Inhibition of Muscle Phosphorylase a by 5-Gluconolactone

5-Gluconolactone is a potent inhibitor of rabbit muscle phosphorylase a in the presence of saturating AMP. Kinetic analysis leads to the conclusion that the inhibitor binds most strongly to the enzyme-glycogen-P i complex ( K api = 0.025 m m ), but weaker binding is also observed with the enzyme-glycogen-α- d -glucopyranose 1-phosphate complex, the enzyme-glycogen complex, the enzyme-P i complex, and the free enzyme. Dissociation constants for each of the observed inhibitor complexes have been evaluated. The kinetic data is consistent with a model in which 5-gluconolactone binds to the site in the enzyme that normally binds the glucosyl residue that is transferred between polysaccharide and P i in the catalytic reaction. 5-Gluconolactone may be an analogue of the substrate part of the transition state of the phosphorylase reaction.