Influence of Carbohydrates on Phosphorylase Structure and Activity. I. Activation by Preincubation with Glycogen*

Preincubation of Phosphorylase a at 20° with glycogen results in an approximately 6-fold increase in maximal velocity with glucose 1-phosphate with no apparent alteration in the Michaelis constant. Activation does not appear related to formation of a better primer. The dependence of the extent of activation with protein concentration, temperature, and ionic strength suggests that activation is related to enzyme dissociation and is consistent with the conversion of a less active tetrameric form of Phosphorylase a or b to a more active dimeric species. Activation is not related to priming capacity of polysaccharide since amylopectin or its β-amylase limit dextrin, or hydrolyzed amylose, were found to be more efficient activators than glycogen. Although hydrolyzed amylose was found to be a competitive inhibitor with respect to glycogen in the activity test, activation by hydrolyzed amylose is not inhibited by glycogen, a weaker activator. These data show that activation by amylose is not a simple consequence of binding at the primer site and suggest that activation is a result of binding at an additional or activation site on the enzyme.