighted text reading, 2,4-dienoyl-Co A reductase. A curved arrow shows that N A D P H plus H plus is added and N A D P plus is lost. This yields italicized trans end italics Greek letter delta superscript 3. It is a 10-carbon zigzag chain with carbons 1 through 5 and carbon 10 numbered in red. C 1 is double bonded to O to the upper right and bonded to S to the lower right that is further bonded to Co A. There is a trans double bond between C 3 and C 4. There are no other double bonds. An arrow points down accompanied by blue highlighted text reading, enoyl-Co A isomerase. This yields italicized trans end italics Greek letter delta superscript 2. It is a 10-carbon zigzag chain with carbons 1 through 4 numbered. There is a red double bond between C 2 and C 3. There are no other double bonds. An arrow pointing down is labeled Greek letter beta oxidation (four cycles). This arrow points to text reading, 5 acetyl-Co A.

Complete Oxidation of Odd-Number Fatty Acids Requires Three Extra Reactions

Although most naturally occurring lipids contain fatty acids with an even number of carbon atoms, fatty acids with an odd number of carbons are common in the lipids of many plants and some marine organisms. Cattle and other ruminant animals form large amounts of the three-carbon propionate $({CH}_{3} — {CH}_{2} — {COO}^{-})$ $left-parenthesis CH Subscript 3 Baseline em-dash CH Subscript 2 Baseline em-dash COO Superscript minus Baseline right-parenthesis$ during fermentation of carbohydrates in the rumen. The propionate is absorbed into the blood and oxidized by the liver and other tissues.

Long-chain odd-number fatty acids are oxidized in the same pathway as the even-number acids, beginning at the carboxyl end of the chain. However, the substrate for the last pass through the β-oxidation sequence is a fatty acyl–CoA with a five-carbon fatty acid. When this is oxidized and cleaved, the products are acetyl-CoA and propionyl-CoA. The acetyl-CoA can be oxidized in the citric acid cycle, of course, but propionyl-CoA enters a different pathway, having three enzymes.

Propionyl-CoA is first carboxylated to form the d stereoisomer of methylmalonyl-CoA by propionyl-CoA carboxylase, which contains the cofactor biotin (Fig. 17-12). In this enzymatic reaction, as in the pyruvate carboxylase reaction (see Fig. 16-16), bicarbonate ion ${(HCO}_{3}^{-})$ $left-parenthesis HCO Subscript 3 Superscript minus Baseline right-parenthesis$ is activated by attachment to biotin before its transfer to the substrate, in this case the propionate moiety. Formation of the carboxybiotin intermediate requires energy, which is provided by ATP. The d-methylmalonyl-CoA thus formed is enzymatically epimerized to its l stereoisomer by methylmalonyl-CoA epimerase. The l-methylmalonyl-CoA then undergoes an intramolecular rearrangement to form succinyl-CoA, which can enter the citric acid cycle. This rearrangement is catalyzed by methylmalonyl-CoA mutase, which requires as its coenzyme $5^{'}$ $bold 5 prime$ -deoxyadenosylcobalamin, or coenzyme $B_{12}$ $bold upper B bold 12$ , which is derived from vitamin $B_{12}$ $upper B Subscript 12$ (cobalamin). Box 17-2 describes the role of coenzyme $B_{12}$ $upper B Subscript 12$ in this remarkable exchange reaction.

A figure shows the conversion of propionyl-Co A, which has three carbons, to produce L-methylmalonyl-Co A and succinyl-Co A. — FIGURE 17-12 Oxidation of propionyl-CoA produced by β oxidation of odd-number fatty acids. The sequence involves the carboxylation of propionyl-CoA to d-methylmalonyl-CoA and conversion of the latter to succinyl-CoA. This conversion requires epimerization of d- to l-methylmalonyl-CoA, followed by a remarkable reaction in which substituents on adjacent carbon atoms exchange positions (see Box 17-2).

Propionyl-Co A is a vertical three-carbon chain with the top C bonded to H to the left, above, and to the right; the middle C bonded to H to the left and right; and the bottom C bonded to S to the lower left further bonded to Co A and double bonded to O to the lower right. An arrow points down accompanied by blue highlighted text reading, propionyl-CoA carboxylase. A curved line shows the addition of red highlighted H C O 3 minus. Below, a curved arrow shows the addition of A T P and removal of A D P plus P i with red highlighted biotin shown at the inflection point of the arrow. This yields D-methylmalonyl-Co A, which is similar except that the central C is bonded to a red highlighted substituent to its left of C bonded to O minus to its upper left and double bonded to O to its lower left. Upward- and downward-pointing arrows are accompanied by blue highlighted text reading methylmalonyl-Co A epimerase. This yields L-methylmalonyl-Co A, a vertical three-carbon chain with the top C bonded to H in a light red box on its left, H above, and H to the right; the middle C bonded to C on its left in a blue box that is double bonded to O to the upper left and bonded to S to the lower left further bonded to Co A, all within the light blue box, and to H on its right; and the bottom C highlighted in red connected to the middle C by a red bond. The bottom C is connected by a red bond to O minus to its lower left and by a red double bond to red highlighted O to its lower right. Right- and left-pointing arrows show a reversible reaction that produces succinyl Co A. Red highlighted coenzyme B subscript 12 is above the right-pointing arrow, and blue highlighted methylmalonyl-Co A mutase is below the left-pointing arrow. Succinyl Co A is a vertical three-carbon chain with the top C bonded to C in a blue box to its left further double bonded to O to the upper left and bonded to S to the lower left further bonded to Co A, all within the blue box, and bonded to H above and to the right; the middle C is bonded to H in a red box on the left and to H on the right; and the bottom C is highlighted in red connected to the middle C by a red bond and is connected by a red bond to red highlighted O minus to the lower left and by a red double bond to red highlighted O to the lower right.

