20.6 Biosynthesis of Starch, Sucrose, and Cellulose in Chapter 20 Photosynthesis and Carbohydrate Synthesis in Plants

20.6 Biosynthesis of Starch, Sucrose, and Cellulose

During active photosynthesis in bright light, a plant leaf produces more carbohydrate (as triose phosphates) than it needs for generating energy or synthesizing precursors. The excess is converted to sucrose and transported to other parts of the plant, to be used as fuel or stored. In most plants, starch is the main storage form of carbohydrate, but in a few plants, such as sugar beet and sugarcane, sucrose is the primary storage form. The synthesis of sucrose and starch occurs in different cellular compartments (cytosol and plastids, respectively), and these processes are coordinated by a variety of regulatory mechanisms that respond to changes in light level and photosynthetic rate. The synthesis of sucrose and starch is important to the plant but also to humans: starch provides more than 80% of human dietary calories worldwide.

ADP-Glucose Is the Substrate for Starch Synthesis in Plant Plastids and for Glycogen Synthesis in Bacteria

Starch, like glycogen, is a high molecular weight polymer of d-glucose in $(α 1 \to 4)$ $left-parenthesis alpha Baseline 1 right-arrow 4 right-parenthesis$ linkage. It is synthesized in chloroplasts for temporary storage as one of the stable end products of photosynthesis, and for long-term storage it is synthesized in amyloplasts of the nonphotosynthetic parts of plants: seeds, roots, and tubers (underground stems).

The mechanism of glucose activation in starch synthesis is similar to that in glycogen synthesis, described in Chapter 15. An activated sugar nucleotide, in this case ADP-glucose, is formed by condensation of glucose 1-phosphate with ATP in a reaction made essentially irreversible by the presence in plastids of inorganic pyrophosphatase. Starch synthase then transfers glucose residues from ADP-glucose to preexisting starch molecules. The monomeric units are almost certainly added to the nonreducing end of the growing polymer, as they are in glycogen synthesis.

The amylose of starch is unbranched, but amylopectin has numerous $(α 1 \to 6)$ $left-parenthesis alpha Baseline 1 right-arrow 6 right-parenthesis$ -linked branches (see Fig. 7-13). Chloroplasts contain a branching enzyme, similar to the glycogen-branching enzyme that introduces the $(α 1 \to 6)$ $left-parenthesis alpha Baseline 1 right-arrow 6 right-parenthesis$ branches of amylopectin. Taking into account the hydrolysis by inorganic pyrophosphatase of the ${PP}_{i}$ $PP Subscript i$ produced during ADP-glucose synthesis, the overall reaction for starch formation from glucose 1-phosphate is

\begin{matrix} {Starch}_{n} & + & glucose 1 -phosphate & + & ATP \to \\ {starch}_{n + 1} + ADP + 2 P_{i} \\ Δ G^{'} ° = - 50 kJ/mol \end{matrix}

$StartLayout 1st Row 1st Column upper S t a r c h Subscript n 2nd Column plus 3rd Column g l u c o s e 1 hyphen p h o s p h a t e 4th Column plus 5th Column upper A upper T upper P right-arrow 2nd Row 1st Column Blank 2nd Column Blank 3rd Column Blank 4th Column Blank 5th Column s t a r c h Subscript n plus 1 Baseline plus upper A upper D upper P plus 2 normal upper P Subscript normal i 3rd Row 1st Column Blank 2nd Column Blank 3rd Column Blank 4th Column Blank 5th Column normal upper Delta upper G Superscript prime Baseline degree equals negative 50 kJ slash mol EndLayout$

Starch synthesis is regulated at the level of ADP-glucose formation, as discussed below.

UDP-Glucose Is the Substrate for Sucrose Synthesis in the Cytosol of Leaf Cells

Most of the triose phosphate generated by ${CO}_{2}$ $CO Subscript 2$ fixation in plants is converted to sucrose (Fig. 20-41) or starch. In the course of evolution, sucrose may have been selected as the transport form of carbon because of its unusual linkage between the anomeric C-1 of glucose and the anomeric C-2 of fructose. This bond is not hydrolyzed by amylases or other common carbohydrate-cleaving enzymes, and the unavailability of the sucrose molecule’s anomeric carbons prevents it from reacting nonenzymatically (as does glucose) with amino acids and proteins.

A figure shows the steps of sucrose synthesis. — FIGURE 20-41 Sucrose synthesis. Sucrose is synthesized from UDP-glucose and fructose 6-phosphate, which are synthesized from triose phosphates in the plant cell cytosol. The sucrose 6-phosphate synthase of most plant species is allosterically regulated by glucose 6-phosphate and $P_{i}$ $upper P Subscript i$ .

FIGURE 20-41 Sucrose synthesis. Sucrose is synthesized from UDP-glucose and fructose 6-phosphate, which are synthesized from triose phosphates in the plant cell cytosol. The sucrose 6-phosphate synthase of most plant species is allosterically regulated by glucose 6-phosphate and $P_{i}$ $upper P Subscript i$ .

Two molecules are shown at the top. U D P-glucose on the left is a six-membered ring with O at the upper right vertex, C 1 bonded to H above and O below bonded to a box labeled U D P, C 2 bonded to H above and O H below, C 3 bonded to O H above and H below, C 4 bonded to H above and O H below, C 5 bonded to C 6 above and H below, and C 6 bonded to 2 H and O H. C 1 on the right side vertex is numbered. Fructose 6-phosphate is a five-membered ring with O at the top vertex, C 2 on the right side vertex bonded to C 1 above bonded to 2 H and O H and bonded to O H below, C 3 bonded to H above and O H below, C 4 bonded to O H above and H below, C 4 bonded to H above and C 5 below, and C 5 bonded to 2 H and to O bonded to a circle labeled P. C 2 on the left side vertex is numbered. An arrow labeled sucrose 6-phosphate synthase points down accompanied by an arrow branching off to show the loss of U D P. This yields sucrose 6-phosphate, shown as the same two molecules joined together by a bond. C 1 of the six-membered ring is now bonded to H above and to O below further bonded to C 2 above of the five-membered ring that is also bonded to the same C 1 above as before the bond formed. An arrow labeled sucrose 6-phosphate phosphatase points down accompanied by an arrow branching off to show the loss of P subscript i end subscript. This yields sucrose, which is similar except that the right-hand vertex of the five-membered ring is now bonded to H above and C H 2 O H below.

