A model for the behaviour of phosphorylase b. The generation of different binding sites via intermediate enzymatic states.

The model given in this paper can be applied to enzymatic systems which have more than two conformational states in equilibrium and which clearly exhibit heterogeneity in the binding of one ligand. The model we propose makes possible quantitative interpretation of our experimental results and of those of many other workers as well. In some cases calorimetric, dialysis and kinetic magnitudes, when plotted against ligand concentration, give multiregional or "stepwise" curves. We suggest that such a behaviour arises because total occupation of one class of binding sites completely moves the enzyme towards a different conformational state in which the affinity for the ligand is greatly increased by the formation of a new class of binding sites. Our calorimetric results for the interaction between some nucleotides and phosphorylase b closely conform to our model.