Symmetry conditions for binding processes.

Symmetry conditions are derived for global and local binding processes in biological macromolecules. It is shown that the conditions applying in the case of the macromolecule as a whole are decoupled from those referring to individual sites. In the case of two sites, the global binding curve is always symmetric, and the individual-site binding curves are always asymmetric, unless the two sites are identical or independent. In the case of three sites or more, individual-site binding curves can show symmetric or asymmetric behavior. The conditions derived for symmetry in the local description of binding processes also apply to the case of linkage among different ligands and to steady-state kinetics. Application to the analysis of oxygen binding to human hemoglobin under physiological conditions provides a model-independent interpretation of the asymmetric nature of the binding curve. Asymmetry of the global binding curve can coexist with symmetric or asymmetric binding to the individual alpha and beta chains. If the binding curves of the two chains are symmetric, then subunit heterogeneity and asymmetric interactions must exist in the hemoglobin tetramer. On the other hand, if the binding curves of the two chains are asymmetric, then subunit heterogeneity and asymmetric interactions are not necessary for global asymmetric binding.