15.2 Breakdown and Synthesis of Glycogen in Chapter 15 The Metabolism of Glycogen in Animals

15.2 Breakdown and Synthesis of Glycogen

As we saw in Chapter 14 (Fig. 14-9), glycogen obtained in the diet is broken down by α-amylases, hydrolytic enzymes that act in the mouth and gut to convert glycogen to free glucose. (Dietary starch is hydrolyzed in a similar way.) But glycogen stored in cells (endogenous glycogen) is degraded by a different pathway. We begin with the breakdown of cellular glycogen to glucose 1-phosphate (glycogenolysis), then turn to synthesis of glycogen (glycogenesis).

Glycogen Breakdown Is Catalyzed by Glycogen Phosphorylase

In skeletal muscle and liver, the glucose units of the outer branches of glycogen enter the glycolytic pathway through the action of three enzymes: glycogen phosphorylase, glycogen debranching enzyme, and phosphoglucomutase. Glycogen phosphorylase catalyzes the reaction in which an $(α 1 \to 4)$ $left-parenthesis alpha Baseline 1 right-arrow 4 right-parenthesis$ glycosidic linkage between two glucose residues at a nonreducing end of glycogen undergoes attack by inorganic phosphate $left-parenthesis upper P Subscript i Baseline right-parenthesis$ $left-parenthesis upper P Subscript i Baseline right-parenthesis$ , removing the terminal glucose residue as α-d-glucose 1-phosphate (Fig. 15-3). This phosphorolysis reaction is different from the hydrolysis of glycosidic bonds by amylase during intestinal degradation of dietary glycogen and starch. In phosphorolysis, some of the energy of the glycosidic bond is conserved in the formation of the phosphate ester glucose 1-phosphate.

A figure shows the structures of molecules during the removal of a glucose residue from the nonreducing end of a glycogen chain by glycogen phosphorylase, producing glucose 1-phosphate and a shortened glycogen molecule. — FIGURE 15-3 Removal of a glucose residue from the nonreducing end of a glycogen chain by glycogen phosphorylase. This process is repetitive; the enzyme removes successive glucose residues, creating a new nonreducing end, until it reaches the fourth glucose unit from a branch point (see Fig. 15-4).

A chain of three glucose molecules is shown that extends beyond the illustration to the right. The left-hand glucose molecule is enclosed in a red box and labeled nonreducing end. The carbons of this glucose molecule are labeled. The structure of the left-hand glucose molecule is a six-membered ring with O at the upper right vertex, C 1 bonded to H above and to O below further bonded outside of the light red bond to C 4 of the glucose molecule to its right, C 2 and C 4 are bonded to H above the right and O H below the ring, C 3 is bonded to O H above the ring and H below the ring, C 5 is bonded to C 6 above the ring and H below the ring, and C 6 is bonded to 2 H and O H. The middle ring has a similar structure to the left-hand glucose molecule except that C 4 is bonded to O below further bonded to C 1 of the left-hand ring and C 1 is bonded to O below further bonded to C 4 of the right-hand ring. The right-hand ring is similar to the middle ring except that C 4 is bonded to O below further bonded to C 1 of the middle ring and C 1 is bonded to O below further bonded outside of the frame. Text below the right-hand ring reads, glycogen chain (glucose) subscript italicized n. An arrow pointing down is joined by red highlighted P subscript I and to the left of a blue oval labeled glycogen phosphorylase. The product has glucose 1-phosphate in a red box to the left and a glycogen molecule on the right. Glucose 1-phosphate has a similar structure to glucose except that C 1 is bonded to H above and to O below that is further bonded outside of the red box to red highlighted P bonded to red highlighted O minus above and to the right and double bonded to red highlighted O below. Two molecules of glycogen are shown to the right. The left-hand molecule of glycogen, formerly the middle molecule in the glycogen chain above, is labeled nonreducing end and has numbered carbons. It has a similar structure, except that C 4 is bonded to H above and O H below. The right-hand molecule of the glycogen molecule is unchanged. Text below reads, glycogen shortened by one residue (glucose) subscript italicized n end italics minus 1.

Pyridoxal phosphate is an essential cofactor in the glycogen phosphorylase reaction. It is covalently attached near the enzyme active site, where its phosphate group acts as a general acid catalyst, promoting attack by $P_{i}$ $upper P Subscript i$ on the glycosidic bond. (This is an unusual role for pyridoxal phosphate; its more typical role is as a cofactor in amino acid metabolism; see Fig. 18-6.)

Glycogen phosphorylase acts repetitively on the nonreducing ends of glycogen branches until it reaches a point four glucose residues away from an $(α 1 \to 6)$ $left-parenthesis alpha Baseline 1 right-arrow 6 right-parenthesis$ branch point, where its action stops. Further degradation by glycogen phosphorylase can occur only after the debranching enzyme, formally known as oligo $(α 1 \to 6)$ $left-parenthesis alpha Baseline 1 right-arrow 6 right-parenthesis$ to $(α 1 \to 4)$ $left-parenthesis alpha Baseline 1 right-arrow 4 right-parenthesis$ glucantransferase, catalyzes two successive reactions that transfer branches (Fig. 15-4) to form straight chains. Once these branches are transferred and the glucosyl residue at C-6 is hydrolyzed, glycogen phosphorylase activity can continue.

