Theory of sedimentation for ligand-mediated heterogeneous association-dissociation reactions

Theoretical analytical sedimentation patterns have been computed for ligand-mediated heterogeneous association-dissociation reactions between macromolecules. Involvement of either a single kind of ligand or two different ligands acting in a stepwise fashion has been considered. Self-association, mediated in a stepwise fashion by two different ligands, has also been examined. The conclusion reached is that such interactions have the potentiality for exhibiting as many as three or four sedimenting peaks despite rapid rates of reaction. In general, the peaks correspond to different equilibrium compositions and not to individual macromolecular species; that is to say, they constitute a reaction boundary. Their resolution depends upon generation of concentration gradients of ligand(s) along the centrifuge cell by chemical reequilibration during sedimentation of the several macromolecular species. The implications of these findings for fundamental studies on subunit proteins and protein assemblies and for conventional applications of ultracentrifugation are discussed.