The Role of Pyridoxal Phosphate in Glycogen Phosphorylase

Glycogen phosphorylase catalyzes the sequential 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 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.

Studies on the reactivation of apoglycogen phosphorylase with a variety of analogs of pyridoxal phosphate have shown that the catalytic moiety is the 5'-phosphate group - only analogs with a reversibly protonatable dianion in this position have any activity. In the nonactivated form of phosphorylase b, the phosphate is monoprotonated (-OPO3H-); when the enzyme has been activated, either allosterically or by phosphorylation (phosphorylase a), it is dianionic (-OPO32-). A glutamate residue in the active site acts as the proton acceptor or donor for this transition between the inactive and active forms of the cofactor.

The initial stage in the phosphorolysis of glycogen is protonation of the glycosidic oxygen of the polysaccharide by inorganic phosphate. The resultant oxycarbonium ion is stabilized by the inorganic phosphate. The role of pyridoxal phosphate is as a proton shuttle or buffer to stabilize the oxycarbonium-phosphate ion pair, permitting covalent binding of the phosphate to the oxycarbonium ion to form glucose 1-phosphate (Palm et al., 1990).

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