Glycogen phosphorylase metabolism vitamin

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. 

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. 

There maybe multiple metabolic pools of the vitamin, with very different rates of turnover. In this case, short-term and long-term studies will give very different estimates of the fractional rate of turnover of the toted body pool. As discussed in Section 9.6.1, this is known to be a problem with vitamin Be, because some 80% of the total body pool is associated with muscle glycogen phosphorylase and has a much lower fractional turnover rate than the remedning 20%. 

FIG. 1. Interrelationships between vitamin B6 and phosphorylase metabolism. The low rate of turnover of glycogen phosphorylase (gpb) and the lack of exchange of free and protein-bound PLP mean that exchange into the muscle pool is largely controlled by the kinetics of turnover of the enzyme. At present, it is not known whether resolution of the holo-enzyme is a prerequisite or consequence of phosphorylase degradation. Reproduced with permission of llie Biochemical Journal. 

If the phosphorylase pool plays an important part in vitamin B6 kinetics, it might be anticipated that this metabolism would be disturbed in patients suffering from MciVdle s disease, a rare metabolic myopathy caused by an absence of functional muscle glycogen phosphorylase. The absence of this enzyme means that patients cannot break down their muscle glycogen reserves. Other energy sources within the muscle are rapidly depleted... 

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. 

Vitamin Bg has a central role in amino acid metabolism as the coenzyme for a variety of reactions, including transamination and decarboxylation. It is also the coenzyme of glycogen phosphorylase and acts to modulate the activity of steroid and other hormones (including retinoids and vitamin D) that act by regulation of gene expression. 

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