Kinetic Studies of the Activation of Muscle Phosphorylase Phosphatase

Abstract

Activation of phosphorylase phosphatase by glucose, glucose 6-phosphate, and glycogen has been investigated by kinetic studies.The carbohydrates exert their effect by binding to phosphorylase a since no effect could be demonstrated with these compounds on the alternative substrate, the phosphorylated tetradecapeptide.Glucose and glucose-6-P activate the reaction by changing the V,,, of the reaction.Glucose-6-P binding has no effect on affinity, whereas glucose binding to phosphorylase a causes a slight increase in the apparent Km value.The binding constant for glucose-6-P was evaluated as 3.0 mu.Glycogen activates phosphorylase phosphatase solely by changing the Km value.Circular dichroism studies show that phosphorylases a and b possess different conformations, but activators do not change the circular dichroism signal.A possible physiological role for glucose-6-P in the control of phosphorylase phosphatase is presented.