A figure shows the steps of glycogen breakdown near an (Greek letter alpha 1 arrow symbol pointing right to 6) branching point, converting glycogen with an (Greek letter alpha 1 arrow symbol pointing right to 6) linkage to an unbranched (Greek letter alpha 1 arrow symbol pointing right to 4) polymer. — FIGURE 15-4 Glycogen breakdown near an $(α 1 \to 6)$ $left-parenthesis bold-italic alpha Baseline 1 right-arrow 6 right-parenthesis$ branch point. Following sequential removal of terminal glucose residues by glycogen phosphorylase (see Fig. 15-3), glucose residues near a branch are removed in a two-step process that requires a bifunctional debranching enzyme. First, the transferase activity of the enzyme shifts a block of three glucose residues from the branch to a nearby nonreducing end, to which the segment is reattached in $(α 1 \to 4)$ $left-parenthesis alpha Baseline 1 right-arrow 4 right-parenthesis$ linkage. The single glucose residue remaining at the branch point, in $(α 1 \to 6)$ $left-parenthesis alpha Baseline 1 right-arrow 6 right-parenthesis$ linkage, is then released as free glucose by the $(α 1 \to 6)$ $left-parenthesis alpha Baseline 1 right-arrow 6 right-parenthesis$ glucosidase activity of the debranching enzyme. The glucose residues are shown in shorthand form.

FIGURE 15-4 Glycogen breakdown near an $(α 1 \to 6)$ $left-parenthesis bold-italic alpha Baseline 1 right-arrow 6 right-parenthesis$ branch point. Following sequential removal of terminal glucose residues by glycogen phosphorylase (see Fig. 15-3), glucose residues near a branch are removed in a two-step process that requires a bifunctional debranching enzyme. First, the transferase activity of the enzyme shifts a block of three glucose residues from the branch to a nearby nonreducing end, to which the segment is reattached in $(α 1 \to 4)$ $left-parenthesis alpha Baseline 1 right-arrow 4 right-parenthesis$ linkage. The single glucose residue remaining at the branch point, in $(α 1 \to 6)$ $left-parenthesis alpha Baseline 1 right-arrow 6 right-parenthesis$ linkage, is then released as free glucose by the $(α 1 \to 6)$ $left-parenthesis alpha Baseline 1 right-arrow 6 right-parenthesis$ glucosidase activity of the debranching enzyme. The glucose residues are shown in shorthand form.

Glycogen is shown as a linear chain of 12 hexagons with a dashed line extending from its right end to the right to indicate further bonding and a bond from the tenth carbon from the left to a 9-hexagon chain above that begins with a red hexagon above its bond to the strand below, then has, from right to left, three yellow hexagons followed by five blue hexagons. The left-hand ends of both strands are labeled. The bond connecting the tenth hexagon in the bottom strand has a bond extending right from its upper left vertex before bending left to bond to the right side vertex to the red hexagon above. This bond is labeled (Greek letter alpha 1 arrow symbol pointing right to 6) linkage. A bracket enclosing this entire structure reads, glycogen. A downward arrow is accompanied by text reading, glycogen phosphorylase. This yields two rows of five hexagons with dots at their right vertices labeled glucose 1-phosphate molecules. To their right, a seven-hexagon chain has a dashed line extending from its right end to indicate further bonding and a bond connecting its fifth hexagon with a red hexagon above further bonded to a chain of three yellow glycogens to its left. A downward arrow is accompanied by text reading, transferase activity of debranching enzyme. This yields a ten-hexagon chain with dashed lines extending from its right side to indicate further bonding and a bond from its eighth hexagon to a red hexagon above. The left three hexagons are yellow. A downward arrow is accompanied by text reading, (Greek letter alpha 1 arrow symbol pointing right to 6) glucosidase activity of debranching enzyme. An arrow curves away to show the loss of a red hexagon labeled glucose. This yields a ten-hexagon chain with dashed lines extending from its right side. The left three hexagons are yellow. Text below reads, unbranched (Greek letter alpha 1 arrow symbol pointing to the right 4) polymer; unbranched for further phosphorylase action.

Glucose 1-Phosphate Can Enter Glycolysis or, in Liver, Replenish Blood Glucose

Glucose 1-phosphate, the end product of the glycogen phosphorylase reaction, is converted to glucose 6-phosphate by phosphoglucomutase, which catalyzes the reversible reaction:

Glucose 1 -phosphate ⇌ glucose 6 -phosphate

$Glucose 1 hyphen phosphate right harpoon over left harpoon glucose 6 hyphen phosphate$

Initially phosphorylated at a Ser residue, the enzyme donates its phosphoryl group to C-6 of the substrate, then accepts a phosphoryl group from C-1 (Fig. 15-5).

A figure shows how phosphoglucomutase catalyzes the conversion of glucose 1-phosphate to glucose 6-phosphate in two steps. — FIGURE 15-5 Reaction catalyzed by phosphoglucomutase. The reaction begins with the enzyme phosphorylated on a Ser residue. In step , the enzyme donates its phosphoryl group (blue) to glucose 1-phosphate, producing glucose 1,6-bisphosphate. In step , the phosphoryl group at C-1 of glucose 1,6-bisphosphate (red) is transferred back to the enzyme, re-forming the phosphoenzyme and producing glucose 6-phosphate.

FIGURE 15-5 Reaction catalyzed by phosphoglucomutase. The reaction begins with the enzyme phosphorylated on a Ser residue. In step , the enzyme donates its phosphoryl group (blue) to glucose 1-phosphate, producing glucose 1,6-bisphosphate. In step , the phosphoryl group at C-1 of glucose 1,6-bisphosphate (red) is transferred back to the enzyme, re-forming the phosphoenzyme and producing glucose 6-phosphate.

