on 23-Mar-2018 (Fri)

Blood glucose can be obtained from three primary sources: the diet, degradation of glycogen, and gluconeogenesis.

gluconeogenesis (see p. 117) can provide sustained synthesis of glucose, but it is somewhat slow in responding to a falling blood glucose level. Therefore, the body has developed mechanisms for storing a supply of glucose in a rapidly mobilizable form, namely, glycogen. In the absence of a dietary source of glucose, this sugar is rapidly released from liver and kidney glycogen. Similarly, muscle glycogen is extensively degraded in exercising muscle to provide that tissue with an important energy source. When glycogen stores are depleted, specific tissues synthesize glucose de novo, using amino acids from the body’s proteins as a primary source of carbons for the gluconeogenic pathway.

The function of muscle glycogen is to serve as a fuel reserve for the synthesis of adenosine triphosphate (ATP) during mus- cle contraction. That of liver glycogen is to maintain the blood glucose concentration, particularly during the early stages of a fast

Glycogen is a branched-chain polysaccharide made exclusively from α-D-glucose. The primary glycosidic bond is an α(1→4) linkage. After an average of eight to ten glucosyl residues, there is a branch con- taining an α(1→6) linkage

Note: Glycogen synthe sis and degradation are cytosolic processes that go on con- tinuously. The differences between the rates of these two processes determine the levels of stored glycogen during specific physiologic states.

Glycogen is synthesized from molecules of α-D-glucose. The process occurs in the cytosol, and requires energy supplied by ATP (for the phos- phorylation of glucose) and uridine triphosphate (UTP).

α-D-Glucose attached to uridine diphosphate (UDP) is the source of all the glucosyl residues that are added to the growing glycogen molecule. UDP-glucose (Figure 11.4) is synthesized from glucose 1-phosphate and UTP by UDP-glucose pyrophosphorylase (Figure 11.5). The high-energy bond in pyrophosphate (PP i ), the second product of the reaction, is hydrolyzed to two inorganic phosphates (P i ) by pyrophosphatase , which ensures that the UDP-glucose pyro - phosphorylase reaction proceeds in the direction of UDP-glucose production.

Glycogen synthase is responsible for making the α(1→4) linkages in glycogen. This enzyme cannot initiate chain synthesis using free glucose as an acceptor of a molecule of glucose from UDP-glucose. Instead, it can only elongate already existing chains of glucose. Therefore, a fragment of glycogen can serve as a primer in cells whose glycogen stores are not totally depleted.

glycogenin , can serve as an acceptor of glucose residues from UDP-glucose (see Figure 11.5). The side chain hydroxyl group of a specific tyrosine serves as the site at which the initial glucosyl unit is attached. The reaction is catalyzed by glycogenin itself via autoglucosylation; thus, glycogenin is an enzyme. Glycogenin then catalyzes the transfer of the next few molecules of glucose from UDP-glucose, producing a short, α(1→4)- linked glucosyl chain. This short chain serves as a primer that is able to be elongated by glycogen synthase as described below [Note: Glycogenin stays associated with and forms the core of a glycogen granule.

Elongation of a glycogen chain involves the transfer of glucose from UDP-glucose to the nonreducing end of the growing chain, forming a new glycosidic bond between the anomeric hydroxyl of carbon 1 of the activated glucose and carbon 4 of the accepting glucosyl residue (see Figure 11.5). [Note: The nonreducing end of a carbohydrate chain is one in which the anomeric carbon of the terminal sugar is linked by a glycosidic bond to another compound, making the termi- nal sugar nonreducing (see p. 84).] The enzyme responsible for making the α(1→4) linkages in glycogen is glycogen synthase. [Note: The UDP released when the new α(1→4) glycosidic bond is made can be phosphorylated to UTP by nucleoside diphosphate kinase (UDP + ATP UTP + ADP, see p. 296).]

