The interaction of muscle glycogen phosphorylase b with glycogen

Interaction of muscle glycogen phosphorylase b (EC 2.4.1.1) with glycogen was studied by sedimentation, stopped-flow and temperature-jump methods. The equilibrium enzyme concentration was determined by sedimentation in an analytical ultracentrifuge equipped with absorption optics and a photoelectric scanning system. The maximum adsorption capacity of pig liver glycogen is 3.64 μmol dimeric glycogen phosphorylase b per g glycogen, which corresponds to 20 dimeric enzyme molecules per average glycogen molecule of M r 5.5 · 10 6 . Microscopic dissociation constants were determined for the enzyme-glycogen complex within the temperature range from 12.7 to 30.0°C. Enzyme-glycogen complexing is accompanied by increasing light scattering and its increment depends linearly on the concentration of the binding sites on a glycogen particle that are occupied by the enzyme. Complex formation and relaxation kinetics are in accordance with the proposed bimolecular reaction scheme. The monomolecular dissociation rate constant of the complex increases as the temperature increases from 12.7 to 30.0°C, whereas the bimolecular rate constant changes sligtly and is about 10 8 M −1 · s −1 . These data point to the possibility of diffusional control of the complex formation.