A curved white lobe extends down from a larger structure above and is labeled phosphoglucomutase. The surrounding region is blue. Within this white lobe, glucose 1-phosphate is shown as a six-membered ring with O at the upper right vertex, C 1 at the right vertex bonded to H above and to O below further bonded to red highlighted P O subscript 3 superscript 2 minus. C 2 and C 4 bonded to H above and O H below, C 3 bonded to O H above and H below, C 5 bonded to C 6 above and H below, and C 6 bonded to 2 H to the left and O H to the right. S e r in the blue region to the upper right of the white lobe is bonded to O within the lobe further bonded to red highlighted P O subscript 3 superscript 2 minus, from which a blue arrow points to O H bonded to C 6 to represent step 1. Right- and left-pointing arrows indicate that the reaction is reversible. This yields glucose 1,6-bisphosphate, which is similar except that C 6 is bonded to 2 H to the left and to O on the right further bonded to blue highlighted P O subscript 3 superscript 2 minus. S e r is shown in the same location with a bond to O H within the white lobe. An arrow points from the red highlighted P O subscript 3 superscript 2 minus bonded to O bonded to C 1 to the O H bound to S e r to represent step 2. Right- and left-pointing arrows indicate that the reaction is reversible. This yields glucose 6-bisphosphate, which is similar to glucose 1,6-bisphosphate except that C 1 is bonded to H above and O H below.

The glucose 6-phosphate formed from glycogen in skeletal muscle can enter glycolysis and serve as an energy source to support muscle contraction. In liver, glycogen breakdown serves a different purpose: to release glucose into the blood when the blood glucose level drops, as it does between meals. This requires the enzyme glucose 6-phosphatase, present in liver and kidney but not in other tissues. The enzyme is an integral protein of the endoplasmic reticulum, with its active site on the lumenal side of the ER. Glucose 6-phosphate formed in the cytosol is transported into the ER lumen by a specific transporter (T1) (Fig. 15-6) and hydrolyzed at the lumenal surface by glucose 6-phosphatase. The resulting $P_{i}$ $upper P Subscript i$ and glucose are carried back into the cytosol by two different transporters (T2 and T3), and the glucose enters the blood via the plasma membrane transporter, GLUT2. Notice that by having the active site of glucose 6-phosphatase in the ER lumen, the cell separates this reaction from the process of glycolysis, which takes place in the cytosol and would be aborted by the action of glucose 6-phosphatase. Genetic defects in either glucose 6-phosphatase or T1 lead to serious derangement of glycogen metabolism, resulting in type Ia glycogen storage disease (Box 15-1).

A figure shows the reactions involved in the hydrolysis of glucose 6-phosphate by glucose 6-phosphatase of the E R and their locations in the cell. — FIGURE 15-6 Hydrolysis of glucose 6-phosphate by glucose 6-phosphatase of the liver ER. The catalytic site of glucose 6-phosphatase faces the lumen of the ER. A glucose 6-phosphate (G6P) transporter (T1) carries the substrate from the cytosol to the lumen, where glucose 6-phosphatase releases $P_{i}$ $upper P Subscript i$ . The products, glucose and $P_{i}$ $upper P Subscript i$ , pass to the cytosol on specific transporters (T2 and T3). Glucose leaves the cell via the GLUT2 transporter in the plasma membrane.

FIGURE 15-6 Hydrolysis of glucose 6-phosphate by glucose 6-phosphatase of the liver ER. The catalytic site of glucose 6-phosphatase faces the lumen of the ER. A glucose 6-phosphate (G6P) transporter (T1) carries the substrate from the cytosol to the lumen, where glucose 6-phosphatase releases $P_{i}$ $upper P Subscript i$ . The products, glucose and $P_{i}$ $upper P Subscript i$ , pass to the cytosol on specific transporters (T2 and T3). Glucose leaves the cell via the GLUT2 transporter in the plasma membrane.

The left part of the figure shows a rounded structure made of membrane that is connected by a narrower stalk to a wider region below. The region outside of the structure is purple and labeled cytosol. The region inside of the structure is labeled E R lumen. A green channel protein at the upper left of the rounded top of the membrane resembles a rounded structure with a cutout at the top that has been cut in half and is labeled G 6 P transporter (T 1). An arrow points from G 6 P in the cytosol through the channel in the G 6 P transporter to G 6 P in the E R lumen, from which a curved arrow runs up across the bottom of an orange rectangle in the membrane labeled glucose 6-phosphate, after which the arrow splits to show two products: glucose and P subscript i. Two blue oval channel proteins extend through the right-hand side of the membrane. The top blue protein, labeled glucose transporter (T 2), is thicker than the bottom blue protein, labeled P subscript I transporter (T 3). Glucose has an arrow pointing through the glucose transporter to glucose in the cytosol. P subscript i has an arrow pointing through the P i transporter to the cytosol. To the right of glucose, a vertical piece of plasma membrane contains a blue protein labeled G L U T 2 with two crescent-shaped sides that are together to the right, toward the outside of the membrane, and that curve apart to the left, where they are inside of the membrane. A hexagonal opening is visible on the left side of this protein. An arrow points from glucose through G L U T 2, after which is passes into a red vertical rectangle labeled capillary and points downward to text beneath the bottom open end of the capillary reading, increased blood glucose concentration.

