Isoelectric focusing of interacting systems. IV. interaction of macromolecules with each other and with ligands.

The theoretical isoelectric focusing behavior for rapidly reversible, bimolecular complexing between two macromolecules depends upon the relative value of the isoelectric point of the complex. When it is intermediate in value, the transient patterns exhibit three peaks. As equilibrium is approached the central peak of complex disappears leaving two reactant peaks. When the isoelectric point is acidic or alkaline to both reactants, the equilibrium pattern also shows two peaks; but in this case only one is pure reactant, the other being a reaction zone. The two cases can be distinguished by varying the relative amounts of reactants. Transient patterns for ligand-binding exhibit a peak of unliganded protein and a reaction zone. As the charged ligand is driven out of the focusing column the reaction zone disappears, so that the equilibrium pattern shows only a peak of unliganded protein. In general, the isoelectric point of the complex cannot be determined from the transient patterns.