Box 17-2

Coenzyme $B_{12}$ $bold upper B Subscript bold 1 bold 2$ : A Radical Solution to a Perplexing Problem

In the methylmalonyl-CoA mutase reaction (see Fig. 17-12), the group $— CO — S-CoA$ $em-dash CO em-dash upper S hyphen CoA$ at C-2 of the original propionate exchanges position with a hydrogen atom at C-3 of the original propionate (Fig. 1a). Coenzyme $B_{12}$ $upper B Subscript 12$ is the cofactor for this general type of reaction (Fig. 1b). These coenzyme $B_{12}$ $upper B Subscript 12$ –dependent processes are among the very few enzymatic reactions in biology in which there is an exchange of an alkyl or substituted alkyl group (X) with a hydrogen atom on an adjacent carbon, with no mixing of the transferred hydrogen atom with the hydrogen of the solvent, $H_{2} O$ $upper H Subscript 2 Baseline upper O$ . How can the hydrogen atom move between two carbons without mixing with the enormous excess of hydrogen atoms in the solvent?

FIGURE 1

Part a shows L-methylmalonyl-Co A, which has a left-end C bonded to H above, H to the left, H in a red box below, and a central C to the right further bonded to H above, C in a blue box below that is further bonded, and a right-hand C that is double bonded to O to the upper right and bonded to O minus to the lower right. C in the blue box is double bonded to O to its lower left and bonded to S to its lower right that is further bonded to Co A, all within the blue box. Right- and left-pointing arrows indicate a reversible reaction. Coenzyme B subscript 12 is shown above the arrows and blue highlighted methylmalonyl-Co A mutase is shown beneath the arrows. This yields succinyl-Co A, which has a similar structure except that the left-hand C is bonded to C in the blue box and its substituents and the middle C is bonded to H in the red box. Part b shows C that is bonded above, to the left, to H in a red box below, and to C on the right that is bonded above, to the right, and to X in a blue box below. Right- and left-pointing arrows indicate a reversible reaction. Coenzyme B subscript 12 is shown above the arrows. This yields C that is bonded above, to the left, to X in a blue box below, and to C on the right that is bonded above, to the right, and to H in a red box below.

Coenzyme $B_{12}$ $upper B Subscript 12$ is the cofactor form of vitamin $upper B Subscript 12$ $upper B Subscript 12$ , which is unique among all the vitamins in that it contains an essential trace element, cobalt. The complex corrin ring system of vitamin $B_{12}$ $upper B Subscript 12$ (colored blue in Fig. 2), to which cobalt (as ${Co}^{3 +}$ $Co Superscript 3 plus$ ) is coordinated, is chemically related to the porphyrin ring system of heme (see Fig. 5-1). A fifth coordination position of cobalt is filled by dimethylbenzimidazole ribonucleotide (shaded yellow), bound covalently by its $3^{'}$ $3 prime$ -phosphate group to a side chain of the corrin ring, through aminoisopropanol. The formation of this complex cofactor occurs in one of only two known reactions in which triphosphate is cleaved from ATP (Fig. 3); the other reaction is the formation of S-adenosylmethionine from ATP and methionine (see Fig. 18-18).

FIGURE 2

5 prime-deoxyadenosine is shown highlighted in red at the upper left. It has a five-membered ring with O substituted for C at the top vertex and the carbons numbered. It has C 1 prime bonded to H above and to N of the upper right of a five-membered ring below, C 2 prime and C 3 prime bonded to O H above and H below, C 4 prime bonded to H above and to C 5 prime below, and C 5 prime bonded to C o superscript 3 plus in the center of the corrin ring system below. The five-membered ring shares its left side with a six-membered ring. It has N substituted for C at its upper right vertex at position 9 and further bonded to C 1 prime of the ring above, a double bond at its lower right side between positions 7 and 8, a double bond at its left side between positions 4 and 5, and N substituted for C at its lower right vertex at position 7. The six-membered ring has a double bond at its right side between positions 4 and 5 shared with the five-membered ring, a double bond at its lower left side between positions 1 and 6, a double bond at its upper left side between positions 2 and 3, N substituted for C at its lower left vertex at position 1, N substituted for C at its top vertex at position 3, and its bottom vertex, C 6, bonded to N H 2. A yellow box beneath the corrin ring system is labeled dimethylbenzimidazole ribonucleotide. It has a five-membered ring with O substituted for C at the top vertex; C 1 prime bonded to H above and C H 2 O H below; C 2 prime bonded to O above further bonded to P to the right double bonded to O below, bonded to O minus to the right, and bonded to O above further bonded to C at the beginning of aminoisopropanol; C 3 prime bonded to O H above and H below; and C 4 prime bonded to N at the lower right vertex of a five-membered ring above and to H below. The five-membered ring shares its left side with a benzene ring, has N substituted for C at position 1, has a double bond between C 8 and C 9 at the left side shared with the left-hand ring, has a double bond at its upper right side between positions 2 and 3, and has N substituted for C in position 3 that is connected by a dashed arrow that becomes a solid red arrow that points to C o superscript 3 plus in the venter of the corrin ring system. The six-membered ring is a benzene ring that shares its right side with the five-membered ring and has C H 3 bonded to the upper and lower left vertices in positions 5 and 6. The aminoisopropanol group has C bonded to O of the phosphate of dimethylbenzimidazole ribonucleotide. This C is bonded to H, to C H 3 to the right, and to C H 2 above further bonded to N H that is bonded above the aminoisopropanol group to C double bonded to O further bonded to C H 2 bonded to C H 2 bonded to C 13 of the corrin ring system. The corrin ring system consists of 8 rings or partial rings arranged around C o superscript 3 plus in the center. The lower left-hand ring has its left side vertex bonded to 2 C H 3, its bottom vertex bonded below to C H 2 C H 2 C double bonded to O and to N H 2 and above to blue highlighted H, a right side bond shared with the central ring, N at the upper right vertex with a red arrow pointing to C o superscript 3 plus in the center, and a double bond across its top side shared with the center side ring. The center bottom ring has a double bond at its lower left vertex, a bottom vertex bonded to C H 3, a right side that shares a double bond with the right side ring, and N at its upper right vertex with a red arrow pointing to C o superscript 3 plus in the center. The right side ring has a bottom vertex bonded below to C H 2 of the chain leading to the aminoisopropanol group and bonded above to blue highlighted C H 3, a right side vertex bonded above to C H 2 bonded to C double bonded to O and bonded to N H 2 and below to H, and a top right vertex bonded to H with a long bond across the central region to the bottom vertex of top right ring, which also has a blue bond up to blue highlighted C H 3. The top right ring has a right side vertex bonded above to C H 2 bonded to C double bonded to O and bonded to N H 2 and bonded below to C H 3, a top vertex bonded above to C H 2 C H 2 C double bonded to O and bonded to N H 2 and to the left to H, a left side shared with the central ring, and a lower left vertex with N bonded to C o superscript 3 plus. The middle upper ring has a double bond at its upper right side, a top vertex bonded to C H 3, a left side with a double bond shared with the upper left ring, and N at the lower left vertex with an arrow to C o superscript 3 plus. The left side ring has a top vertex bonded above to C H 2 C double bonded to O and bonded to N H 2 and bonded below to blue highlighted C H 3, a left side vertex bonded to above to H and below to C H 2 C H 2 C double bonded to O and bonded to N H 2, and a bottom side shared with a partial central ring that has a double bond at its upper left side.