BOX 15-1 Medicine

Carl and Gerty Cori: Pioneers in Glycogen Metabolism and Disease

Much of what is written in present-day biochemistry textbooks about the metabolism of glycogen was discovered between about 1925 and 1950 by the remarkable husband and wife team of Carl F. Cori and Gerty T. Cori. Both trained in medicine in Europe at the end of World War I (she completed premedical studies and medical school in one year!). They left Europe together in 1922 to establish research laboratories in the United States, first for nine years in Buffalo, New York, at what is now the Roswell Park Comprehensive Cancer Center, then from 1931 until the end of their lives at Washington University in St. Louis.

In their early physiological studies of the origin and fate of glycogen in animal muscle, the Coris demonstrated the conversion of glycogen to lactate in tissues; movement of lactate in the blood to the liver; and, in the liver, reconversion of lactate to glycogen — a pathway that came to be known as the Cori cycle (see Fig. 23-17). Pursuing these observations at the biochemical level, they showed that glycogen was mobilized in a phosphorolysis reaction catalyzed by the enzyme they discovered, glycogen phosphorylase. They identified the product of this reaction (the “Cori ester”) as glucose 1-phosphate and showed that it could be reincorporated into glycogen in the reverse reaction. Although this did not prove to be the reaction by which glycogen is synthesized in cells, it was the first in vitro demonstration of the synthesis of a macromolecule from simple monomeric subunits, and it inspired others to search for polymerizing enzymes. Arthur Kornberg, discoverer of the first DNA polymerase, said of his experience in the Coris’s lab, “Glycogen phosphorylase, not base pairing, was what led me to DNA polymerase.”

FIGURE 1 The Coris in Gerty Cori’s laboratory, around 1947.

Gerty Cori became interested in human genetic diseases in which too much glycogen is stored in the liver. She was able to identify the biochemical defect in several of these diseases and to show that the diseases could be diagnosed by assays of the enzymes of glycogen metabolism in small samples of tissue obtained by biopsy. Table 1 summarizes what we now know about 13 genetic diseases of this sort.

TABLE 1 Glycogen Storage Diseases of Humans
Type (name)	Enzyme affected	Primary organ/cells affected	Symptoms
Type 0	Glycogen synthase	Liver	Low blood glucose, high ketone bodies, early death
Type Ia (von Gierke)	Glucose 6-phosphatase	Liver	Enlarged liver, kidney failure
Type Ib	Microsomal glucose 6-phosphate translocase	Liver	As in type Ia; also high susceptibility to bacterial infections
Type Ic	Microsomal $P_{i}$ $upper P Subscript i$ transporter	Liver	As in type Ia
Type II (Pompe)	Lysosomal glucosidase	Skeletal and cardiac muscle	Infantile form: death by age 2; juvenile form: muscle defects (myopathy); adult form: as in muscular dystrophy
Type IIIa (Cori or Forbes)	Debranching enzyme	Liver, skeletal and cardiac muscle	Enlarged liver in infants; myopathy
Type IIIb	Liver debranching enzyme (muscle enzyme normal)	Liver	Enlarged liver in infants
Type IV (Andersen)	Branching enzyme	Liver, skeletal muscle	Enlarged liver and spleen, myoglobin in urine
Type V (McArdle)	Muscle phosphorylase	Skeletal muscle	Exercise-induced cramps and pain; myoglobin in urine
Type VI (Hers)	Liver phosphorylase	Liver	Enlarged liver
Type VII (Tarui)	Muscle PFK-1	Muscle, erythrocytes	As in type V; also hemolytic anemia
Type VIb, VIII, or IX	Phosphorylase kinase	Liver, leukocytes, muscle	Enlarged liver
Type XI (Fanconi-Bickel)	Glucose transporter (GLUT2)	Liver	Failure to thrive, enlarged liver, rickets, kidney dysfunction

Carl and Gerty Cori shared the Nobel Prize in Physiology or Medicine in 1947 with Bernardo Houssay of Argentina, who was cited for his studies of hormonal regulation of carbohydrate metabolism. The Cori laboratories in St. Louis became an international center of biochemical research in the 1940s and 1950s, and at least six scientists who trained with the Coris became Nobel laureates: Arthur Kornberg (for DNA synthesis, 1959), Severo Ochoa (for RNA synthesis, 1959), Luis Leloir (for the role of sugar nucleotides in polysaccharide synthesis, 1970), Earl Sutherland (for the discovery of cAMP in the regulation of carbohydrate metabolism, 1971), Christian de Duve (for subcellular fractionation, 1974), and Edwin Krebs (for the discovery of phosphorylase kinase, 1991).

Because muscle and adipose tissue lack glucose 6-phosphatase, they cannot convert the glucose 6-phosphate formed by glycogen breakdown to glucose, and these tissues therefore do not contribute glucose to the blood.

The Sugar Nucleotide UDP-Glucose Donates Glucose for Glycogen Synthesis

Many of the reactions in which hexoses are transformed or polymerized involve sugar nucleotides, compounds in which the anomeric carbon of a sugar is activated by attachment to a nucleotide through a phosphate ester linkage. Sugar nucleotides are the substrates for polymerization of monosaccharides into disaccharides, glycogen, starch, cellulose, and more complex extracellular polysaccharides. They are also key intermediates in the production of the aminohexoses and deoxyhexoses found in some of these polysaccharides, and in the synthesis of vitamin C (l-ascorbic acid). The role of sugar nucleotides in the biosynthesis of glycogen and many other carbohydrate derivatives was discovered in 1953 by the Argentine biochemist Luis Leloir.

