Allosteric Properties of Phosphorylase b

Abstract

Kinetic data for the inhibition of phosphorylase b by adenosine di- and triphosphate and glucose 6-phosphate have been compared with predictions from the theoretical model for allosteric transitions proposed by Monod, Wyman, and Changeux (6). The substrate, inorganic orthophosphate, shows homotropic cooperativity at low concentrations, and this cooperativity is increased as the concentration of the activator, AMP, is decreased. The homotropic cooperativity of the substrates, Pi or glucose-1-P, is expressed more strongly and at higher concentrations as the above inhibitors are added, and these effects are proportional to the concentration of the inhibitor. The activator shows homotropic cooperativity at low concentrations. Decreasing the substrate concentration does not appear to affect the strength of this cooperativity although the apparent Km values are increased. Adding an inhibitor appears to increase the homotropic cooperativity of AMP and causes it to be expressed at a higher concentration. The inhibitor, ATP, shows homotropic cooperativity which, measured indirectly by kinetic methods, appears to increase as Pi or AMP is decreased. AMP and substrate act in concert to antagonize the inhibition by ATP. Uridine diphosphate glucose, which is a competitive inhibitor for Pi or glucose-1-P, activates the enzyme in the presence of ATP at low substrate concentrations. With some exceptions in the realm of the homotropic cooperativity of activator or inhibitor, these results are in qualitative agreement with the theoretical model cited above. Quantitative agreement has been demonstrated for several situations.