FIGURE 3

A T P is shown as a five-membered ring with O substituted for C at the top vertex and the carbons numbered. A T P has C 1 prime bonded to H above and to N at the upper right of a five-membered ring below, C 2 prime and C 3 prime bonded to O H above and H below, C 4 prime bonded to H above and to C 5 prime below, and C 5 prime bonded to 2 H and to O bonded to P double bonded to O above, bonded to O minus below, and bonded to P to the right double bonded to O above, bonded to O minus below, and bonded to P on the right double bonded to O above, bonded to O minus below, and bonded to O minus to the right. The five-membered ring shares its left side with a six-membered ring. It has N substituted for C at its upper right vertex at position 9 and further bonded to C 1 prime of the ring above, a double bond at its lower right side between positions 7 and 8, a double bond at its left side between positions 4 and 5, and N substituted for C at its lower right vertex at position 7. The six-membered ring has a double bond at its right side between positions 4 and 5 shared with the five-membered ring, a double bond at its lower left side between positions 1 and 6, a double bond at its upper left side between positions 2 and 3, N substituted for C at its lower left vertex at position 1, N substituted for C at its top vertex at position 3, and its bottom vertex, C 6, bonded to N H 2. Red highlighted C o with radiating lines extending from it is labeled cobalamin and has an arrow pointing to C 5 prime. An arrow points down with a branching off to show the loss of three phosphates. These are O minus bonded to P double bonded to O above, bonded to O minus below, and bonded to O to the right bonded to P double bonded to O above, bonded to O minus below, and bonded to O to the right further bonded to P double bonded to O above, bonded to O minus below, and bonded to O minus to the right. The downward arrow shows that the reaction yields coenzyme B subscript 12. This has a similar structure to A T P, except that C 5 prime is now bonded to C o surrounded by radiating lines instead of to a chain of three phosphates.

Vitamin $B_{12}$ $upper B Subscript 12$ is usually isolated as cyanocobalamin, because it contains a cyano group (picked up during purification) attached to cobalt in the sixth coordination position. In $5^{'}$ $bold 5 bold prime$ -deoxyadenosylcobalamin, the cofactor for methylmalonyl-CoA mutase, the cyano group is replaced by the $5^{'}$ $bold 5 bold prime$ -deoxyadenosyl group (red in Fig. 2), covalently bound through C- $5^{'}$ $5 prime$ to the cobalt. The three-dimensional structure of the cofactor was determined by Dorothy Crowfoot Hodgkin in 1956, using x-ray crystallography.

Dorothy Crowfoot Hodgkin, 1910–1994

The key to understanding how coenzyme $B_{12}$ $upper B Subscript 12$ catalyzes hydrogen exchange lies in the properties of the covalent bond between cobalt and C- $5^{'}$ $5 prime$ of the deoxyadenosyl group (Fig. 2). This is a relatively weak bond; merely illuminating the compound with visible light is enough to break this $Co — C$ $Co em-dash upper C$ bond. (This extreme photolability probably accounts for the absence of vitamin $B_{12}$ $upper B Subscript 12$ in plants.) Dissociation produces a $5^{'}$ $5 prime$ -deoxyadenosyl radical and the ${Co}^{2 +}$ $Co Superscript 2 plus$ form of the vitamin. By generating free radicals in this way, $5^{'}$ $5 prime$ -deoxyadenosylcobalamin initiates a series of transformations such as that illustrated in Figure 4 — a postulated mechanism for the reaction catalyzed by methylmalonyl-CoA mutase and several other coenzyme $B_{12}$ $upper B Subscript 12$ –dependent transformations. In this postulated mechanism, the migrating hydrogen atom never exists as a free species and is thus never free to exchange with the hydrogen of surrounding water molecules.