In contrast, glycogen has branches located, on average, eight glucosyl residues apart, resulting in a highly branched, tree-like structure (see Figure 11.3) that is far more soluble than the unbranched amylose. Branching also increases the number of nonreducing ends to which new glucosyl residues can be added (and also, as described later, from which these residues can be removed), thereby greatly accelerating the rate at which glycogen synthesis can occur, and dramatically increasing the size of the molecule

Branches are made by the action of the branching enzyme, amylo-α(1→4) →α(1→6)-transglucosidase . This enzyme removes a chain of six to eight glucosyl residues from the nonreducing end of the glycogen chain, breaking an α(1→4) bond to another residue on the chain, and attaches it to a non-terminal glucosyl residue by an α(1→6) linkage, thus func- tioning as a 4:6 transferase . The resulting new, nonreducing end (see “j” in Figure 11.5), as well as the old nonreducing end from which the six to eight residues were removed (see “o” in Figure 11.5), can now be further elongated by glycogen synthase .

a separate set of cytosolic enzymes is required. When glycogen is degraded, the primary product is glucose 1-phosphate, obtained by breaking α(1→4) glycosidic bonds. In addition, free glucose is released from each α(1→6)-linked glucosyl residue.

Glycogen phosphorylase sequentially cleaves the α(1→4) glycosidic bonds between the glucosyl residues at the nonreducing ends of the glycogen chains by simple phosphorolysis (producing glucose 1- phosphate) until four glucosyl units remain on each chain before a branch point (Figure 11.7). [Note: This enzyme contains a molecule of covalently bound pyridoxal phosphate (PLP) that is required as a coenzyme.] The resulting structure is called a limit dextrin, and phosphorylase cannot degrade it any further

Branches are removed by the two enzymic activities of a single bifunctional protein, the debranching enzyme (see Figure 11.8). First, oligo-α(1→4)→α(1→4)-glucan transferase activity removes the

outer three of the four glucosyl residues attached at a branch. It next transfers them to the nonreducing end of another chain, lengthening it accordingly. Thus, an α(1→4) bond is broken and an α(1→4) bond is made, and the enzyme functions as a 4:4 transferase . Next, the remaining single glucose residue attached in an α(1→6) linkage is removed hydrolytically by amylo-α(1→6)-glucosidase activity, releas- ing free glucose. The glucosyl chain is now available again for degradation by glycogen phosphorylase until four glucosyl units from the next branch are reached

Glucose 1-phosphate, produced by glycogen phosphorylase , is con- verted in the cytosol to glucose 6-phosphate by phosphogluco - mutase

In the liver, glucose 6-phosphate is transported into the endoplasmic reticulum (ER) by glucose 6-phosphate translocase . There it is converted to glucose by glucose 6-phosphatase —the same enzyme used in the last step of gluconeogenesis (see p. 121). The glucose then moves from the ER to the cytosol. Hepato cytes release glycogen-derived glucose into the blood to help maintain blood glucose levels until the gluconeogenic pathway is actively producing glucose. [Note: In the muscle, glucose 6-phosphate cannot be dephosphorylated because of a lack of glucose 6-phosphatase . Instead, it enters glycolysis, pro- viding energy needed for muscle contraction.]

Because of the importance of maintaining blood glucose levels, the syn- thesis and degradation of its glycogen storage form are tightly regulated. In the liver, glycogenesis accelerates during periods when the body has been well fed, whereas glycogenolysis accelerates during periods of fasting. In skeletal muscle, glycogenolysis occurs during active exercise, and glycogenesis begins as soon as the muscle is again at rest. Regulation of glycogen synthesis and degradation is accomplished on two levels. First, glycogen synthase and glycogen phosphorylase are hormonally regulated to meet the needs of the body as a whole. Second, the pathways of glycogen synthesis and degradation are allosterically controlled to meet the needs of a particular tissue.