A figure shows the structure of U D P-glucose (a sugar nucleotide) with the D-glucosyl group and uridine highlighted in red boxes.

A red box on the right is labeled uridine. It contains a five-membered ring with O at its top vertex, C 1 prime at its right side vertex bonded to N above at the bottom vertex of a six-membered ring and H below, C 2 prime and C 3 prime bonded to H above and O H below, C 4 prime bonded to C 5 prime above and to H below, and C 5 prime bonded to 2 H within the red box and to O to the left outside of the red box. The six-membered ring has N substituted for C at the bottom vertex at position 1, a double bond on the right side between positions 5 and 6, the lower left vertex at position 2 double bonded to O, N substituted for C at the upper left vertex at position 3 and further bonded to H, and the top vertex at position 4 double bonded to O. The O bonded to C 5 prime of the five-membered ring is further bonded to P above that is double bonded to O above, bonded to O minus to the right, and bonded to O to the left and is further bonded to P double bonded to O below, bonded to O minus to the left, and bonded to O above within a red box on the left labeled D-glucosyl group that is further bonded to the right side vertex of a six-membered ring. This six-membered ring has O at its upper right vertex, C 1 bonded to H above and to O outside of the ring below that is further bonded, C 2 and C 4 bonded to H above and O H below, C 3 bonded to O H above and H below, C 5 bonded to C 6 above and to H below, and C 6 bonded to 2 H and O H.

The suitability of sugar nucleotides for biosynthetic reactions stems from several properties:

Their formation is metabolically irreversible, contributing to the irreversibility of the synthetic pathways in which they are intermediates. The condensation of a nucleoside triphosphate with a hexose 1-phosphate to form a sugar nucleotide has a small positive free-energy change, but the reaction releases ${PP}_{i}$ $PP Subscript i$ , which is immediately hydrolyzed by inorganic pyrophosphatase (Fig. 15-7), in a reaction that is strongly exergonic $(Δ G^{'} ° = - 19.2 kJ/mol)$ $left-parenthesis upper Delta upper G Superscript prime Baseline degree equals negative 19.2 kJ slash mol right-parenthesis$ . This keeps the cellular concentration of ${PP}_{i}$ $PP Subscript i$ low, ensuring that the actual free-energy change in the cell is favorable. In effect, rapid removal of the product, driven by the large, negative free-energy change of ${PP}_{i}$ $PP Subscript i$ hydrolysis, pulls the synthetic reaction forward. This is a common strategy in biological polymerization reactions.
Although the chemical transformations of sugar nucleotides do not involve the atoms of the nucleotide itself, the nucleotide moiety has many groups that can undergo noncovalent interactions with enzymes; the additional free energy of binding can contribute significantly to catalytic activity (Chapter 6; see also p. 294).
Like phosphate, the nucleotidyl group (UDP or ADP, for example) is an excellent leaving group, facilitating nucleophilic attack by activating the sugar carbon to which it is attached.
By “tagging” some hexoses with nucleotidyl groups, cells can set them aside in a pool for a particular purpose (glycogen synthesis, for example), separate from hexose phosphates destined for another purpose (such as glycolysis).

A figure shows the reactions through which a sugar phosphate and N T P are used to produce a sugar nucleotide and 2 molecules of inorganic phosphate. — FIGURE 15-7 Formation of a sugar nucleotide. A condensation reaction occurs between a nucleoside triphosphate (NTP) and a sugar phosphate. The negatively charged oxygen on the sugar phosphate serves as a nucleophile, attacking the α phosphate of the nucleoside triphosphate and displacing pyrophosphate. The reaction is pulled in the forward direction by the hydrolysis of ${PP}_{i}$ $PP Subscript i$ by inorganic pyrophosphatase.

FIGURE 15-7 Formation of a sugar nucleotide. A condensation reaction occurs between a nucleoside triphosphate (NTP) and a sugar phosphate. The negatively charged oxygen on the sugar phosphate serves as a nucleophile, attacking the α phosphate of the nucleoside triphosphate and displacing pyrophosphate. The reaction is pulled in the forward direction by the hydrolysis of ${PP}_{i}$ $PP Subscript i$ by inorganic pyrophosphatase.

The sugar phosphate has a rectangle labeled sugar connected by a red bond to red highlighted O of a red highlighted phosphate. This O is bonded to P double bonded to O above and bonded to O minus to the right and below. It is added to N T P, which is shown as three phosphates bonded to a rectangle labeled ribose bonded to a rectangle labeled base. The full molecule is shown as O minus 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 to the right bonded to P double bonded to O above, bonded to O below, and bonded to O to the right further bonded to ribose further bonded to base. A red arrow points from the right-hand O minus of the sugar phosphate to the right-hand P of the N T P and a second red arrow points from the bond between this P and the O to its left to the same O. An arrow pointing downward is labeled N D P-sugar pyrophosphorylase and splits to show two products: pyrophosphate and sugar nucleotide. Pyrophosphate (P P subscript i) is shown as 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 and bonded to O minus to the right and below. An arrow labeled inorganic pyrophosphatase, accompanied by the addition of H 2 O, shows that the reaction yields 2 molecules of phosphate (P subscript i), shown as P double bonded to O above, bonded to O H to the right, and bonded to O minus below and to the left. The sugar nucleotide (N D P-sugar) is shown as a rectangle labeled sugar connected by a red bond to red highlighted O bonded to red highlighted P double bonded to red highlighted O above, bonded to red highlighted O minus below, and bonded to red highlighted O to the right further bonded to nonhighlighted P double bonded to O above, bonded to O minus below, and bonded to O to the right further bonded to a rectangle labeled ribose bonded to a rectangle labeled base.