MECHANISM FIGURE 4

A circle of clockwise arrows contains text in the center as follows: The migrating hydrogen in steps and never exists as a free species and does not exchange with the hydrogen of surrounding water molecules. Coenzyme B subscript 12 is shown at the eleven o’clock position as a box labeled deoxyadenosine with a bond down to C H 2 with H 2 highlighted in red and a bond down to C o 3 plus in the center of a rectangle with N at each corner that each have a red arrow pointing to C o 3 plus. An arrow points from this position to a 5 prime-deoxyadenosyl free radical at the one o’clock position beneath text that reads, The C o to C bond undergoes homolytic cleavage yielding C o 2 plus and the 5 prime-deoxyadenosyl free radical. A substrate is added to an arrow that runs clockwise from the reactant at the one o’clock position to the three o’clock position. The substrate is C further bonded to the left and above, bonded to red highlighted H below, and bonded to C to the right bonded above and to the right and bonded to blue highlighted X below. The reactant is labeled 5 prime-deoxyadenosyl free radical. It is shown as a rectangle with N at each vertex, each with an arrow pointing in to C o superscript 2 plus in the center. Above, a rectangle labeled deoxyadenosine has a bond down to C with a single red dot below and bonds to red H to the left and right. Accompanying text reads, The radical is converted to 5 prime-deoxyadenosine by abstraction of a hydrogen atom from the substrate, producing a substrate radical. The product is a rectangle labeled deoxyadenosine with a bond down to C with bonds to red H to the left, right, and below above a similar rectangle with N in each corner that each have a red arrow pointing to C o superscript 2 plus in the center. To the right, the substrate radical is shown as a molecule that is similar to the substrate except that the left-hand C has a single red dot below instead of a bond to a red H. An arrow labeled radical rearrangement points down to a product at the six o’clock position. Accompanying text reads, The substrate radical is rearranged producing a new radical with the carbon skeleton of the product. For methylmalonyl-Co A mutase, the migrating group (X) is – C O – S – Co A. This yields a rectangle labeled deoxyadenosine with a bond down to C bonded to red H to the left, right, and below above a rectangle with N at each corner that each have a red arrow pointing to C o superscript 2 plus in the center. The product like radical resembles the substrate radical except that the left-hand C is now bonded to blue highlighted X and the right-hand C now has a single red dot. An arrow points clockwise to the nine o’clock position accompanied by text reading, A hydrogen from the 5 prime – C H 3 of the deoxyadenosine is returned to the product like radical, forming product. A branch from the main arrow points to the product, which has a left-hand C bonded to the left, above, to blue highlighted X below, and to C to the right that is bonded above, to the right, and to red highlighted H below. The clockwise arrow ends at the 5 prime-deoxyadenosyl free radical. There is a rectangle labeled deoxyadenosine bonded to C below with a single red dot below and bonded to red highlighted H on each side above a rectangle with N at each corner that each have a red arrow pointing to C o superscript 2 plus in the center. A clockwise arrow points up to the eleven o’clock position accompanied by text reading, The bond between the 5 prime – C H 2 of the deoxyadenosyl radical and cobalt is re-formed, regenerating the B subscript 12 cofactor in its stable C o superscript 3 plus form. This yields coenzyme B subscript 12, shown as a box labeled deoxyadenosine with a bond down to C H 2 with H 2 highlighted in red and a bond down to C o 3 plus in the center of a rectangle with N at each corner that each have a red arrow pointing to C o 3 plus.

Vitamin $B_{12}$ $upper B Subscript 12$ is not made by plants or animals and can be synthesized only by a few species of microorganisms. It is required by healthy people in only minute amounts, about $3 μ g/day$ $3 mu g slash day$ . Vitamin $B_{12}$ $upper B Subscript 12$ deficiency results in pernicious anemia, caused by a failure to absorb vitamin $B_{12}$ $upper B Subscript 12$ efficiently from the intestine, where it is synthesized by intestinal bacteria or obtained from digestion of meat. Individuals with this disease do not produce sufficient amounts of intrinsic factor, a glycoprotein essential to vitamin $B_{12}$ $upper B Subscript 12$ absorption. The pathology in pernicious anemia includes reduced production of erythrocytes, reduced levels of hemoglobin, and severe, progressive impairment of the central nervous system. Administration of large doses of vitamin $B_{12}$ $upper B Subscript 12$ alleviates these symptoms in at least some cases.

About 1 in 100,000 babies are born with a genetic deficiency in propionyl-CoA carboxylase, making them unable to metabolize the propionyl-CoA that results from catabolism of odd-number fatty acids, fatty acids with methyl branches, and the amino acids isoleucine, valine, threonine, and methionine. The absence of functional propionyl-CoA carboxylase leads to an accumulation of propionyl-CoA in mitochondria, depleting the available supply of coenzyme A for continuing β-oxidation and other metabolism. The propionyl-CoA is esterified to carnitine, transported out of the mitochondria through the carnitine shuttle, and eventually released to the blood as propionate, which severely acidifies the blood and urine, a condition called propionic acidemia. Symptoms generally manifest in the first few days of life, with vomiting, low blood glucose, seizures, and neurologic abnormalities. The condition is diagnosed by detection of propionate and its metabolites in the blood and urine. Treatments include severe restriction of dietary protein, supplying carnitine, and the use of antibiotics targeted at gut bacteria that produce odd-chain and branched-chain fatty acids. Some individuals who are defective in the incorporation of biotin into the enzyme benefit from treatment with high doses of biotin.

Fatty Acid Oxidation Is Tightly Regulated

Oxidation of fatty acids consumes a precious fuel, and it is regulated so as to occur only when the organism’s need for energy requires it. In the liver, fatty acyl–CoA formed in the cytosol has two major pathways open to it: (1) β oxidation by enzymes in mitochondria or (2) conversion into triacylglycerols and phospholipids by enzymes in the cytosol. The pathway taken depends on the rate of transfer of long-chain fatty acyl–CoA into mitochondria. The three-step carnitine shuttle by which fatty acyl groups are carried into the mitochondrial matrix as fatty acyl–carnitine (Fig. 17-6) is rate-limiting for fatty acid oxidation and is an important point of regulation. Once fatty acyl groups have entered the mitochondrion, they are committed to oxidation to acetyl-CoA.

