Glycogen phosphorylase metabolism pyridoxal phosphate

INTERRELATIONSHIPS BETWEEN METABOLISM OF GLYCOGEN PHOSPHORYLASE AND PYRIDOXAL PHOSPHATE—IMPLICATIONS IN MCARDLE S DISEASE... 

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.)... 

Pantothenic acid is present in coenzyme A and acyl carrier protein, which act as carriers for acyl groups in metabolic reactions. Pyridoxine, as pyridoxal phosphate, is the coenzyme for several enzymes of amino acid metabolism, including the aminotransferases, and of glycogen phosphorylase. Biotin is the coenzyme for several carboxylase enzymes. 

All aminotransferases have the same prosthetic group and the same reaction mechanism. The prosthetic group is pyridoxal phosphate (PLP), the coenzyme form of pyridoxine, or vitamin B6. We encountered pyridoxal phosphate in Chapter 15, as a coenzyme in the glycogen phosphorylase reaction, but its role in that reaction is not representative of its usual coenzyme function. Its primary role in cells is in the metabolism of molecules with amino groups. 

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. 

Unlike other pyridoxal phosphate-dependent enzymes, in which it is the carbonyl group that is essential for catalysis, the internal Schiff base between pyridoxal phosphate and lysine in glycogen phosphorylase can be reduced with sodium borohydride without affecting catalytic activity. Thus, while pyridoxal phosphate is essential for phosphorylase activity, it does not act by the same kind of mechanism as in amino acid metabolism. 

Biochemical functions pyridoxal phosphate is essential for aminotransferase (transamination) reactions in amino aoid metabolism and so is needed both for biosynthesis of the non-essential amino acids and also for amino acid oxidation for energy metabolism. Also an Important component of glycogen phosphorylase... 

Vitamin Bg occurs in the three forms shown below. In the form of pyridoxal phosphate the vitamin acts as a coenzyme for more than sixty enzymes concerned with amino acid metabolism, e.g. aminotransferases, decarboxylases, deaminases and desulphurases. It also plays a role in the absorption of amino acids and is a constituent of glycogen phosphorylase. 

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. 

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