Glycogen synthesis takes place in virtually all animal tissues but is especially prominent in the liver and skeletal muscles. The starting point for synthesis of glycogen is glucose 6-phosphate. This can be derived from free glucose in a reaction catalyzed by the isozymes hexokinase I and II in muscle and hexokinase IV (glucokinase) in liver:

Glucose + ATP \to glucose 6 -phosphate + ADP

$Glucose plus ATP right-arrow glucose 6 hyphen phosphate plus ADP$

However, some ingested glucose takes a more roundabout path to glycogen. It is first taken up by erythrocytes and converted to lactate via glycolysis; the lactate is then taken up by the liver and converted to glucose 6-phosphate by gluconeogenesis.

To start glycogen synthesis, the glucose 6-phosphate is converted to glucose 1-phosphate in the phosphogluco-mutase reaction:

Glucose 6 -phosphate ⇌ glucose 1 -phosphate

$Glucose 6 hyphen phosphate right harpoon over left harpoon glucose 1 hyphen phosphate$

The product is then converted to UDP-glucose by the action of UDP-glucose pyrophosphorylase, in a key step of glycogen biosynthesis:

Glucose 1 -phosphate + UTP \to UDP-glucose + {PP}_{i}

$Glucose 1 hyphen phosphate plus UTP right-arrow UDP hyphen glucose plus PP Subscript i Baseline$

Notice that this enzyme is named for the reverse reaction; in the cell, the reaction proceeds in the direction of UDP-glucose formation, because the pyrophosphate concentration is kept low by its immediate hydrolysis by inorganic pyrophosphatase (Fig. 15-7).

UDP-glucose is the immediate donor of glucose residues in the reaction catalyzed by glycogen synthase, which promotes the transfer of the glucose residue from UDP-glucose to a nonreducing end of a branched glycogen molecule, forming an $(α 1 \to 4)$ $left-parenthesis alpha Baseline 1 right-arrow 4 right-parenthesis$ linkage (Fig. 15-8). The overall equilibrium of the path from glucose 6-phosphate to glycogen lengthened by one glucose unit greatly favors synthesis of glycogen.

A figure shows how U D P-glucose is added to the nonreducing end of a glycogen chain in a reaction catalyzed by glycogen synthase. — FIGURE 15-8 Glycogen synthesis. A glycogen chain is elongated by glycogen synthase. The enzyme transfers the glucose residue of UDP-glucose to the nonreducing end of a glycogen branch to make a new $(α 1 \to 4)$ $left-parenthesis alpha Baseline 1 right-arrow 4 right-parenthesis$ linkage.

FIGURE 15-8 Glycogen synthesis. A glycogen chain is elongated by glycogen synthase. The enzyme transfers the glucose residue of UDP-glucose to the nonreducing end of a glycogen branch to make a new $(α 1 \to 4)$ $left-parenthesis alpha Baseline 1 right-arrow 4 right-parenthesis$ linkage.

U D P-glucose has a red highlighted glucose molecule bonded to two non-highlighted phosphate groups that are further bonded to a five-membered ring bonded to uracil. The five-membered ring has O at the top vertex, C 1 prime at the right vertex bonded to a rectangle labeled uracil above and H below, C 2 prime and C 3 prime bonded to H above and O H below, C 4 prime bonded to C 5 prime above and H below, and C 5 prime bonded to 2 H and to O to the left further bonded to P above double bonded to O above, bonded to O minus to the right, and bonded to O to the left further bonded to P bonded to O minus above, double bonded to below, and bonded to red highlighted O to the left bonded to C 1 of the red highlighted glucose molecule. This molecule has C 1 bonded to H above and O below further bonded, C 2 and C 4 bonded to H above and O H below, C 3 bonded to O H above and H below, C 5 bonded to C 6 above and H below, and C 6 bonded to 2 H and O H. Two glucose molecules at the left side of a glycogen chain are shown above text reading, nonreducing end of a glycogen chain with italicized n end italics residues (italicized n end italics > 4). These glucose molecules have the same structure as the red highlighted glucose except that the left-hand glucose has C 1 bonded to H above and to O below further bonded to C 4 of the middle glucose and the middle glucose has C 4 bonded to H above and to O below further bonded to the left-hand glucose and C 1 bonded to H above and to O below that is further bonded to the right. An arrow from the lone pair of electron on the hydroxyl group at C 4 of the non-reducing end of a glycogen chain points toward phosphorus of the UDP-glucose chain. Arrows from U D P glucose and the nonreducing end of a glycogen chain come together to point down next to a purple oval labeled glycogen synthase. An arrow curves away to show the loss of U D P. This yields a chain of three glucose molecules at the nonreducing end of a glycogen chain. The right-hand molecule is labeled new nonreducing end and is red, representing the red glucose from U D P glucose and similar except that C 1 is bonded to O below that is further bonded to C 4 of the central glucose in the chain. The central glucose is similar except that C 4 is bonded to H above and to O below that is further bonded to C 1 of the red highlighted glucose. The right-hand glucose is unchanged. Text beneath the central glucose molecule reads, elongated glycogen with italicized n end italics plus 1 residues.