Sucrose is synthesized in the cytosol, beginning with dihydroxyacetone phosphate and glyceraldehyde 3-phosphate exported from the chloroplast. After condensation of two triose phosphates to form fructose 1,6-bisphosphate (catalyzed by aldolase), hydrolysis by fructose 1,6-bisphosphatase yields fructose 6-phosphate. Sucrose 6-phosphate synthase then catalyzes the reaction of fructose 6-phosphate with UDP-glucose to form sucrose 6-phosphate (Fig. 20-41). Finally, sucrose 6-phosphate phosphatase removes the phosphate group, making sucrose available for export to other tissues. The reaction catalyzed by sucrose 6-phosphate synthase is a low-energy process $(Δ G^{'} ° = - 5.7 kJ/mol)$ $left-parenthesis normal upper Delta upper G Superscript prime Baseline degree equals negative 5.7 kJ slash mol right-parenthesis$ , but the hydrolysis of sucrose 6-phosphate to sucrose is sufficiently exergonic $(Δ G^{'} ° = - 16.5 kJ/mol)$ $left-parenthesis normal upper Delta upper G Superscript prime Baseline degree equals negative 16.5 kJ slash mol right-parenthesis$ to make the overall synthesis of sucrose thermodynamically favorable. Sucrose synthesis is regulated and closely coordinated with starch synthesis, as we shall see.

One remarkable difference between the cells of plants and animals is the absence in the plant cell cytosol of the enzyme inorganic pyrophosphatase, which catalyzes the reaction

{PP}_{i} + H_{2} O \to 2 P_{i} Δ G^{'} ° = - 19.2 kJ/mol

$upper P upper P Subscript normal i Baseline plus normal upper H 2 normal upper O right-arrow 2 normal upper P Subscript normal i Baseline normal upper Delta upper G Superscript prime Baseline degree equals negative 19.2 kJ slash mol$

For many biosynthetic reactions that liberate ${PP}_{i}$ $PP Subscript i$ , pyrophosphatase activity makes the process more favorable energetically, tending to make these reactions irreversible. In plants, this enzyme is present in plastids but absent from the cytosol. As a result, the cytosol of leaf cells contains a substantial concentration of ${PP}_{i}$ $PP Subscript i$ — enough (~0.3 mm) to make reactions such as that catalyzed by UDP-glucose pyrophosphorylase (see Fig. 15-7) readily reversible.

Conversion of Triose Phosphates to Sucrose and Starch Is Tightly Regulated

Triose phosphates produced by the Calvin cycle in bright sunlight, as we have noted, may be stored temporarily in the chloroplast as starch, or converted to sucrose and exported to nonphotosynthetic parts of the plant, or both. The balance between the two processes is tightly regulated, and both must be coordinated with the rate of ${CO}_{2}$ $CO Subscript 2$ fixation. Five-sixths of the triose phosphate formed in the Calvin cycle must be recycled to ribulose 1,5-bisphosphate (Fig. 20-32); if more than one-sixth of the triose phosphate is drawn out of the cycle to make sucrose and starch, the cycle will slow or stop. However, insufficient conversion of triose phosphate to starch or sucrose would tie up phosphate, leaving a chloroplast deficient in $P_{i}$ $upper P Subscript i$ , which is also essential for operation of the Calvin cycle.

The flow of triose phosphates into sucrose is regulated by the activity of fructose 1,6-bisphosphatase (FBPase-1) and the enzyme that effectively reverses its action, ${PP}_{i}$ $PP Subscript i$ -dependent phosphofructokinase (PP-PFK-1). These enzymes are therefore critical points for determining the fate of triose phosphates produced by photosynthesis. Both enzymes are regulated by fructose 2,6-bisphosphate (F26BP), which inhibits FBPase-1 and stimulates PP-PFK-1. In vascular plants, the concentration of F26BP varies inversely with the rate of photosynthesis (Fig. 20-42). Phosphofructokinase-2, responsible for F26BP synthesis, is inhibited by dihydroxyacetone phosphate or 3-phosphoglycerate and is stimulated by fructose 6-phosphate and $P_{i}$ $upper P Subscript i$ . During active photosynthesis, dihydroxyacetone phosphate is produced and $P_{i}$ $upper P Subscript i$ is consumed, resulting in inhibition of PFK-2 and lowered concentrations of F26BP. This favors greater flux of triose phosphate into fructose 6-phosphate formation and sucrose synthesis. With this regulatory system, sucrose synthesis occurs when the level of triose phosphate produced by the Calvin cycle exceeds that needed to maintain operation of the cycle.

A figure shows how fructose 2,6-bisphosphate regulates sucrose synthesis. — FIGURE 20-42 Fructose 2,6-bisphosphate as regulator of sucrose synthesis. The concentration of the allosteric regulator fructose 2,6-bisphosphate in plant cells is regulated by the products of photosynthetic ${CO}_{2}$ $CO Subscript 2$ assimilation and by $P_{i}$ $upper P Subscript i$ . Dihydroxyacetone phosphate and 3-phosphoglycerate produced by ${CO}_{2}$ $CO Subscript 2$ assimilation inhibit phosphofructokinase-2 (PFK-2), the enzyme that synthesizes the regulator; $P_{i}$ $upper P Subscript i$ stimulates PFK-2. The concentration of the regulator is therefore inversely proportional to the rate of photosynthesis. In the dark, the concentration of fructose 2,6-bisphosphate increases and stimulates the glycolytic enzyme ${PP}_{i}$ $PP Subscript i$ -dependent phosphofructokinase-1 (PP-PFK-1), while inhibiting the gluconeogenic enzyme fructose 1,6-bisphosphatase (FBPase-1). When photosynthesis is active (in the light), the concentration of the regulator drops and the synthesis of fructose 6-phosphate and sucrose is favored.

