At the same time, as phosphorylase is activated by a rise in concentration of cAMP (via phosphorylase kinase), glycogen synthase is converted to the inactive form; both effects are mediated via cAMP-dependent protein kinase(Figure 1). Thus, inhibition of glycogenolysis enhances net glycogenesis, and inhibition of glycogenesis enhances net glycogenolysis. Also, the dephosphorylation of phosphorylase a, phosphorylase kinase, and glycogen synthase b is catalyzed by a single enzyme with broad specificity—protein phosphatase-1. In turn, protein phosphatase-1 is inhibited by cAMP-dependent protein kinase via inhibitor-1. Insulin ability to inhibit glycogenolysis is via its ability to increase protein phosphatase-1 and lower cAMP. Thus, glycogenolysis can be terminated and glycogenesis can be stimulated, or vice versa, synchronously, because both processes are dependent on the activity of cAMP dependent protein kinase and the availability of cAMP. Both phosphorylase kinase and glycogen synthase may be reversibly phosphorylated at more than one site by separate kinases and phosphatases. These secondary phosphorylations modify the sensitivity of the primary sites to phosphorylation and dephosphorylation (multisite phosphorylation). They allow increases in glucose and insulin by way of increased glucose 6-phosphate (secondarily to an increase in plasma glucose that activates glucokinase by translocating it from the nucleus to the glycogen granule in the liver), to have effects that act to amplify the effects of insulin to attenuate glycogen mobilization or augment glycogen synthesis (see Figures 2 and 3).

Fig1. Coordinated control of glycogenolysis and glycogenesis by cAMP-dependent protein kinase. The reactions that lead to glycogenolysis as a result of an increase in cAMP concentrations are shown with bold arrows, and those that are inhibited by activation of protein phosphatase-1 are shown with dashed arrows. The reverse occurs when cAMP concentrations decrease as a result of phosphodiesterase activity, leading to glycogenesis.

Fig2. Control of glycogen phosphorylase in muscle. The sequence of reactions arranged as a cascade allows amplification of the hormonal signal at each step. (G6P, glucose 6-phosphate; n, number of glucose residues.)

Fig3. Control of glycogen synthase in muscle.(G6P, glucose-6-phosphate; GSK, glycogen synthase kinase; n, number of glucose residues.)

The ability to rapidly fine tune hepatic glycogen metabolism is critical as a failure to rapidly increase hepatic glucose production in response to an increase in glucose demand (ie, exercise) will cause hypoglycemia. It can cause death if severe enough, and even moderate hypoglycemia will prematurely terminate exercise. Gluconeogenesis is a slower process and requires a rise in gluconeogenic substrate supply that helps to sustain glucose production during sustained exercise. In fact, in moderate intensity exercise glucose barely changes because of the tight coupling between liver glycogenolysis, gluconeogenesis, and muscle glucose demand. This occurs because of a rise in glucagon and fall in insulin that drives liver glucose production (remember during exercise glucose uptake in muscle goes up independent of insulin, so even though insulin is falling glucose uptake in exercising muscle increases).

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مواضيع ذات صلة

Gluconeogenesis & the Control of Blood Glucose: Biomedical Importance

cAMP Activates Glycogen Phosphorylase

Cyclic AMP Integrates The Regulation of Glycogenolysis & Glycogenesis

Glycogenolysis is not the Reverse of Glycogenesis, it is A Separate Pathway

Glycogenesis Occurs Mainly in Muscle & Liver

The Oxidation of Pyruvate to Acetyl-COA is The Irreversible Route from Glycolysis to The Citric Acid Cycle

Glycolysis is Regulated at Three Nonequilibrium Reactions

Tissues That Function Under Hypoxic Conditions or have Intrinsically High Rates of Glucose Oxidation Produce Lactate

Glycolysis Constitutes The Main Pathway of Glucose Utilization

Glycolysis Can Function Under Anaerobic Conditions

Regulation of the Citric Acid Cycle Depends Primarily on a Supply of Oxidized Cofactors

The Citric Acid Cycle Takes Part in Fatty Acid Synthesis