Glycogen Glycogenolysis

The liver uses two mechanisms for endogenous glucose production, the mobilization of intracellular glycogen (glycogenolysis) and the synthesis of glucose from noncarbohydrate precursors (gluconeogenesis). 

Glycogen Synthesis and Degradation The Structure of Glycogen Glycogenolysis Glycogen... 

The breakdown of glycogen (glycogenolysis) requires two enzymes, glycogen phosphorylase and (... 

Glycogen phosphorylase catalyzes the rate-limiting step in glycogenolysis by promoting the phosphorylytic cleavage by inorganic phosphate (phosphorylysis cf hy-... 

Not only is phosphorylase activated by a rise in concentration of cAMP (via phosphorylase kinase), but glycogen synthase is at the same time converted to the inactive form both effects are mediated via cAMP-dependent protein kinase. Thus, inhibition of glycogenolysis enhances net glycogenesis, and inhibition of glycogenesis enhances net glycogenolysis. Furthermore,... 

Glucose is also formed from liver glycogen by glycogenolysis (Chapter 18). 

Glycogenolysis The process by which glycogen is broken down to glucose in body tissues. 

There were also less concrete considerations. In the early 1950s glycogenolysis was still believed to be completely reversible. UTP dependency and the glycogen synthase reactions had not yet been discovered nor had phosphofructokinase been shown to act irreversibly. The mechanism of protein synthesis was still a mystery. Laboratories studying proteolysis had shown that the peptide bond could be resynthesized by peptidases, although under very restricted conditions. Reversibility seemed to be an accepted property of the major metabolic pathways. 

Glycogen phosphorylase isoenzymes have been isolated from liver, brain and skeletal muscle. All forms are subject to covalent control with conversion of the inactive forms (GP-b) to the active forms (GP-a) by phosphorylation on specific serine residues. This phosphorylation step, mediated by the enzyme phosphorylase kinase, is initiated by glucagon stimulation of the hepatocyte. Indeed, the same cAMP cascade which inhibits glycogen synthesis simultaneously stimulates glycogenolysis, giving us an excellent example of reciprocal control. 

Glycogenolysis and glycogen synthesis P-oxidation of fatty acids transamination and deamination of amino acids Cori cycle and glucose-alanine cycle, which recycles substrates between muscle and liver. 

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