1. Activation of protein kinase A: Binding of glucagon or epinephrine to their specific hepatocyte GPCR, or of epinephrine to specific myocyte GPCR, results in the G protein-mediated acti- vation of adenylyl cyclase . This enzyme catalyzes the synthesis of cAMP, which activates cAMP-dependent protein kinase A ( PKA ), as described on page 95. PKA is a tetramer, having two regula- tory subunits (R) and two catalytic subunits (C). cAMP binds to the regulatory subunit dimer, releasing individual catalytic sub- units that are active (Figure 11.9). PKA then phosphorylates sev- eral enzymes of glycogen metabolism, affecting their activity. [Note: When cAMP is removed, the inactive tetramer, R 2 C 2 , is again formed.] 2. Activation of phosphorylase kinase: Phosphorylase kinase exists in two forms: an inactive “b” form and an active “a” form. Active PKA phosphorylates the inactive “b” form of phosphorylase kinase , producing the active “a” form (see Figure 11.9). [Note: The phosphorylated enzyme can be inactivated by the hydrolytic removal of its phosphate by protein phosphatase-1 . This enzyme is activated by a signal cascade initiated by insulin (see p. 311). Insulin also activates the phosphodiesterase that degrades cAMP, thus opposing the effects of glucagon and epinephrine.] 3. Activation of glycogen phosphorylase: Glycogen phosphorylase also exists in two forms: the dephosphorylated, inactive “b” form and the phosphorylated, active “a” form. Active phosphorylase kinase phosphorylates glycogen phosphorylase b to its active “a” form, which then begins glycogenolysis (see Figure 11.9). Phosphorylase a is reconverted to phosphorylase b by the hydrol- ysis of its phosphate by protein phosphatase-1 . [Note: Protein phosphatase-1 is inactivated by inhibitor proteins that bind in response to their phosphorylation and activation by PKA .] 4. Summary of the regulation of glycogen degradation: The cascade of reactions listed above results in glycogenolysis. The large num- ber of sequential steps serves to amplify the effect of the hormonal signal, that is, a few hormone molecules binding to their receptors result in a number of protein kinase A molecules being activated that can each activate many phosphorylase kinase molecules. This causes the production of many active glycogen phosphorylase a molecules that can degrade glycogen.

The regulated enzyme in glycogenesis is glycogen synthase . It also exists in two forms, the active “a” form and the inactive “b” form. However, for glycogen synthase , in contrast to phosphorylase kinas e and glycogen phosphorylase , the active form is dephosphorylated

whereas the inactive form is phosphorylated (Figure 11.10). Glycogen synthase a is converted to the inactive “b” form by phos - phorylation at several sites on the enzyme, with the level of inactiva- tion proportional to its degree of phosphorylation.

protein phosphatase-1

A. Activation of glycogen degradation by cAMP-directed pathway

B. Inhibition of glycogen synthesis by a cAMP-directed pathway

C. Allosteric regulation of glycogen synthesis and degradation

1. Regulation of glycogen synthesis and degradation in the well- fed state: In the well-fed state, glycogen synthase b in both liver and mus- cle is allosterically activated by glucose 6-phosphate which is present in elevated concentrations (Figure 11.11). In contrast, glycogen phosphorylase a is allosterically inhibited by glucose 6- phosphate, as well as by ATP, a high-energy signal in the cell. [Note: In liver, but not muscle, non-phosphorylated glucose is also an allosteric inhibitor of glycogen phosphorylase a , making it a better substrate for protein phosphatase-1 .]

2. Activation of glycogen degradation by calcium:

phosphorylase kinase b , which is activated by the Ca 2+ -calmodulin complex without the need for the kinase to be phosphorylated by PKA. [Note: Epinephrine at β-adrenergic receptors signals through a rise in cAMP, not calcium

Calcium activation of muscle phosphorylase kinase: During muscle contraction, there is a rapid and urgent need for ATP. This energy is supplied by the degradation of muscle glycogen to glucose, which can then enter glycolysis. Nerve impulses cause membrane depolarization, which promotes Ca 2+ release from the sarcoplasmic reticulum into the sarcoplasm of myocytes. The Ca 2+ binds calmodulin and the complex acti- vates muscle phosphorylase kinase b

Calcium activation of liver phosphorylase kinase

3. Activation of glycogen degradation in muscle by AMP: Muscle glycogen phosphorylase is active in the presence of the high AMP concentrations that occur in the muscle under extreme conditions of anoxia and ATP depletion. AMP binds to glycogen phosphory- lase b , causing its activation without phos phory lation (see Figure 11.9). [Note: Recall that AMP also activates PFK-1 of glycolysis

CHAPTER SUMMARY