Glycogen phosphorylase reactions involving

Whereas pyridoxal phosphate is used primarily for reactions involving amino acids, it is also required for the glycogen phosphorylase reaction, in which it acts as a general acid/base catalyst. 

The control of glycogen phosphorylase by the phosphorylation-dephosphorylation cycle was discovered in 1955 by Edmond Fischer and Edwin Krebs50 and was at first regarded as peculiar to glycogen breakdown. However, it is now abundantly clear that similar reactions control most aspects of metabolism.51 Phosphorylation of proteins is involved in control of carbohydrate, lipid, and amino acid metabolism in control of muscular contraction, regulation of photosynthesis in plants,52 transcription of genes,51 protein syntheses,53 and cell division and in mediating most effects of hormones. 

Glycogen phosphorylase, an enzyme involved in the metabolism of the carbohydrate polymer glycogen, catalyzes the reaction ... 

The cofactor of glycogen phosphorylase is pyridoxal 5 -phosphate (PLP). This cofactor, linked via a Schiff base to a lysine residue (Lys680 in the rabbit sequence), is tightly bound to the enzyme and cannot be resolved from the apo-enzyme unless powerful denaturants are used. The use of PLP in the phosphorylase reaction is unusual and involves the 5 -phosphate group rather than the aldehydic group that is used more conunonly in, for example, transaminases (Johnson, 1 2). 

An important extended example of enzyme interconversion cycles is the reciprocal control of glycogen metabolism involving glycogen synthase and glycogen phosphorylase (Section 11.5). The activities of both enzymes are regulated in concert by phosphorylation and dephosphorylation reactions so that when the synthetic pathway is in operation, the degradative pathway is reciprocally r uced. 

The metabolically active vitamer is pyridoxal phosphate, which is involved in many reactions of amino acid metabolism, where the carbonyl group is the reactive moiety, in glycogen phosphorylase, where it is the phosphate group that is important in catalysis, and in the release of hormone receptors from tight nuclear binding, where again it is the carbonyl group that is important. 

In nature a cascade of enzyme-catalyzed reactions is involved for the biosynthesis of starch. When selecting the appropriate enzymes and reaction circumstances reactions with multiple enzymes can be performed in vitro. Synthetic glycogen was first synthesized in vitro by Cori [146] in 1943 via the cooperative action of muscle phosphorylase and branching enzymes isolated from rat liver and rabbit heart. 

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