An essential arginine residue in the active-site pocket of glycogen phosporylase

Abstract

Phosphorylases a and b (EC 2.4.1.1) were inactivated by selective modification of arginyl residues on reaction with 2,3-butanedione in sodium borate buffer. The rate of inactivation was slightly greater for phosphorylase a than b. The course of inactivation followed pseudo-first-order kinetics with some deviations at low rates or at more than 60% inactivation. The rate of inactivation was first order with respect to butanedione concentration. The inactivation was partially reversible, and ultracentrifugal studies showed no change in subunit association or dissociation. Amino acid analyses indicated that several arginines were modified during inactivation and that no other amino acid was affected. Protection from inactivation was provided by the substrate glucose 1-phosphate (G1P), alone or together with the allosteric activator AMP, as well as by the competitive inhibitor UDP-glucose. The rate of inactivation of phosphorylase b was also retarded by the presence of AMP alone. Glycogen did not have any appreciable effect on inactivation. The Km of G1P for phosphorylase a remained constant over the course of inactivation, while the Km values of G1P and AMP for phosphorylase b increased. The modification of cross-linked tetragonal microcrystals of phosphorylase a followed the same trend as the enzyme in solution, although the rate of inactivation was slower. The X-ray crystallography studies at 6 A (1 A = 0.1 nm) resolution, of butanedione-treated cross-linked tetragonal crystals of phosphorylase a showed a large new peak of electron density at the end of a long side chain in the active-site pocket. The substrates G1P and arsenate, as well as UDP-glucose, had previously been shown to bind in that location. Other, small peaks of electron density were found in locations on the outside of the protein molecule. UDP-glucose failed to bind to the active site of crystals which had been treated with butanedione, while AMP, which also binds in the active-site pocket, showed a lower occupancy. This work indicates the presence of a functional arginine residue at the binding site for G1P in glycogen phosphorylases a and b.