Glycogen phosphorylase metabolism cofactor

Pyridoxal phosphate is a coenzyme for many enzymes involved in amino acid metabolism, especially in transamination and decarboxylation. It is also the cofactor of glycogen phosphorylase, where the phosphate group is catalytically important. In addition, vitamin Bg is important in steroid hormone action where it removes the hormone-receptor complex from DNA binding, terminating the action of the hormones. In vitamin Bg deficiency, this results in increased sensitivity to the actions of low concentrations of estrogens, androgens, cortisol, and vitamin D. 

Pyridoxal phosphate is an essential cofactor in the glycogen phosphorylase reaction its phosphate group acts as a general acid catalyst, promoting attack by Pj on the glycosidic bond. (This is an unusual role for this cofactor its more typical role is as a cofactor in amino acid metabolism see Fig. 18-6.)... 

Vitamin Be has a central role in the metabolism of amino acids in transaminase reactions (and hence the interconversion and catabolism of amino acids and the synthesis of nonessential amino acids), in decarboxylation to yield biologically active amines, and in a variety of elimination and replacement reactions. It is also the cofactor for glycogen phosphorylase and a variety of other enzymes. In addition, pyridoxal phosphate, the metabolically active vitamer, has a role in the modulation of steroid hormone action and the regulation of gene expression. 

This increased phosphorylase activity lead to a breakdown of glycogen in the adrenal cortical cells, producing glucose-l-phosphate and glucose-6-phosphate. The latter substance when metabolized via the pentose pathway produces NADPH, and it was proposed that this nucleotide tiien stimulated corticosteroidogenesis through its role as a cofactor in many of the biosynthetic steps. 

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