How pyridoxal 5'-phosphate could function in glycogen phosphorylase catalysis.

A mechanism for the phosphorylase reaction is proposed which offers a plausible explanation for the essential role of pyridoxal 5'-phosphate in glycogen phosphorylases: in the forward direction, phosphorolysis of alpha-1,4-glycosidic bonds in oligo- or polysaccharides is started by protonation of the glycosidic oxygen by the substrate orthophosphate followed by stabilization of the incipient oxocarbonium ion and subsequent covalent binding to form alpha-glucose 1-phosphate. In the reverse direction, protonation of the phosphate of glucose 1-phosphate destabilizes the glycosidic bond and promotes formation of a glucosyl oxocarbonium ion-phosphate anion pair. In the subsequent step the phosphate anion facilitates the nucleophilic attack of a terminal glucosyl residue on the carbonium ion bringing about alpha-1,4-glycosidic bond formation and primer elongation. Both in the forward and reverse reactions, the phosphate of the cofactor pyridoxal 5'-phosphate acts as a general acid (PL-OPO3H- or PL-OPO3(2-) and protonates the substrate phosphate functioning as proton shuttle. Thus in glycogen phosphorylases, phosphates which directly interact with each other have replaced a pair of amino acid carboxyl groups functioning in catalysis of carbohydrases.