The Role of Pyridoxal Phosphate in Glycogen Phosphorylase

Glycogen phosphorylase catalyzes the sequentied phosphorolysis of glycogen to release glucose 1-phosphate it is thus the key enzyme in the utilization of tissue glycogen reserves. 

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

Studies on the reactivation of apoglycogen phosphorylase with a variety of anedogs of pyridoxal phosphate have shown that the catedytic moiety is the 5 -phosphate group - only antdogs with a reversibly protonatable dianion in this position have any activity. In the nonactivated form of phosphorylase b, the phosphate is monoprotonated (-OPOsH ) when the enzyme htis been activated, either edlosterictdly or by phosphorylation (phosphoryltise a), it is dianionic (-OPOs ). A glutamate residue in the active site acts as the proton acceptor or donor for this trtmsition between the inactive and active forms of the cofactor. 

3 The Role of Pyridoxal Phosphate in Steroid Hormone Action 

Steroid hormones act by binding to, and activating, nuclear receptors that then bind to hormone response elements on DNA, increasing (or sometimes decreasing) the transcription of specific genes. 

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