Location of pyridoxal phosphate in glycogen phosphorylase a.

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Abstract
The pyridoxal 5''-phosphate cofactor of glycogen phosphorylase a (1,4-.alpha.-D-glucan:orthophosphate .alpha.-glucosyltransferase, EC 2.4.1.1.) was positioned on the protomer with X-ray diffraction data, chemical markers and sequence information. The electron density was computed from 3.0 .ANG. resolution phases calculated from 4 heavy-atom derivatives. The cofactor is buried inside the protomer adjacent to the glucose-binding site. The phosphoryl substrates Pi and glucose-1-P [phosphate] each bind at 2 sites on the protomer. At low concentrations, Pi and glucose-1-P bind in the same location as does the allosteric effector AMP, near the monomer-monomer interface and some 30 .ANG. from the glucose site. At high concentrations glucose-1-P also binds strongly at the glucose site, with its P only 7.2 .ANG. from that of the cofactor. Pi can also bind at this site. Implications for the participation of the pyridoxal phosphate in the catalytic mechanism are discussed in the light of these structural findings as well as indirect evidence in the literature.