Regulation of Glycogenesis and Glycogenolysis

REGULATION OF GLYCOGENESIS AND GLYCOGENOLYSIS

Because of the importance of maintaining blood glucose levels, the synthesis 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 (by phosphorylation/dephosphorylation) to meet the needs of the body as a whole. [Note: Phosphorylation of glycogen phosphorylase is catalyzed by glycogen phosphorylase kinase.] Second, these same enzymes are allosterically regulated (by effector molecules) to meet the needs of a particular tissue.

A. Activation of glycogen degradation

The binding of hormones, such as glucagon or epinephrine, to plasma membrane G protein–coupled receptors (GPCRs) signals the need for glycogen to be degraded, either to elevate blood glucose levels or to provide energy for exercising muscle.

Figure 11.9 Stimulation and inhibition of glycogen degradation. AMP = adenosine monophosphate; cAMP = cyclic AMP; GTP = guanosine triphosphate; P = phosphate; PPi = pyrophosphate; R = regulatory subunit; C = catalytic subunit.

1. Activation of protein kinase A: Binding of glucagon or epinephrine to their specific hepatocyte GPCR, or of epinephrine to a specific myocyte GPCR, results in the G protein–mediated activation of adenylyl cyclase. This enzyme catalyzes the synthesis of cyclic adenosine monophosphate (cAMP), which activates cAMP-dependent protein kinase A (PKA), as described on page 95. PKA is a tetramer, having two regulatory subunits (R) and two catalytic subunits (C). cAMP binds to the regulatory subunits, releasing individual catalytic subunits that are active (Figure 11.9). PKA then phosphorylates several enzymes of glycogen metabolism, affecting their activity. [Note: When cAMP is removed, the inactive tetramer, R 2C2, 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).

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 is the only enzyme that phosphorylates glycogen phosphorylase b to its active “a” form, which then begins glycogenolysis (see Figure 11.9).

4. Summary of the regulation of glycogen degradation: The cascade of reactions listed above results in glycogenolysis. The large number of sequential steps serves to amplify the effect of the hormonal signal (that is, a few hormone molecules binding to their receptors results in a number of PKA 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.

5. Maintenance of the phosphorylated state: The phosphate groups added to phosphorylase kinase and phosphorylase in response to cAMP are maintained because the enzyme that hydrolytically removes the phosphate, protein phosphatase-1 (PP1), is inactivated by inhibitor proteins that are also phosphorylated and activated in response to cAMP (see Figure 11.9). [Note: PP1 is activated by a signal cascade initiated by insulin. Insulin also activates the phosphodiesterase that degrades cAMP, and, thus, insulin opposes the effects of glucagon and epinephrine.]

B. Inhibition of glycogen synthesis

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 kinase and 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 phosphorylation at several sites on the enzyme, with the level of inactivation proportional to its degree of phosphorylation. Phosphorylation is catalyzed by several different protein kinases that are regulated by cAMP or other signaling mechanisms (see C. below). Glycogen synthase b can be reconverted to the “a” form by PP1. Figure 11.11 summarizes the covalent regulation of glycogen metabolism.

Figure 11.10 Hormonal regulation of glycogen synthesis. [Note: In contrast to glycogen phosphorylase, glycogen synthase is inactivated by phosphorylation.] cAMP = cyclic adenosine monophosphate; P = phosphate; PPi = pyrophosphate; R = regulatory subunit; C = catalytic subunit.

Figure 11.11 Summary of the hormone-mediated covalent regulation of glycogen metabolism. cAMP = cyclic AMP; PKA = protein kinase A.

C. Allosteric regulation of glycogen synthesis and degradation

In addition to hormonal signals, glycogen synthase and glycogen phosphorylase respond to the levels of metabolites and energy needs of the cell. Glycogenesis is stimulated when substrate availability and energy levels are high, whereas glycogenolysis is increased when glucose and energy levels are low. This allosteric regulation allows a rapid response to the needs of a cell and can override the effects of hormone-mediated covalent regulation.

1. Regulation of glycogen synthesis and degradation in the well- fed state:

In the well-fed state, glycogen synthase b in both liver and muscle is allosterically activated by glucose 6-phosphate, which is present in elevated concentrations (Figure 11.12). 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, nonphosphorylated glucose is also an allosteric inhibitor of glycogen phosphorylase a, making it a better substrate for PP1.]

Figure 11.12 Allosteric regulation of glycogen synthesis and degradation. A. Liver. B. Muscle. P = phosphate.

2. Activation of glycogen degradation by calcium: Ca²⁺ is released into the cytoplasm in muscle in response to neural stimulation and in liver in response to epinephrine binding to α1-adrenergic receptors. The Ca²⁺ binds to calmodulin (CaM), the most widely distributed member of a family of small, calcium-binding proteins. The binding of four molecules of Ca²⁺ to CaM triggers a conformational change such that the activated Ca²⁺–CaM complex binds to and activates protein molecules, often enzymes, that are inactive in the absence of this complex (see Figure 11.13). Thus, CaM functions as an essential subunit of many complex proteins. One such protein is the tetrameric phosphorylase kinase, whose b form is activated by the binding of Ca²⁺ to its δ subunit (CaM) without the need for the kinase to be phosphorylated by PKA. [Note: Epinephrine at β-adrenergic receptors signals through  rise in cAMP, not Ca²⁺.]

Figure 11.13 Calmodulin mediates many effects of intracellular Ca2+.

a. 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 Ca²⁺ release from the sarcoplasmic reticulum into the sarcoplasm of myocytes. The Ca²⁺ binds the CaM subunit, and the complex activates muscle phosphorylase kinase b (see Figure 11.9).

b. Calcium activation of liver phosphorylase kinase: During physiologic stress, epinephrine is released from the adrenal medulla and signals the need for blood glucose. This glucose initially comes from hepatic glycogenolysis. Binding of epinephrine to hepatocyte α-adrenergic GPCRs activates a phospholipid-dependent cascade that results in movement of Ca²⁺ from the ER into the cytoplasm. A Ca²⁺–CaM complex forms and activates hepatic phosphorylase kinase b. [Note: The released Ca²⁺ also helps to activate protein kinase C that can phosphorylate (therefore, inactivate) glycogen synthase a.]

3. Activation of glycogen degradation in muscle: Muscle glycogen phosphorylase is active in the presence of the high adenosine monophosphate (AMP) concentrations that occur under extreme conditions of anoxia and ATP depletion. AMP binds to glycogen phosphorylase b, causing its activation without phosphorylation (see Figure 11.9). [Note: Recall that AMP also activates phosphofructokinase-1 of glycolysis, allowing glucose from glycogenolysis to be oxidized.]