FIGURE 20-42 Fructose 2,6-bisphosphate as regulator of sucrose synthesis. The concentration of the allosteric regulator fructose 2,6-bisphosphate in plant cells is regulated by the products of photosynthetic ${CO}_{2}$ $CO Subscript 2$ assimilation and by $P_{i}$ $upper P Subscript i$ . Dihydroxyacetone phosphate and 3-phosphoglycerate produced by ${CO}_{2}$ $CO Subscript 2$ assimilation inhibit phosphofructokinase-2 (PFK-2), the enzyme that synthesizes the regulator; $P_{i}$ $upper P Subscript i$ stimulates PFK-2. The concentration of the regulator is therefore inversely proportional to the rate of photosynthesis. In the dark, the concentration of fructose 2,6-bisphosphate increases and stimulates the glycolytic enzyme ${PP}_{i}$ $PP Subscript i$ -dependent phosphofructokinase-1 (PP-PFK-1), while inhibiting the gluconeogenic enzyme fructose 1,6-bisphosphatase (FBPase-1). When photosynthesis is active (in the light), the concentration of the regulator drops and the synthesis of fructose 6-phosphate and sucrose is favored.

An oval chloroplast is shown at the top and divided in half. The left side of the chloroplast is labeled light. The chloroplast is green. Text inside reads active photosynthesis and an arrow points down to 3-phosphoglycerate and dihydroxyacetone phosphate. The right side of the chloroplast is labeled dark. The inside of the chloroplast is white. Text inside reads no photosynthesis and an arrow points down to P subscript i end subscript. Dashed arrows point down to reactions below. Beneath the chloroplast, a yellow box labeled fructose 2,6-bisphosphate has upward- and downward-pointing arrows below to a yellow box labeled fructose 6-phosphate below. The upward arrow is labeled P F K – 2 and is accompanied by a curved arrow showing the addition of an orange oval labeled A T P and loss of A D P. A dashed arrow from 3-phosphoglycerate and dihydroxyacetone phosphate in the light half of the photosynthesis points to a red “X” in a red circle next to this reaction. A dashed arrow from P subscript i end subscript in the dark half of the chloroplast points down to a green triangle enclosed in a green circle next to the same reaction. The downward-pointing arrow is labeled F B P-ase -2 and is accompanied by a curved arrow showing the addition of H 2 O and loss of P subscript i end subscript. An arrow points from the yellow box labeled fructose 6-phosphate to a yellow box labeled sucrose to the left. Upward- and downward-pointing arrows extending from fructose 6-phosphate to fructose 1,6-bisphosphate below. The upward arrow is labeled F B P ase – 1 and accompanied by an arrow branching off to show the loss of P subscript i. A dashed arrow from fructose 2,6-bisphosphate through a gray box labeled dark on the left leads to a red “X” in a red circle next to this reaction. The downward arrow is labeled P P – P F K – 1 and is accompanied by an arrow showing the addition of P P subscript i end subscript and loss of P subscript i end subscript. A dashed arrow points down from fructose 2,6-bisphosphate through a gray box labeled dark on the right to a green triangle enclosed in a green circle next to this reaction. Upward- and downward-pointing arrows extend from fructose 1,6-bisphosphate to triose phosphate. To the left, a blue upward arrow is labeled gluconeogenesis. To the right, a red downward arrow is labeled glycolysis.

Sucrose synthesis is also regulated at the level of sucrose 6-phosphate synthase, which is allosterically activated by glucose 6-phosphate and inhibited by $P_{i}$ $upper P Subscript i$ . This enzyme is further regulated by phosphorylation and dephosphorylation; a protein kinase phosphorylates the enzyme on a specific Ser residue, making it less active, and a phosphatase reverses this inactivation by removing the phosphate (Fig. 20-43). Inhibition of the kinase by glucose 6-phosphate, and of the phosphatase by $P_{i}$ $upper P Subscript i$ , enhances the effects of these two compounds on sucrose synthesis. When hexose phosphates are abundant, sucrose 6-phosphate synthase is activated by glucose 6-phosphate; when $P_{i}$ $upper P Subscript i$ is elevated (as when photosynthesis is slow), sucrose synthesis is slowed. During active photosynthesis, triose phosphates are converted to fructose 6-phosphate, which is rapidly equilibrated with glucose 6-phosphate by phosphohexose isomerase. Because the equilibrium lies far toward glucose 6-phosphate, as soon as fructose 6-phosphate accumulates, the level of glucose 6-phosphate rises and sucrose synthesis is stimulated.

A figure shows how sucrose phosphate synthase is regulated by phosphorylation. — FIGURE 20-43 Regulation of sucrose phosphate synthase by phosphorylation. A protein kinase (SPS kinase) specific for sucrose phosphate synthase (SPS) phosphorylates a Ser residue in SPS, inactivating it; a specific phosphatase (SPS phosphatase) reverses this inhibition. The kinase is inhibited allosterically by glucose 6-phosphate, which also activates SPS allosterically. The phosphatase is inhibited by $P_{i}$ $upper P Subscript i$ , which also inhibits SPS directly. Thus, when the concentration of glucose 6-phosphate is high as a result of active photosynthesis, SPS is activated and produces sucrose phosphate. A high $P_{i}$ $upper P Subscript i$ concentration, which occurs when photosynthetic conversion of ADP to ATP is slow, inhibits sucrose phosphate synthesis.