Glycogen synthase cannot make the $(α 1 \to 6)$ $left-parenthesis alpha Baseline 1 right-arrow 6 right-parenthesis$ bonds found at the branch points of glycogen; these are formed by the glycogen-branching enzyme, also called amylo $(1 \to 4)$ $left-parenthesis 1 right-arrow 4 right-parenthesis$ to $(1 \to 6)$ $left-parenthesis 1 right-arrow 6 right-parenthesis$ transglycosylase, or glycosyl $(4 \to 6)$ $left-parenthesis 4 right-arrow 6 right-parenthesis$ transferase. The glycogen-branching enzyme catalyzes transfer of a terminal fragment of 6 or 7 glucose residues from the nonreducing end of a glycogen branch having at least 11 residues to the C-6 hydroxyl group of a glucose residue at a more interior position of the same or another glycogen chain, thus creating a new branch (Fig. 15-9). Further glucose residues may be added to the new branch by glycogen synthase. The biological effect of branching is to increase the number of nonreducing ends. This increases the number of sites accessible to glycogen phosphorylase and glycogen synthase, both of which act only at nonreducing ends.

A figure shows how glycogen-branching enzyme forms a new branch point during branch synthesis in glycogen. — FIGURE 15-9 Branch synthesis in glycogen. The glycogen-branching enzyme forms a new branch point during glycogen synthesis.

A chain of 11 hexagons is shown bonded to the left side of a circular glycogen core on the right. All of the hexagons have O at the upper right vertex. The left-hand hexagon is labeled nonreducing end and has C 4 bonded to O H below. Each hexagon has a bond extending down from C 1. For all except the right-hand hexagon, this bond extends to O, from which a bond extends up to C 4 of the next hexagon. For the right-hand hexagon, this bond connects to the glycogen core. Text beneath one of the bonds connecting two hexagons is labeled (Greek letter alpha 1 arrow symbol pointing right to 4). A dashed red arrow begins above C 1 of the seventh arrow from the left and extends to the upper left corner of the eleventh hexagon bound to the glycogen core. The three hexagons to the left of the eleventh hexagon, between the hexagons indicated by the dashed arrow, are yellow. An arrow labeled glycogen-branching enzyme points down. This yields a chain of seven hexagons with the left vertex of the left-hand hexagon bonded to O H below the ring at the nonreducing end and with the right side vertex of the right-hand hexagon bonded to O below further bonded to the upper left vertex of the hexagon adjacent to the glycogen core. This bond between the right-hand hexagons of the upper and lower chains is labeled (Greek letter alpha 1 arrow symbol pointing right to 6).

Glycogenin Primes the Initial Sugar Residues in Glycogen

Glycogen synthase cannot initiate a new glycogen chain de novo. It requires a primer, usually a preformed $(α 1 \to 4)$ $left-parenthesis alpha Baseline 1 right-arrow 4 right-parenthesis$ polyglucose chain. So, how is a new glycogen molecule initiated? The intriguing protein glycogenin (Fig. 15-10) is both the primer on which new chains are assembled and the enzyme that catalyzes their assembly. The first step in the synthesis of a new glycogen molecule is the transfer of a glucose residue from UDP-glucose to the hydroxyl group of ${Tyr}^{194}$ $Tyr Superscript 194$ of glycogenin. Each subunit of the glycogenin homodimer glycosylates ${Tyr}^{194}$ $Tyr Superscript 194$ of the other subunit. The glycosidic bond in the product has the same configuration about the C-1 of glucose as the substrate UDP-glucose, suggesting that the transfer of glucose from UDP to ${Tyr}^{194}$ $Tyr Superscript 194$ occurs in two steps. The first step is probably a nucleophilic attack by $Asp Superscript 162$ $Asp Superscript 162$ , forming a temporary intermediate with inverted configuration. A second nucleophilic attack by ${Tyr}^{194}$ $Tyr Superscript 194$ then restores the starting configuration. The nascent chains are extended by the sequential addition of seven more glucose residues, each derived from UDP-glucose; the reactions are catalyzed by the chain-extending activity of glycogenin. At this point, glycogen synthase takes over, further extending the glycogen chain. Glycogenin remains buried within the β-particle, covalently attached to the two reducing ends of the glycogen molecule.

A two-part figure shows the structure of the glycogenin protein in part a and the reactions catalyzed by glycogenin in part b. — FIGURE 15-10 Glycogenin. (a) The protein is a homodimer. The substrate, UDP-glucose, is bound in a region near the amino terminus and is some distance from the ${Tyr}^{194}$ $Tyr Superscript 194$ residues — 15 Å from the Tyr in the same monomer, 12 Å from the Tyr in the dimeric partner. Each UDP-glucose is bound through its phosphates to a ${Mn}^{2+}$ $Mn Superscript 2 plus$ ion, which is essential to catalysis. ${Mn}^{2+}$ $Mn Superscript 2 plus$ is believed to function as an electron-pair acceptor (Lewis acid) to stabilize the leaving group, UDP. (b) Glycogenin catalyzes two distinct reactions. Initial attack by the hydroxyl group of ${Tyr}^{194}$ $Tyr Superscript 194$ on C-1 of the glucosyl moiety of UDP-glucose results in a glucosylated Tyr residue. The C-1 of another UDP-glucose molecule is now attacked by the C-4 hydroxyl group of the terminal glucose, and this sequence repeats to form a nascent glycogen molecule of eight glucose residues attached by $(α 1 \to 4)$ $left-parenthesis alpha Baseline 1 right-arrow 4 right-parenthesis$ glycosidic linkages. [(a) Data from PDB ID 1LL2, B. J. Gibbons et al., *J. Mol. Biol*. 319:463, 2002.]