Figure 17-13 illustrates, with numbered steps described below, the coordinated down-regulation of β oxidation when carbohydrate levels are sufficient. Ingestion of a high-carbohydrate meal raises the blood glucose level and thus triggers the release of insulin. Insulin-dependent protein phosphatase dephosphorylates the enzyme acetyl-CoA carboxylase (ACC), activating it. ACC catalyzes the formation of malonyl-CoA (the first intermediate of cytosolic fatty acid synthesis; see Figs 21-1 and 21-2), and malonyl-CoA inhibits carnitine acyltransferase 1, thereby preventing fatty acid entry into the mitochondrial matrix through the carnitine shuttle. The inhibition of carnitine acyltransferase 1 by malonyl-CoA ensures that the oxidation of fatty acids is inhibited whenever the liver is amply supplied with glucose as fuel and is actively making triacylglycerols from excess glucose. Two of the enzymes of β oxidation () are also regulated by metabolites that signal energy sufficiency. When the $[NADH] / [{NAD}^{+}]$ $left-bracket NADH right-bracket slash left-bracket NAD Superscript plus Baseline right-bracket$ ratio is high, β-hydroxyacyl-CoA dehydrogenase is inhibited; in addition, high concentrations of acetyl-CoA inhibit thiolase.

Malonyl-Co A has C double bonded to O to the upper right, bonded to O minus to the lower right, and bonded to C H 2 to the right further bonded to C double bonded to O to the upper right and bonded to S to the lower right further bonded to Co A.

A figure shows how fatty acid synthesis and breakdown are regulated in a coordinated way. — FIGURE 17-13 Coordinated regulation of fatty acid synthesis and breakdown. The steps are explained in the text.

The figure shows fatty acid synthesis on the left and fatty acid Greek letter beta oxidation on the right. Dietary carbohydrate is shown at the upper left corner with an arrow pointing down to glucose and a dashed blue arrow pointing right to high blood glucose. An arrow labeled glycolysis, pyruvate dehydrogenase complex points from glucose to acetyl-Co A. High blood glucose has a dashed blue arrow numbered 1 beneath it that points to upward arrow symbol insulin above a dashed blue arrow labeled 2 that points to a green circle containing a green triangle above phosphatase next to a curved arrow pointing down from a blue oval labeled A C C with a bond to a red circle labeled P above and the word inactive below to a similar blue oval below without a bound circle labeled P and with short radiating lines extending out in all directions. The curved arrow has a branch to show the loss of P subscript i. Acetyl Co A produced from glucose is to the lower left of the blue oval labeled A C C ,and a curved arrow numbered 3 curves up against A C C and then down to malonyl-Co A, which has a blue arrow labeled multistep pointing down to fatty acids. A dashed arrow labeled 4 points from malonyl Co A into the fatty acid Greek letter beta oxidation half of the figure and ends at a red circle containing a red X next to an arrow labeled carnitine acyl-transferase 1 that points down from fatty acyl-Co A to fatty acyl-carnitine below. To the right of the high blood glucose pathway, there is a pathway associated with low blood glucose. Beneath low blood glucose, a dashed red arrow points to upward arrow symbol glucagon, from which a dashed red arrow next to the number 5 points down to a green circle around a green triangle above P K A, A M P K to the right of a red curved arrow labeled 6 that runs from active A C C up to inactive A C C. Fatty acyl-carnitine is shown just outside of a mitochondrion. A red arrow labeled 7 points from fatty acyl-carnitine outside of the mitochondrion to fatty acyl-carnitine in the mitochondrial matrix. A series of reactions take place in the mitochondrion. Co A at the top has a black arrow pointing down that meets a curved red arrow from fatty acyl-carnitine. A black arrow curves away to show the loss of carnitine, and the red arrow continues to fatty acyl-Co A at the start of a bracket labeled 8 Greek letter beta oxidation. A red arrow points down next to F A D H 2, followed by another red arrow, followed by a red arrow next to N A D H, followed by a final red arrow pointing down to acetyl Co A.

Figure 17-13 also shows that conversely, when blood glucose levels drop between meals, glucagon release activates protein kinase A (PKA), which phosphorylates and inactivates ACC. Recall from Chapter 14 that in a low-energy state, [AMP] rises relative to [ATP], activating AMP kinase (AMPK). Activated AMPK also phosphorylates and inactivates ACC. The concentration of malonyl-CoA falls, and the inhibition of fatty acid entry into mitochondria is relieved. Fatty acids enter the mitochondrial matrix, allowing β oxidation to replenish the supply of ATP. Because glucagon also triggers the mobilization of fatty acids in adipose tissue, a supply of fatty acids begins arriving in the blood.

Transcription Factors Turn on the Synthesis of Proteins for Lipid Catabolism

In addition to the various short-term regulatory mechanisms that modulate the activity of existing enzymes, transcriptional regulation can change the number of molecules of the enzymes of fatty acid oxidation on a longer time scale — minutes to hours. The PPAR family of nuclear receptors are transcription factors that affect many metabolic processes in response to a variety of fatty acid–like ligands. (They were originally recognized as peroxisome proliferator-activated receptors, then were found to function more broadly.) $PPAR α$ $PPAR alpha$ acts in muscle, adipose tissue, and liver to turn on a set of genes essential for fatty acid oxidation, including the fatty acid transporter, carnitine acyltransferases 1 and 2; the fatty acyl–CoA dehydrogenases for short, medium, long, and very long acyl chains; and related enzymes. This response is triggered when a cell or an organism has an increased demand for energy from fat catabolism, such as during a fast between meals or under conditions of longer-term starvation. Glucagon, released in response to low blood glucose, can act through cAMP and the transcription factor CREB to turn on certain genes for lipid catabolism.