A blue oval labeled sucrose 6-phosphate synthase has a bond to C H 2 O bonded to a red circle labeled P. Text below reads, less active. An arrow labeled S P S phosphatase curves down along the right to a similar blue oval labeled sucrose 6-phosphate synthase with a bond to C H 2 O H above and labeled more active. A curved arrow meets the arrow labeled S P S phosphatase and shows the addition of H 2 O and loss of P subscript I end subscript. An arrow pointing left from P subscript i end subscript to a red “X” in a red circle in the inflection point of this curved arrow. At the bottom of the cycle of reactions, glucose 6-phosphate has a dashed arrow up to a green triangle enclosed in a green circle beneath the more active form of sucrose 6-phosphate synthase. P subscript i end subscript to its right has a dashed arrow up to a red “X” in a red circle beneath the more active form of sucrose 6-phosphate synthase. An arrow labeled S P S kinase curves up from the more active form to the less active form of sucrose 6-phosphate synthase and is accompanied by a curved arrow showing the addition of an orange oval labeled A T P and loss of A D P. A dashed arrow points from glucose 6-phosphate to the left to a red “X” in a red circle in the inflection point of this curved arrow.

The key regulatory enzyme in starch synthesis is ADP-glucose pyrophosphorylase (Fig. 20-44); it is activated by 3-phosphoglycerate, which accumulates during active photosynthesis, and inhibited by $P_{i}$ $upper P Subscript i$ , which accumulates when light-driven condensation of ADP and $P_{i}$ $upper P Subscript i$ slows. When sucrose synthesis slows, 3-phosphoglycerate formed by ${CO}_{2}$ $CO Subscript 2$ fixation accumulates, activating this enzyme and stimulating the synthesis of starch.

A figure shows the regulation of A D P-glucose pyrophosphorylase by 3-phosphoglycerate and P subscript i end subscript. — FIGURE 20-44 Regulation of ADP-glucose pyrophosphorylase by 3-phosphoglycerate and $P_{i}$ $bold upper P Subscript bold i$ . This enzyme, which produces the precursor for starch synthesis, is rate-limiting in starch production. The enzyme is stimulated allosterically by 3-phosphoglycerate (3-PGA) and inhibited by $P_{i}$ $upper P Subscript i$ ; in effect, the ratio $[3 -PGA] / [P_{i}]$ $left-bracket 3 hyphen PGA right-bracket slash left-bracket upper P Subscript i Baseline right-bracket$ , which rises with increasing rates of photosynthesis, controls starch synthesis at this step.

FIGURE 20-44 Regulation of ADP-glucose pyrophosphorylase by 3-phosphoglycerate and $P_{i}$ $bold upper P Subscript bold i$ . This enzyme, which produces the precursor for starch synthesis, is rate-limiting in starch production. The enzyme is stimulated allosterically by 3-phosphoglycerate (3-PGA) and inhibited by $P_{i}$ $upper P Subscript i$ ; in effect, the ratio $[3 -PGA] / [P_{i}]$ $left-bracket 3 hyphen PGA right-bracket slash left-bracket upper P Subscript i Baseline right-bracket$ , which rises with increasing rates of photosynthesis, controls starch synthesis at this step.

The figure is divided into two halves with dim light or darkness on the left and bright light on the right. In the dim light or darkness side, a yellow box labeled A D P plus P subscript i end subscript has an arrow labeled slow pointing to a yellow box labeled A T P. A yellow box labeled glucose 1-phosphate plus A T P has left- and right-pointing arrows labeled A D P-glucose pyrophosphorylase that extend into the bright light half to a yellow box labeled A D P-glucose plus P P subscript i. A dashed arrow points from A D P plus P subscript i end subscript to a red “X” in a red circle above the arrows labeled A D P-glucose pyrophosphorylase. In the bright light side, an arrow points down from photosynthesis to a yellow box labeled 3-phosphoglycerate. A dashed arrow points to a green triangle enclosed in a green circle above the arrows labeled A D P-glucose pyrophosphorylase.

The Glyoxylate Cycle and Gluconeogenesis Produce Glucose in Germinating Seeds

Many plants store lipids (oils) and proteins in their seeds, to be used as sources of energy and as biosynthetic precursors during germination, before photosynthetic capacity has developed. These stored components are converted to carbohydrates by the combined action of several pathways. Glucogenic amino acids (see Table 14-4) derived from the breakdown of stored seed proteins are transaminated and oxidized to succinyl-CoA, pyruvate, oxaloacetate, fumarate, and α-ketoglutarate (Chapter 18) — all good starting materials for gluconeogenesis. Active gluconeogenesis in germinating seeds provides glucose for the synthesis of sucrose, polysaccharides, and many metabolites derived from hexoses. In plant seedlings, sucrose provides much of the chemical energy needed for initial growth.

Triacylglycerols stored in seeds also provide fuel for the germinating plants. They are hydrolyzed to free fatty acids, which undergo β oxidation to acetyl-CoA in specialized peroxisomes called glyoxysomes that develop during seed germination (see Fig. 17-14). The acetyl-CoA formed from seed oils enters the glyoxylate cycle (Fig. 20-45), which brings about the net conversion of acetate to succinate or other four-carbon intermediate of the citric acid cycle:

2 Acetyl-CoA + {NAD}^{+} + 2 H_{2} O \to succinate + 2 CoA + NADH + H^{+}

$2 upper A c e t y l hyphen upper C o upper A plus upper N upper A upper D Superscript plus Baseline plus 2 normal upper H 2 normal upper O right-arrow s u c c i n a t e plus 2 upper C o upper A plus upper N upper A upper D upper H plus normal upper H Superscript plus$

A figure shows the how stored fatty acids are converted to sucrose in germinating seeds through the glyoxylate cycle. — FIGURE 20-45 Conversion of stored fatty acids to sucrose in germinating seeds through the glyoxylate cycle. This pathway begins in specialized peroxisomes called glyoxysomes. The citrate synthase, aconitase, and malate dehydrogenase of the glyoxylate cycle are isozymes of the citric acid cycle enzymes; isocitrate lyase and malate synthase are unique to the glyoxylate cycle. Notice that two acetyl groups enter the cycle and four carbons leave as succinate. Succinate is exported to mitochondria, where it is converted to oxaloacetate by enzymes of the citric acid cycle. Oxaloacetate enters the cytosol and serves as the starting material for gluconeogenesis and for synthesis of sucrose, the transport form of carbon in plants.

