The Kinetics of Glycogen Phosphorylases from Brain and Muscle

Abstract

A kinetic study has been made of phosphorylase a from rabbit brain and of phosphorylase b from rabbit brain and muscle. In the case of brain phosphorylase a the kinetic data indicate strong cooperative interaction between glycogen and phosphate sites and between glycogen and adenylate sites. Under the experimental conditions there was found little evidence of interaction between phosphate and adenylate sites, although from a few experiments under different conditions and from the work of others, some interaction is possible. Although there are presumably at least two glycogen sites and two phosphate sites in the active form of the enzyme, there was no sign of interaction of one glycogen site with another or one phosphate site with another. The kinetic behavior of brain phosphorylase a is similar to that reported earlier for muscle phosphorylase a except for the failure to show the strong phosphate-adenylate interaction characteristic of the latter. The muscle enzyme also has much greater affinity for glycogen and somewhat lower affinity for adenylate and phosphate. The kinetic behavior of phosphorylase b is more complicated than that of phosphorylase a. The data for muscle phosphorylase b can be adequately explained by a formulation involving two binding sites each for 5'-adenylate and inorganic phosphate, and one site for glycogen. Random order of addition is indicated with marked interaction between many of the sites. Of the 17 independent kinetic constants demanded by this formulation 10 have been provisionally evaluated. The kinetic pattern of brain phosphorylase b resembles that of the muscle enzyme but the apparent affinities for phosphate and adenylate are greater. Raising the temperature to 38° profoundly decreases apparent affinity of all four enzymes for glycogen, with comparatively little effect on apparent affinity for phosphate.