Catalytic site of glycogen phosphorylase: structural changes during activation and mechanistic implications

The structure of the activated form of glycogen phosphorylase has been probed at two different levels: in the crystalline state, by X-ray crystallography, and in solution, by kinetic studies on a modified enzyme using substrate analogues. Addition of a good, nondegradable substrate analogue, glucose cyclic 1,2-phosphate, to crystals of phosphorylase a, previously washed to remove glucose, results in partial activation. Changes occur in the lattice constants, and considerable structural rearrangement is observed upon difference Fourier analysis. The analogue is found at essentially the same site as glucose but with a 1-angstrom translation within the active site. Several protein conformational changes occur in response to this binding and activation, the most prominent being an order arrow right disorder transition affecting a beta -hairpin loop (residues 282-286) at the entrance to the active site.