A glyoxysome is shown as an orange rectangle above a gray oval mitochondrion. At the top of the glyoxysome, three arrows labeled b oxidation point down from fatty acid to acetyl Co A, shown as C H 3 bonded to C double bonded to O above and bonded to S to the right further bonded to Co A. Except for S bonded to Co A, acetyl Co A is enclosed in a light red box. Acetyl Co A is added to the glyoxylate cycle at the twelve o’clock position, where it joins an arrow labeled citrate synthase that extends from oxaloacetate at the eleven o’clock position to aconitase at the two o’clock position. Oxaloacetate is shown as C double bonded to O to the left, bonded to C O O minus to the right, and bonded to C H 2 below further bonded to C O O minus. Citrate is a vertical three-carbon vertical chain with C 1 bonded to 2 H and C O O minus, C 2 bonded to O H on the left and to C O O minus on the right, and C 3 bonded to 2 H and to C O O minus on the right. An arrow labeled aconitase points down to isocitrate at the four o’clock position. Isocitrate is a vertical three-carbon chain with C 1 bonded to 2 H and C O O minus on the right, C 2 bonded to H and to C O O minus on the right, and C 3 bonded to H, to O H on the left, and to C O O minus on the right. An arrow labeled isocitrate lyase points from isocitrate to glyoxylate at just past the six o’clock position with an arrow that branches off to succinate, shown as a two-carbon vertical chain in a blue box with C 1 bonded to 2 H and to C O O minus on the right and C 2 bonded to 2 H and to C O O minus to the right. An arrow labeled malate synthase points from glyoxylate to malate at the eight o’clock position with a curved line showing the addition of acetyl Co A that has the same structure as above. Malate is a vertical four-carbon chain with C 1 forming C O O minus, C 2 bonded to O H to the left and H to the right, C 3 bonded to 2 H, and C 4 forming C O O minus. An arrow labeled malate dehydrogenase points from malate to oxaloacetate accompanied by a curved arrow showing the addition of N A D plus and loss of an orange oval labeled N A D H. An arrow points from succinate at the lower right to succinate at the two o’clock position of the citric acid cycle in the mitochondrion. Arrows point from succinate to fumarate, from fumarate to malate, and from malate to oxaloacetate. Two arrows point from oxaloacetate. A blue arrow continues the left side of the citric acid cycle by pointing to citrate, from which a blue arrow points to isocitrate, then a blue arrow points to alpha-ketoglutarate, then a blue arrow points to succinyl Co A, then a blue arrow points back to succinate. The other arrow points from oxaloacetate in the mitochondrion to oxaloacetate in the cytosol. Oxaloacetate is shown as a four-carbon horizontal chain with C 1 forming C O O minus, C 2 double bonded to O above, C 3 bonded to 2 H, and C 4 forming C O O minus. An arrow points down to phosphoenolpyruvate with an arrow branching away to show the loss of C O 2. Two arrows labeled gluconeogenesis point down from phosphoenolpyruvate to fructose 6-phosphate, from which another arrow points down to glucose 6-phosphate. Arrows from fructose 6-phosphate and glucose 6-phosphate join to yield sucrose.

In the glyoxylate cycle, acetyl-CoA condenses with oxaloacetate to form citrate, and citrate is converted to isocitrate, exactly as in the citric acid cycle. The next step, however, is not the breakdown of isocitrate by isocitrate dehydrogenase but the cleavage of isocitrate by isocitrate lyase, forming succinate and glyoxylate. The glyoxylate then condenses with a second molecule of acetyl-CoA to yield malate, in a reaction catalyzed by malate synthase. The malate is subsequently oxidized to oxaloacetate, which can condense with another molecule of acetyl-CoA to start another turn of the cycle. The succinate passes into the mitochondrial matrix, where it is converted by citric acid cycle enzymes to oxaloacetate. The oxaloacetate moves into the cytosol and can be converted to phosphoenolpyruvate by PEP carboxykinase, then to fructose 6-phosphate, the precursor of sucrose, by gluconeogenesis. Thus, reaction sequences carried out in three subcellular compartments (glyoxysomes, mitochondria, and cytosol) are integrated for the production of fructose 6-phosphate or sucrose from stored lipids.

Enzymes common to the citric acid and glyoxylate cycles have two isozymes, one specific to mitochondria, the other to glyoxysomes. Physical separation of the glyoxylate cycle and β-oxidation enzymes from the mitochondrial citric acid cycle enzymes prevents further oxidation of acetyl-CoA to ${CO}_{2}$ $CO Subscript 2$ . Each turn of the glyoxylate cycle consumes two molecules of acetyl-CoA and produces one molecule of succinate, which is then available for biosynthetic purposes. Hydrolysis of stored triacylglycerols also produces glycerol 3-phosphate, which can enter the gluconeogenic pathway, after its oxidation to dihydroxyacetone phosphate (see Fig. 14-16).

We noted in Chapter 14 that animal cells can carry out gluconeogenesis from three- and four-carbon precursors, but not from the two acetyl carbons of acetyl-CoA. Because the pyruvate dehydrogenase reaction is effectively irreversible (see Section 16.1) and animals do not have the enzymes specific to the glyoxylate cycle (isocitrate lyase and malate synthase), they have no way to convert acetyl-CoA to pyruvate or oxaloacetate. So, unlike vascular plants, animals cannot bring about the net synthesis of glucose from fatty acids.

