Catalytic mechanism of glycogen phosphorylase: pyridoxal(5')diphospho(1)-alpha-D-glucose as a transition-state analogue.

Pyridoxal(5')diphospho(1)-alpha-D-glucose was used to reconstitute glycogen phosphorylase beta (1,4-alpha-D-glucan:orthophosphate alpha-D-glucosyltransferase, EC 2.4.1.1) from rabbit muscle, replacing the natural pyridoxal 5'-phosphate coenzyme. Incubation of the reconstituted enzyme alone resulted in the gradual cleavage of the synthetic cofactor to pyridoxal 5'-phosphate, which caused slow reactivation of the enzyme. The addition of maltopentaose or glycogen altered the mode of cleavage; the cofactor was rapidly decomposed to pyridoxal 5'-diphosphate. The radioactive glucose moiety released from pyridoxal(5')diphospho(1)-alpha-D-[14C]glucose was incorporated into the outer chain of glycogen, forming an alpha-1,4-glucosidic linkage. These results show that the glucosyl transfer reaction discovered mimics the normal catalysis of this enzyme, and they strongly support the catalytic mechanism in which the coenzyme phosphate acts as a catalyst by direct interaction with the phosphate of the substrate, forming the pyrophosphate-like transition intermediate.