The principal enzymes controlling glycogen metabolism— glycogen phosphorylase and glycogen synthase—are regulated in opposite directions by allosteric mechanisms, subcellular localization, and covalent modification by reversible phosphorylation and dephosphorylation of enzyme protein in response to hormone action . Phosphorylation of glycogen phosphorylase increases its activity; phosphorylation of glycogen synthase reduces its activity.

Phosphorylation is increased in response to cyclic AMP (cAMP) (Figure 1) formed from ATP by adenylyl cyclase at the inner surface of cell membranes in response to hormones such as epinephrine, norepinephrine, and glucagon. cAMP is hydrolyzed by phosphodiesterase, so terminating hormone action; in liver insulin increases the activity of phosphodiesterase.

Fig1. The formation and hydrolysis of cyclic AMP (3′,5′-adenylic acid, cAMP).

Glycogen Phosphorylase Regulation Is Different in Liver & Muscle

In the liver, the role of glycogen is to provide free glucose for export to maintain the blood concentration of glucose; in muscle, the role of glycogen is to provide a source of glucose 6-phosphate for glycolysis in response to the need for ATP for muscle contraction. In both tissues, the enzyme is activated by phosphorylation catalyzed by phosphorylase kinase (to yield phosphorylase a) and inactivated by dephosphorylation catalyzed by phosphoprotein phosphatase (to yield phosphorylase b), in response to hormonal and other signals.

There is instantaneous overriding of this hormonal control. Active phosphorylase a in both tissues is allosterically inhibited by ATP and glucose-6-phosphate; in liver, but not muscle, free glucose is also an inhibitor. Muscle phosphorylase differs from the liver isoenzyme in having a binding site for 5′ AMP (see Figure 1), which acts as an allosteric activator of the (inactive) dephosphorylated b-form of the enzyme. 5′ AMP acts as a potent signal of the energy state of the muscle cell; it is formed as the concentration of ADP begins to increase (indicating the need for increased substrate metabolism to permit ATP formation), as a result of the reaction of adenylate kinase: 2 × ADP ↔ ATP + 5′ AMP. The differing regulation of muscle and liver glycogen phosphorylase likely reflects the differing roles of glycogen in liver and muscle.

Liver glycogen is released to control plasma glucose. It has to be fast as glucose demands can change rapidly. For example in exercise liver glucose production increases twofold within a few minutes of exercise. Glucose levels would fall if there was a delay. If exogenous glucose is given the liver must rapidly stop making glucose and become a glucose consumer to prevent hyperglycemia and maximize glycogen repletion. In muscle, glycogen is used by the contracting muscle and thus it would be very sensitive to the energy state of the muscle (5′ AMP as well as calcium).

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مواضيع ذات صلة

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

The Citric Acid Cycle Takes Part in Gluconeogenesis, Transamination, & Deamination

Reactions of The Citric Acid Cycle Generate Reducing Equivalents & CO2

The Citric Acid Cycle Provides Substrates for The Respiratory Chain