Cellulose Is Synthesized by Supramolecular Structures in the Plasma Membrane

Cellulose is a major constituent of plant cell walls, providing strength and rigidity and preventing the swelling of the cell and rupture of the plasma membrane that might result when osmotic conditions favor water entry into the cell. Each year, worldwide, plants synthesize more than $10^{11}$ $10 Superscript 11$ metric tons of cellulose, making this simple polymer one of the most abundant compounds in the biosphere. The structure of cellulose in the plant cell wall is simple: linear polymers of thousands of $(β 1 \to 4)$ $left-parenthesis beta Baseline 1 right-arrow 4 right-parenthesis$ -linked d-glucose units, assembled into bundles of at least 18 chains, which co-crystalize to form microfibrils, which may in turn be assembled into larger macrofibrils. (Fig. 20-46).

A figure shows the structure of cellulose. — FIGURE 20-46 Cellulose structure. The plant cell wall is made up in part of cellulose molecules arranged side by side to form crystalline arrays — cellulose microfibrils. Several microfibrils may combine to form larger cellulose macrofibrils. The scanning electron microscope shows macrofibrils, 5 to 12 nm in diameter, laid down on the cell surface in several layers distinguishable by the different orientations of the fibrils.

The figure shows a tree in the upper right with a square to its right showing a close-up of plant cells containing chloroplasts and other structures. A micrograph labeled cellulose microfibrils in plant cell wall shows many diagonal strands with perpendicular strands lined up beneath them. A strand extending from this micrograph is labeled a microfibril and is opened up at its left end to show that it contains many parallel cellulose chains. A close-up of the chains shows that they consist of linked rings. A close-up of two rings shows their structure. The left-hand ring has O at its upper right vertex, C 1 further bonded above and bonded to H below, C 2 bonded to H above and O H below, C 3 bonded to O H above and H below, C 4 bonded to H above and further bonded below, C 5 bonded to C 6 above bonded to 2 H and O H. C 1 is connected by a bond labeled (beta 1 arrow pointing right to 4) to a ring to its right. It has a bond extending up that curves down to O, then continues to curve down before looping up to bond to C 4 of the right-hand ring from beneath. The right-hand ring has the same structure and is further bonded to the right.

As a major component of the plant cell wall, cellulose must be synthesized from intracellular precursors but deposited and assembled outside the plasma membrane. The enzymatic machinery for initiation, elongation, and export of cellulose chains is therefore more complicated than that used to synthesize starch or glycogen (which are not exported).

The complex enzymatic machinery that assembles cellulose chains spans the plasma membrane, with one part on the cytoplasmic side positioned to bind the substrate, UDP-glucose, and elongate the chains, and another part extending to the outside, responsible for exporting the cellulose molecules to the extracellular space. Freeze-fracture electron microscopy shows a cellulose synthesis complex, or rosette, composed of six large particles arranged in a regular hexagon with a diameter of about 30 nm (Fig. 20-47a). Several proteins, including the catalytic subunit of cellulose synthase, make up this structure. The structure of the plant cellulose synthase is similar to that of the bacterium Rhodobacter sphaeroides, which has been determined by x-ray crystallography (Fig. 20-47b).

A two-part figure shows a model for cellulose synthesis by showing a schematic illustration in part a and the structure of cellulose synthase in part b. — FIGURE 20-47 A model for the synthesis of cellulose. (a) Schematic derived from a combination of genetic, electron microscopic, and biochemical studies of *Arabidopsis thaliana* and other vascular plants. (b) The structure of cellulose synthase from the bacterium *Rhodobacter sphaeroides*. The transmembrane part of the protein provides a channel through which the lengthening cellulose polymer (red) is pushed into the periplasm as the chain grows by addition of glucose units on the inside surface of the plasma membrane. Two structures of the enzyme move during the catalytic cycle. The gating loop moves into the substrate-binding site when UDP-glucose binds, then moves out to allow UDP to leave. The finger helix touches the glucose residue at the growing polymer end, then, after a new residue is added, moves so as to touch this new terminal glucose. The glycosyl transferase domain extends into the cytoplasm, where it binds its substrate UDP-glucose. [(b) Data from PDB ID 5EJZ, J. L. W. Morgan et al., *Nature* 531:329, 2016. An extension of the cellulose chain was modeled in.]

Part a shows a series of steps In glucose synthesis. Step 1: Sucrose synthase generates U D P-glucose. An arrow from sucrose to fructose meets an arrow from U D P to U D P glucose. U D P glucose is shown beneath the cytoplasmic side of a membrane beneath a ring of spheres labeled sucrose synthase. Vertical green ovals labeled cellulose synthase extend across the membrane to the extracellular side. Sets of three of these structures, with sucrose synthase below each cellulose synthase, are located in five rounded groups of six dispersed across the membrane. Lines extend up from each oval within a group to form a single strand above. Step 2: Each cellulose synthase of a rosette synthesizes a long cellulose chain outside the plasma membrane. A round structure made of nine cylinders arranged around an open center extends out from beneath the cytoplasmic side of the membrane to the lower right accompanied by an arrow pointing to the lower right. Accompanying text reads, Direction of movement of cellulose synthase complex along plasma membrane. Step 3: Microtubule in cell cortex directs motion of the rosette. Step 4: Parallel cellulose chains crystallize to form fibril. These fibrils extend up from each group of cellulose synthases. A micrograph shows four full rosettes and one partial rosette. The rosettes are circles of six rounded structures around an open center. Text above reads, The transmembrane regions of rosette-type cellulose synthesis complexes, viewed by freeze-fracture electron microscopy of plant cell plasma membrane. Part b shows a horizontal piece of membrane with the region above labeled as periplasm and a long, irregular, vertical green structure extending across the membrane. The lower left corner of the green structure is labeled glycosyl transferase domain. A green strand runs horizontally from the left and then bends upward. This green strand is labeled gating loop. A yellow horizontal helix to its right is labeled finger helix. A long red strand of cellulose made up of many rings begins between the green and yellow strands and curves up through the green structure, is briefly visible outside of the structure as it emerges above the membrane, then runs through more of the green structure before emerging above.

