Role of oligomeric interactions in the cooperativity of crayfish hemocyanin

To examine the effect of interactions between the hexameric units in the dodecameric hemocyanin, crayfish (Procambarus clarki) hemocyanin was partially dissociated. Gel-filtration yielded fractions containing the undissociated dodecamer and a mixture of the hexamer and heptamer (referred to as 'half-dodecamer'). Their O2 equilibria were compared and were analyzed by curve fitting of cooperativity models. The partial dissociation of the dodecamer significantly lowered the cooperativity but little affected the O2 affinities. The O2 equilibrium of the half-dodecamer could be described by the two-state allosteric model of Monod-Wyman-Changeux on the assumption that the cooperative unit is composed of six subunits. We tested the applicability of allosteric models for the dodecamer that deal with the interactions between the hexamers. To reduce the arbitrariness of the fitting, we presumed that some parameters are equal to the two-state model parameters for the half-dodecamer. It was found that the 'nesting model', which assumes allosteric equilibria at two different hierarchical levels, could be applied successfully to the dodecamer.