Another situation that is accompanied by major changes in the expression of the enzymes of fatty acid oxidation is the transition from fetal to neonatal metabolism in the heart. In the fetus, the principal fuels in heart muscle are glucose and lactate, but in the neonatal heart, fatty acids are the main fuel. At the time of this transition, $PPAR α$ $PPAR alpha$ is activated and in turn activates the genes essential for fatty acid metabolism. As we will see in Chapter 23, two other transcription factors in the PPAR family also play crucial roles in determining the enzyme complements — and therefore the metabolic activities — of specific tissues at particular times (see Fig. 23-38).

The major sites of fatty acid oxidation, at rest and during exercise, are skeletal and heart muscle. Endurance training increases $PPAR α$ $PPAR alpha$ expression in muscle, leading to increased levels of fatty acid–oxidizing enzymes and increased oxidative capacity of the muscle.

Genetic Defects in Fatty Acyl–CoA Dehydrogenases Cause Serious Disease

Stored triacylglycerols are typically the chief source of energy for muscle contraction, and an inability to oxidize fatty acids from triacylglycerols has serious consequences for health. The most common genetic defect in fatty acid catabolism in U.S. and northern European populations is due to a mutation in the gene encoding the medium-chain acyl-CoA dehydrogenase (MCAD), mentioned earlier in the chapter. Among northern Europeans, the frequency of carriers (individuals with this recessive mutation on one of the two homologous chromosomes) is about 1 in 40, and about 1 individual in 10,000 has the disease — that is, has two copies of the mutant MCAD allele and is unable to oxidize fatty acids of 6 to 12 carbons. The disease is characterized by recurring episodes of a syndrome that includes fat accumulation in the liver, high blood levels of octanoic acid (8:0), low blood glucose (hypoglycemia), sleepiness, vomiting, and coma. Although individuals may have no symptoms between episodes, the episodes are very serious; mortality due to this disease is 25% to 60% in early childhood. If the genetic defect is detected shortly after birth, the infant can be started on a low-fat, high-carbohydrate diet. With early detection and careful management of the diet — including avoiding long intervals between meals, to prevent the body from turning to its fat reserves for energy — the prognosis for these individuals is good.

More than 20 human genetic defects in fatty acid transport or oxidation have been documented, most much less common than the defect in MCAD. One of the most severe disorders results from loss of the long-chain β-hydroxyacyl-CoA dehydrogenase activity of the trifunctional protein, TFP. Other disorders include defects in the α or β subunits that affect all three activities of TFP and cause serious heart disease and abnormal skeletal muscle.

Peroxisomes Also Carry Out β Oxidation

The mitochondrial matrix is the major site of fatty acid oxidation in animal cells, but other compartments also contain enzymes capable of oxidizing fatty acids to acetyl-CoA, by a pathway similar but not identical to that in mitochondria. In plant cells, the major site of β oxidation is not mitochondria but peroxisomes. In peroxisomes, organelles also found in animal cells, the intermediates for β oxidation of fatty acids are coenzyme A derivatives, and the process consists of four steps, as in mitochondrial β oxidation (Fig. 17-14): (1) dehydrogenation, (2) addition of water to the resulting double bond, (3) oxidation of the β-hydroxyacyl-CoA to a ketone, and (4) thiolytic cleavage by coenzyme A. The identical reactions also occur in glyoxysomes, organelles found only in germinating seeds.

A figure compares beta oxidation in mitochondria with beta oxidation in peroxisomes and glyoxysomes. — FIGURE 17-14 Comparison of β oxidation in mitochondria and in peroxisomes and glyoxysomes. The peroxisomal/glyoxysomal system differs from the mitochondrial system in three respects: (1) the peroxisomal system prefers very-long-chain fatty acids; (2) in the first oxidative step, electrons pass directly to $upper O Subscript 2$ $upper O Subscript 2$ , generating $H_{2} O_{2}$ $upper H Subscript 2 Baseline upper O Subscript 2$ ; and (3) the NADH formed in the second oxidative step cannot be reoxidized in the peroxisome or glyoxysome, so reducing equivalents are exported to the cytosol, eventually entering mitochondria. The acetyl-CoA produced by peroxisomes and glyoxysomes is also exported; the acetate from glyoxysomes serves as a biosynthetic precursor. Acetyl-CoA produced in mitochondria is further oxidized in the citric acid cycle.

FIGURE 17-14 Comparison of β oxidation in mitochondria and in peroxisomes and glyoxysomes. The peroxisomal/glyoxysomal system differs from the mitochondrial system in three respects: (1) the peroxisomal system prefers very-long-chain fatty acids; (2) in the first oxidative step, electrons pass directly to $upper O Subscript 2$ $upper O Subscript 2$ , generating $H_{2} O_{2}$ $upper H Subscript 2 Baseline upper O Subscript 2$ ; and (3) the NADH formed in the second oxidative step cannot be reoxidized in the peroxisome or glyoxysome, so reducing equivalents are exported to the cytosol, eventually entering mitochondria. The acetyl-CoA produced by peroxisomes and glyoxysomes is also exported; the acetate from glyoxysomes serves as a biosynthetic precursor. Acetyl-CoA produced in mitochondria is further oxidized in the citric acid cycle.