In one working model of cellulose synthesis, cellulose chains are initiated by the transfer of a glucose residue from UDP-glucose to a “primer” glucose already bound to cellulose synthase on the cytoplasmic side of the plasma membrane, to form a disaccharide. As addition of further glucose residues lengthens the chain, it is extruded through a channel formed by the transmembrane helices of cellulose synthase and, on the outer surface of the plasma membrane, joins growing chains from neighboring cellulose synthase molecules to form a cellulose microfibril. Polymers of more than 6 to 8 glucose units are insoluble in water, promoting microfibril crystallization. There is no definite length for a cellulose polymer; synthesis is highly processive, and some polymers are as long as 15,000 glucose units.

The UDP-glucose used for cellulose synthesis (step in Fig. 20-47) is generated from sucrose produced during photosynthesis, in a reaction catalyzed by sucrose synthase (named for the reverse reaction):

Sucrose + UDP \to UDP-glucose + fructose

$upper S u c r o s e plus upper U upper D upper P right-arrow upper U upper D upper P hyphen g l u c o s e plus f r u c t o s e$

A membrane-bound form of sucrose synthase may produce a high local concentration of UDP-glucose for cellulose synthesis.

Each of the six particles of the rosette most likely contains three cellulose synthase molecules, each synthesizing a single cellulose chain (step ). The large enzyme complex that catalyzes this process moves along the plasma membrane with directionality often related to the course of microtubules in the cell cortex, the cytoplasmic layer just below the membrane (step ). When these microtubules lie perpendicular to the axis of the plant’s growth, the cellulose microfibrils are laid down similarly to promote elongation. The motion of the cellulose synthase complexes is believed to be driven by energy released in the polymerization reaction, not by a molecular motor such as kinesin.

The fundamental cellulose microfibril made by one rosette-type cellulose synthesis complex is thought to be composed of 18 chains lying side by side with the same (parallel) orientation of nonreducing and reducing ends. The 18 separate polymers coalesce on the outer surface of the cell and crystallize soon after they are polymerized (step ), just prior to integrating into the cell wall.

In UDP-glucose, the glucose is α-linked to the nucleotide, but in cellulose, the glucose residues are $(β 1 \to 4)$ $left-parenthesis beta Baseline 1 right-arrow 4 right-parenthesis$ -linked, so there is an inversion of configuration at the anomeric carbon (C-1) as the glycosidic bond forms. Glycosyltransferases that invert configuration are generally assumed to use a single-displacement mechanism, with nucleophilic attack by the acceptor species at the anomeric carbon of the donor sugar (in this case, UDP-glucose).

Pools of Common Intermediates Link Pathways in Different Organelles

Although we have described metabolic transformations in plant cells in terms of individual pathways, these pathways interconnect so completely that we should instead consider pools of metabolic intermediates shared among these pathways and connected by readily reversible reactions (Fig. 20-48). One such metabolite pool includes the hexose phosphates glucose 1-phosphate, glucose 6-phosphate, and fructose 6-phosphate; a second includes the 5-phosphates of the pentoses ribose, ribulose, and xylulose; a third includes the triose phosphates dihydroxyacetone phosphate and glyceraldehyde 3-phosphate. Metabolite fluxes through these pools change in magnitude and direction in response to changes in the circumstances of the plant, and they vary with tissue type. Transporters in the membranes of each organelle move specific compounds in and out, and the regulation of these transporters presumably influences the degree to which the pools mix.

A figure shows pools of hexose phosphates, pentose phosphates, and triose phosphates. — FIGURE 20-48 Pools of hexose phosphates, pentose phosphates, and triose phosphates. The compounds in each pool are readily interconvertible by reactions that have small standard free-energy changes. When one component of the pool is temporarily depleted, a new equilibrium is quickly established to replenish it. Movement of the sugar phosphates between intracellular compartments is limited; specific transporters must be present in an organelle membrane.

A light red bar begins with glucose 1-phosphate on the left, then shows right- and left-pointing arrows showing a reversible reaction that yields glucose 6-phosphate, from which another set of right- and left-pointing arrows extend to fructose 6-phosphate. An arrow points up from glucose 1-phosphate to A D P-glucose, from which another arrow points up to starch. A second arrow points diagonally from glucose 1-phospahte to U D P-glucose, from which right- and left-pointing arrows extend to sucrose. An arrow points down from glucose 6-phosphate to 6-phosphoglucontate to the lower left, from which an arrow points down to the lower left to ribose 5-phosphate in a light red box. Within this box, forward and reverse arrows extend from ribose 5-phosphate to ribulose 5-phosphate, from which another set of forward and reverse arrows extends to xylulose 5-phosphate. An arrow points down from fructose 6-phosphate to fructose 1,6-bisphosphate, from which forward and reverse arrows extend to glyceraldehyde 3-phosphate to the lower left within a light red box and to dihydroxyacetone phosphate to the lower right within the same light red box. Forward and reverse arrows are also shown between glyceraldehyde 3-phosphate and dihydroxyacetone phosphate.

During daylight hours, triose phosphates produced in photosynthetic leaf tissue (“source” tissues, in which there is a net fixation of ${CO}_{2}$ $CO Subscript 2$ ) move out of the chloroplast and into the cytosolic hexose phosphate pool, where they are converted to sucrose for transport via the plant phloem (sap) to nonphotosynthetic “sink” tissues (Fig. 20-49). In sink tissues such as roots, tubers, and bulbs, sucrose is converted to starch for storage or is used as an energy source via glycolysis. In growing plants, hexose phosphates are also withdrawn from the pool for the synthesis of cell walls. At night, starch is metabolized by glycolysis and oxidative phosphorylation to provide energy for both source and sink tissues.

