A Probe into the Catalytic Activity and Subunit Assembly of Glycogen Phosphorylase

Abstract Limited tryptic attack of phosphorylase a to produce phosphorylase b' is stimulated in the presence of the dissociating agents, glucose and hydrolyzed amylose. Phosphorylase b', a dimer, has been shown to exhibit no tendency to associate into a tetramer under conditions in which phosphorylase b, also a dimer, readily associates. The catalytic properties of phosphorylase b' have been investigated and found to differ strikingly from those of phosphorylase b. The apparent Michaelis constants of phosphorylase b' for substrates and activator were essentially independent of the concentrations of glycogen, inorganic phosphate, and AMP. No activation could be detected with NaF, and little or no inhibition was found with glucose 6-phosphate. The results suggest that the region of the primary structure released by trypsin, which contains the seryl residue phosphorylated in the phosphorylase b to a reaction, is involved in some way in dimer-dimer binding of phosphorylase a, and that this region is an essential part of the molecular structure which is responsible for the regulatory behavior of glycogen phosphorylase.