The left half of the figure is labeled mitochondrion, and the right half is labeled peroxisome/glyoxysome. The mitochondrial side has folded membrane along its length. The reactions start with a molecule that extends across both sides. This is R bonded to C H 2 bonded to C H 2 bonded to C double bonded to O to the upper right and bonded to S to the lower right further bonded to Co A. An arrow points down to R bonded to C bonded to H above and double bonded to C on the right bonded to H below and bonded to C to the right double bonded to O above and bonded to S to the lower right further bonded to Co A. The downward arrow is met by a red curved arrow in the mitochondrion showing the addition of F A D and removal of F A D H 2. Another curved arrow extends up from F A D H 2 to a red box labeled respiratory chain and back to F A D. The red box labeled respiratory chain has a curved arrow to its left showing the addition of O 2 and removal of H 2 O in an invagination in the membrane. An arrow points down from the red box to A T P. On the peroxisome/glyoxysome side, a curved arrow shows that F A D is added and F A D H 2 is removed. Another curved arrow from F A D H 2 encounters an arrow to its right as it is converted back to F A D. The right-hand arrow begins with O 2 and produces H 2 O 2, from which another arrow curves back to show the production of H 2 O plus one half O 2. Another arrow points down from the main product and is joined by H 2 O from both sides, required in both the mitochondrion and in the peroxisome/glyoxysome. This yields R bonded to C bonded to O H above, H below, and C H 2 to the right further bonded to C double bonded to O to the upper right and bonded to S to the lower right further bonded to Co A. Another arrow points down to R bonded to C double bonded to O above and bonded to C H 2 to the right bonded to C double bonded to O to the upper right and bonded to S to the lower right further bonded to Co A. To the left of this arrow, a curved arrow on the mitochondrial side shows that N A D plus is added and N A D H is removed. Another curved arrow extends up from N A D H to a red box labeled respiratory chain and back to N A D plus. The red box labeled respiratory chain has a curved arrow to its left showing the addition of O 2 and removal of H 2 O in an invagination in the membrane. An arrow points down from the red box to A T P. A similar curved arrow on the peroxisome/glyoxysome side shows the addition of N A D plus and removal of N A D H. A dashed red arrow from N A D H to N A D plus is accompanied by text reading, N A D H exported for reoxidation. Another arrow points down from the main product and is joined by Co A – S H from both sides, required in both the mitochondrion and in the peroxisome/glyoxysome. This yields R bonded to C double bonded to O to the upper right and bonded to S to the lower right further bonded to Co A plus C H 3 bonded to C double bonded to O to the upper right and bonded to S to the lower right further bonded to Co A. A dashed arrow to the left shows that this enters the citric acid cycle in the mitochondrion. A dashed arrow to the right shows that acetyl Co A is exported from the peroxisome/glyoxysome.

One difference between the peroxisomal and mitochondrial pathways is in the chemistry of the first step (Fig. 17-14). In peroxisomes, the flavoprotein acyl-CoA oxidase that introduces the double bond passes electrons directly to $upper O Subscript 2$ $upper O Subscript 2$ , producing $H_{2} O_{2}$ $upper H Subscript 2 Baseline upper O Subscript 2$ (thus the name “peroxisomes”). This strong and potentially damaging oxidant is immediately cleaved to $H_{2} O$ $upper H Subscript 2 Baseline upper O$ and $O_{2}$ $upper O Subscript 2$ by catalase. Recall that in mitochondria, the electrons removed in the first oxidation step pass through the respiratory chain to $O_{2}$ $upper O Subscript 2$ to produce $upper H Subscript 2 Baseline upper O$ $upper H Subscript 2 Baseline upper O$ , and this process is accompanied by ATP synthesis. In peroxisomes, the energy released in the first oxidative step of fatty acid breakdown is not conserved as ATP, but is dissipated as heat.

A second important difference between mitochondrial and peroxisomal β oxidation in mammals is in the specificity for fatty acyl–CoAs; the peroxisomal system is much more active on very-long-chain fatty acids such as hexacosanoic acid (26:0) and on branched-chain fatty acids such as phytanic acid and pristanic acid (see Fig. 17-15). These less-common fatty acids are obtained from dietary intake of dairy products, the fat of ruminant animals, meat, and fish. Their catabolism in the peroxisome involves several auxiliary enzymes unique to this organelle. The inability to oxidize these compounds is responsible for several serious human genetic diseases. Individuals with Zellweger syndrome are unable to make peroxisomes and therefore lack all the metabolism unique to that organelle. In X-linked adrenoleukodystrophy (XALD), peroxisomes fail to oxidize very-long-chain fatty acids, apparently due to lack of a functional transporter for these fatty acids in the peroxisomal membrane. Both defects lead to accumulation in the blood of very-long-chain fatty acids, especially 26:0. XALD affects young boys before the age of 10 years, causing loss of vision, behavioral disturbances, and death within a few years.

In mammals, high concentrations of fats in the diet result in increased synthesis of the enzymes of peroxisomal β oxidation in the liver. Liver peroxisomes do not contain the enzymes of the citric acid cycle and cannot catalyze the oxidation of acetyl-CoA to ${CO}_{2}$ $CO Subscript 2$ . Instead, long-chain or branched fatty acids are catabolized in peroxisomes to shorter-chain products, such as hexanoyl-CoA, which are exported to mitochondria and completely oxidized there. As we will see in Chapter 20, the germinating seeds of plants can synthesize carbohydrates and many other metabolites from acetyl-CoA produced in peroxisomes, using a pathway (the glyoxylate cycle) not present in vertebrates.

Phytanic Acid Undergoes α Oxidation in Peroxisomes

Phytanic acid, a long-chain fatty acid with methyl branches, is derived from the phytol side chain of chlorophyll (see Fig. 20-5). The presence of a methyl group on the β carbon of this fatty acid prevents the formation of a β-keto intermediate, making its β oxidation impossible. Humans obtain phytanic acid in the diet, primarily from dairy products and from the fats of ruminant animals; microorganisms in the rumen of these animals produce phytanic acid as they digest plant chlorophyll. The typical western diet includes 50 to 100 mg of phytanic acid per day.