A figure shows the movement of sucrose between source and sink tissues with part a showing sources and part b showing sinks. — FIGURE 20-49 Movement of sucrose between source and sink tissues. (a) In daylight, photosynthetic leaves (source tissue) fix ${CO}_{2}$ $CO Subscript 2$ into triose phosphates via the Calvin cycle in chloroplasts. Some of the triose phosphate is used in the chloroplasts to synthesize starch; the rest is exported to the cytosol, where it can be converted via gluconeogenesis to fructose 6-phosphate and glucose 1-phosphate. Sucrose, synthesized from UDP-glucose and fructose, is exported from leaf mesophyll cells to the plant phloem; the resulting high sucrose content draws water into the phloem by osmosis. The resulting increased turgor pressure (p. 52) pushes the solution in the phloem toward sink tissues. (b) Sucrose moves from the phloem into the sink tissues, where it is converted to starch or cell wall cellulose, or is used as fuel for glycolysis, the citric acid cycle, and oxidative phosphorylation to provide ATP for these nonphotosynthetic tissues. Sugar transport across the plasma membrane and between intracellular compartments is catalyzed by several symporters and antiporters coupled to a proton gradient. [Information from Dr. Gerald Edwards, School of Biological Sciences, Washington State University.]

Part a is labeled source. It shows a green plan cell with an outer cell wall. A leaf cell is shown inside as a lighter green rectangle. On the left there is a vertical oval labeled chloroplast. A wavy yellowish-orange arrow points from light outside of the cell to Calvin cycle at the top of the chloroplast. An arrow also points from C O 2 outside of the cell to the Calvin cycle. An arrow points down from Calvin cycle to triose phosphates, from which an arrow points down to A D P-glucose, from which an arrow points down to starch, from which an arrow points down to a glucose (occurs in darkness) in the gray bottom portion of the chloroplast. An arrow shows that triose phosphates travel through a channel out of the chloroplast. Glucose (occurs in darkness) also passes through a channel to join a series of reactions outside of the chloroplast. An arrow points up from triose phosphates outside of the chloroplast to U D P-glucose plus fructose 6-phosphate. An arrow points down from triose phosphates to A D P-glucose, from which an arrow points down to glucose that left the chloroplast, from which an arrow points down to respiration. An arrow points right from respiration to A T P. An arrow labeled sucrose phosphate synthase points right from fructose 6-phosphate produced from triose phosphates to yield sucrose phosphate, from which an arrow labeled sucrose phosphate phosphatase points to sucrose. Sucrose travels through a sucrose uniporter out of the leaf cell, then through a sucrose-H plus symporter across the cell wall into a vertical column labeled phloem. An arrow shows that H plus travels through the symporter at the same time. An arrow points down from sucrose near the top of the phloem cylinder. Three arrows point from H 2 O into the phloem cylinder. In part b, sucrose travels through two sucrose uniporters into a cell below. Sucrose then travels through two sucrose-H plus symporters into a tuber cell. As sucrose travels through, H plus travels into the cell along with sucrose. An arrow labeled invertase points from sucrose to the addition of glucose plus fructose, from which an arrow points down to glycolysis. An arrow points right from glycolysis to A T P. Outside of the cell, an arrow labeled invertase points from sucrose to glucose plus fructose. Glucose travels through a glucose-H plus symporter that also transports H plus to enter the cell, where it can react with fructose. An arrow labeled sucrose synthase points left from sucrose that entered the cell through the lower sucrose-H plus symporter to the reaction of U D P-glucose plus fructose, from which an arrow points up to glycolysis. From U D P-glucose, an arrow points to cellulose synthesis to the upper left and to starch synthesis to the lower left. A vacuole is shown as a vertical oval to the left. Sucrose inside the vacuole travels out of the vacuole through a channel as H plus moves out through the same channel. Sucrose enters the vacuole through a sucrose hexose antiporter that moves hexose out of the cell at the same time.

SUMMARY 20.6 Biosynthesis of Starch, Sucrose, and Cellulose

Starch synthase in chloroplasts and amyloplasts catalyzes the addition of single glucose residues, donated by ADP-glucose, to the growing polymer chain.
Sucrose is synthesized in the cytosol from UDP-glucose and fructose 1-phosphate, in two steps.
The partitioning of triose phosphates between sucrose synthesis and starch synthesis is regulated by fructose 2,6-bisphosphate (F26BP). [F26BP] varies inversely with the rate of photosynthesis, and F26BP inhibits the synthesis of fructose 6-phosphate, the precursor of sucrose.
The glyoxylate cycle, taking place in the glyoxysomes of germinating seeds of some plants, uses several citric acid cycle enzymes and two additional enzymes: isocitrate lyase and malate synthase. The two decarboxylation steps of the citric acid cycle are bypassed, making possible the net formation of succinate, oxaloacetate, and other cycle intermediates from acetyl-CoA.
Cellulose synthase has a glycosyl transferase activity in its cytoplasmic domain and forms a transmembrane channel through which the growing cellulose chain is extruded. Glucose units are transferred from UDP-glucose to the nonreducing end of the growing chain.
The plant cell shares pools of common intermediates, including hexose-, pentose-, and triose-phosphates. Transporters in the membranes of chloroplasts, mitochondria, and amyloplasts mediate the movement of sugar phosphates between organelles. The direction of metabolite flow through the pools within a leaf changes from day to night.
Sucrose produced in a photosynthetic (source) tissue is exported to nonphotosynthetic (sink) tissue such as roots and tubers via the plant phloem.