FIGURE 15-10 Glycogenin. (a) The protein is a homodimer. The substrate, UDP-glucose, is bound in a region near the amino terminus and is some distance from the ${Tyr}^{194}$ $Tyr Superscript 194$ residues — 15 Å from the Tyr in the same monomer, 12 Å from the Tyr in the dimeric partner. Each UDP-glucose is bound through its phosphates to a ${Mn}^{2+}$ $Mn Superscript 2 plus$ ion, which is essential to catalysis. ${Mn}^{2+}$ $Mn Superscript 2 plus$ is believed to function as an electron-pair acceptor (Lewis acid) to stabilize the leaving group, UDP. (b) Glycogenin catalyzes two distinct reactions. Initial attack by the hydroxyl group of ${Tyr}^{194}$ $Tyr Superscript 194$ on C-1 of the glucosyl moiety of UDP-glucose results in a glucosylated Tyr residue. The C-1 of another UDP-glucose molecule is now attacked by the C-4 hydroxyl group of the terminal glucose, and this sequence repeats to form a nascent glycogen molecule of eight glucose residues attached by $(α 1 \to 4)$ $left-parenthesis alpha Baseline 1 right-arrow 4 right-parenthesis$ glycosidic linkages. [(a) Data from PDB ID 1LL2, B. J. Gibbons et al., *J. Mol. Biol*. 319:463, 2002.]

The surface contour of a protein is shown. It has two roughly spherical halves. The left half is white and the right half is blue. Where the two halves fit together, there are two bluish green pieces containing a wireframe model of a ring labeled T y r superscript 194 with one piece in a projection of the white piece slightly into the blue piece above the second piece in a projection of the blue piece into the white piece below. In the center of the white piece, there is a wireframe model of U D P-glucose that has two rings to the left of an orange region that projects to the left and right lower sides of a green sphere labeled M g 2 plus before ending with another ring. A blue region slightly beneath the orange region below the green sphere is labeled A s p superscript 162 and contains a wireframe bend that connects to a helix that extends back and to the right. The bottom bluish green T y r superscript 194 is to its right. The blue half has a very similar structure that is inverted. U D P-glucose is slightly higher in the molecule, and the green sphere extends beneath it with the blue A s p superscript 162 above with a helix extending up and to its left. Part b shows U D P-glucose. A glucose molecule in a red box has a six-membered ring with O at the upper left vertex, C 1 at the right side vertex bonded to H above and O below further bonded to a chain of two phosphates in a light blue box that is further bonded, C 2 and C 4 bonded to H above and O H below, C 3 bonded to O H above and H below, C 5 bonded to C 6 above and H below, and C 6 bonded to 2 H and O H. In the blue box, the phosphate chain begins with P bonded to O above further bonded to C 1 of the ring, bonded to O minus to the left, double bonded below, and bonded to O on the right further bonded to P double bonded to O above, bonded to O minus to the right, and bonded to O below further bonded to a blue box labeled ribose bonded to a blue box labeled uracil. A benzene ring is bonded to a long blue oval on the right labeled glycogenin and to O H on the left with a red pair of electr4ons on O. This molecule is labeled T y r superscript 194. A red arrow points from the red pair of electrons on O to C 1 of the glucose ring. An arrow labeled glucosyltransferase activity points down. A curved arrow shows that a blue box labeled U D P leaves. A line joining the arrow shows that U D P-glucose is added. This produces a glucose molecule in a red box similar to the one above except that the right vertex is bonded to O outside of the box further bonded to a benzene ring bonded to a long blue oval and the left vertex is bonded to H above and to O H below with a pair of red electrons on the O. A curved red arrow points from the pair of red electrons on O to C 1 of a six-membered glucose ring of a second U D P glucose molecule with a similar structure to the previous molecule except that it does not have regions highlighted in red or blue. The six-membered ring indicated by the red arrow is equivalent to the six-membered ring in a light red box in the previous structure. An arrow pointing down is labeled chain-extending activity and an arrow curves to the right to show the loss of U D P. Text below reads, repeats six times.

SUMMARY 15.2 Breakdown and Synthesis of Glycogen

Glycogen phosphorylase catalyzes phosphorolytic cleavage at the nonreducing ends of glycogen chains, producing glucose 1-phosphate. The debranching enzyme transfers branches onto main chains and releases the residue at the $(α 1 \to 6)$ $left-parenthesis alpha Baseline 1 right-arrow 6 right-parenthesis$ branch as free glucose.
Phosphoglucomutase interconverts glucose 1-phosphate and glucose 6-phosphate. Glucose 6-phosphate can enter glycolysis or, in liver, it can be converted to free glucose by glucose 6-phosphatase in the endoplasmic reticulum, then exported to replenish blood glucose.
The sugar nucleotide UDP-glucose donates glucose residues to the nonreducing end of glycogen in the reaction catalyzed by glycogen synthase, producing short $(α 1 \to 4)$ $left-parenthesis alpha Baseline 1 right-arrow 4 right-parenthesis$ -linked segments. A separate branching enzyme produces the $(α 1 \to 6)$ $left-parenthesis alpha Baseline 1 right-arrow 6 right-parenthesis$ linkages at branch points.
New glycogen particles begin with the autocatalytic formation of a glycosidic bond between the glucose of UDP-glucose and a Tyr residue of the protein glycogenin, followed by addition of several glucose residues to form a primer that can be acted on by glycogen synthase.