Phytanic acid is metabolized in peroxisomes by α oxidation, in which a single carbon is removed from the carboxyl end of the fatty acid (Fig. 17-15). Phytanoyl-CoA is first hydroxylated on its α carbon in a reaction that involves molecular oxygen. The product is decarboxylated to form an aldehyde one carbon shorter, and then oxidized to the corresponding carboxylic acid, which now has no substituent on the β carbon. Further β oxidation produces acetyl-CoA and then propionyl-CoA in successive oxidation cycles. Refsum disease, resulting from a genetic defect in phytanoyl-CoA hydroxylase, leads to the accumulation of very high blood levels of phytanic acid, causing (by unknown mechanisms) severe neurological deficits, including blindness and deafness.

A figure shows five steps involved in the Greek letter alpha oxidation of branched-chain fatty acids in peroxisomes. — FIGURE 17-15 The α oxidation of a branched-chain fatty acid (phytanic acid) in peroxisomes. Phytanic acid has a methyl-substituted β carbon and therefore cannot undergo β oxidation. The combined action of the enzymes shown here removes the carboxyl carbon of phytanic acid to produce pristanic acid, in which the β carbon is unsubstituted, allowing β oxidation. Notice that β oxidation of pristanic acid releases propionyl-CoA, not acetyl-CoA. This is further catabolized as in Figure 17-12. (The details of the reaction that produces pristanal remain controversial.)

Phytanic acid is shown as a 16-carbon zigzag chain. At the right end, there is a red box containing Greek letter beta C bonded to C H 3 above, H, and C H 2 to the right further bonded to C O O H. The entire chain has a methyl group bonded to each fourth peak after Greek letter beta C. An arrow labeled phytanoyl-Co A synthetase points downward accompanied by a curved arrow showing the addition of A T P, Co A – S H and removal of A M P, P P i. This yields phytanoyl-Co A, a 16-carbon zigzag chain with a similar structure except that C O O H has been replaced by C O bonded to S bonded to Co A. An arrow labeled phytanoyl-Co A hydroxylase is accompanied by a curved arrow showing the addition of Greek letter alpha-ketoglutarate, ascorbate and the loss of C O 2, succinate with F e 2 plus shown in the inflection point of the curve. This yields Greek letter alpha-hydroxyphytanoyl-Co A, which is similar except that the second carbon is now bonded to O H below. The right side of the molecule is now Greek letter beta C bonded to C H 3 above and bonded to C H to the right bonded to O H below and bonded to C O to the right further bonded to S bonded to Co A. An arrow labeled Greek letter alpha-hydroxyphytanoyl-Co A lyase has an arrow branching off to show the loss of formyl-Co A. An arrow points from formyl-Co A to formic acid, from which an arrow points down to C O 2 in a red box. The main product is pristanal, which is a 16-carbon zigzag chain. It is similar to the previous chain except that the part in the box is now C H bonded to C H 3 above and bonded to C O O H to the right. An arrow labeled Greek letter beta oxidation points down to the next two products. The first product is 4, 8, 12-trimethyltridecanoyl-Co A. This is a 13-carbon zigzag chain. Its right side has C H bonded to C H 3 above bonded to C H 2 C H 2 C double bonded to O to the upper right and bonded to S to the lower right further bonded to Co A. The rest of the chain is unchanged, with a methyl group extending up from every fourth peak. The second product is propionyl-Co A, which has a red box containing C H 3 C H 2 bonded to C that is further bonded outside of the box with a double bond to O to the upper right and a bond to S to the lower right further bonded to Co A.

SUMMARY 17.2 Oxidation of Fatty Acids

In the first stage of β oxidation, four sequential reactions remove each acetyl-CoA unit, in turn, from the carboxyl end of a saturated fatty acyl–CoA: (1) dehydrogenation of the α and β carbons by acyl–CoA dehydrogenases; (2) hydration of the resulting trans- $Δ^{2}$ $upper Delta squared$ double bond by enoyl-CoA hydratase; (3) dehydrogenation of the resulting l-β-hydroxyacyl-CoA; and (4) cleavage of the resulting β-ketoacyl–CoA by thiolase, to form acetyl-CoA and a fatty acyl–CoA shortened by two carbons.
The shortened fatty acyl–CoA then reenters the β oxidation sequence for removal of another, and then another, acetyl-CoA.
In the second stage of fatty acid oxidation, the acetyl-CoA is oxidized to ${CO}_{2}$ $CO Subscript 2$ in the citric acid cycle. Much of the free energy from fatty acid oxidation is recovered as ATP by oxidative phosphorylation, the final stage of the oxidative pathway.
Oxidation of unsaturated fatty acids requires two additional enzymes: enoyl-CoA isomerase and 2,4-dienoyl-CoA reductase.
Odd-number fatty acids are oxidized by the β-oxidation pathway to yield acetyl-CoA and a molecule of propionyl-CoA. The propionyl-CoA is carboxylated to methylmalonyl-CoA, which is isomerized to succinyl-CoA in a reaction catalyzed by methylmalonyl-CoA mutase, an enzyme requiring coenzyme $B_{12}$ $upper B Subscript 12$ .
To prevent futile cycling, entry of fatty acids into mitochondria is blocked by the first intermediate in fatty acid synthesis, malonyl-CoA.
The transcription factor $PPAR α$ $PPAR alpha$ stimulates the synthesis of several enzymes required in β oxidation when glucose is not available as an energy source.
Individuals who lack medium-chain acyl-CoA dehydrogenase experience fatty liver, high blood levels of octanoic acid, coma, and sometimes death.
Peroxisomes, of plants and animals, and glyoxysomes, of plants, carry out β oxidation in four steps similar to those of the mitochondrial pathway. The first oxidation step, however, transfers electrons directly to $upper O Subscript 2$ $upper O Subscript 2$ , generating $H_{2} O_{2}$ $upper H Subscript 2 Baseline upper O Subscript 2$ . Peroxisomes of animal tissues specialize in the oxidation of very-long-chain fatty acids and branched fatty acids.
The reactions of α oxidation convert branched fatty acids such as phytanic acid to products that can undergo β oxidation, yielding acetyl-CoA and propionyl-CoA.