See also Calmodulin Mechanism of Activating Glycogen Breakdown Glycogen Breakdown

Regulation, Glycogen Breakdown, Kinase Cascade in Fat Mobilization (from Chapter 18), Second Messenger Systems

Figure 16.11: Regulatory cascades affecting glycogen synthesis and mobilization.

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Figure 16.11: Regulatory cascades affecting glycogen synthesis and mobilization.

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Pp1 Glycogen

Figure 16.12: Regulation of glycogen synthase activity through cAMP mediated control of phosphoprotein phosphatase (PP-1) activity.

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Figure 16.12: Regulation of glycogen synthase activity through cAMP mediated control of phosphoprotein phosphatase (PP-1) activity.

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Glycogen Phosphorylase Calmodulin

Glycogen phosphorylase a is the more active form of glycogen phosphorylase. It catalyzes the phosphorolysis of glycogen to form glucose-1-phosphate. Two features distinguish glycogen phosphorylase a from the b form:

1. The a form is derived from the b form by phosphorylation of the b form by the enzyme glycogen phosphorylase b kinase (Figure 13.18).

2. The b form requires AMP for allosteric activation and is thus active only when cells are at a low energy state.

See also: Glycogen Phosphorylase b, Glycogen Breakdown, Mechanism of Activating Glycogen Breakdown, Kinase Cascade, Glycogen Breakdown Regulation, Phosphorolysis

The breakdown of glycogen (glycogenolysis) requires two enzymes, glycogen phosphorylase and ( oil,4 ->otl,4) glucantransferase (a "Debranching Enzyme"). Glycogen phosphorylase catalyzes the phosphorolytic cleavage of oi(l->4) bonds, generating glucose-1-phosphate in the process. The branch points of glycogen are comprised of oi(l->6) bonds, however and glycogen phosphorylase does not cleave within four glucose units of an ot(l->6) bond. Instead, the debranching enzyme, transfers three of the four glucoses to another branch and hydrolytically cleaves the remaining glucose as free glucose (Figure 13.17).

Glycogen phosphorylase is present in two forms, glycogen phosphorylase a (the active form) and glycogen phosphorylase b (the relatively inactive form). Phosphorylase a is phosphorylated at a serine residue whereas phosphorylase b is not The two forms are interconverted by phosporylase b kinase (which puts phosphates on) or a phosphatase (which takes phosphates off).

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