An oxygenation-linked dye binding to Limulus polyphemus hemocyanin

The reaction of Limulus polyphemus hemocyanin with a dye, bromthymol blue, was examined by equilibrium dialysis, spectrophotometric titration and stopped-flow methods. Oxy-hemocyanin contained one binding site per hexamer unit. The dye binding was linked to oxygenation, and the affinity of the dye for the oxy form was about 10 times as high as that for the deoxy form. Conversely, the dye increased the O2 affinity of hemocyanin. Hemocyanin showed a simple hyperbolic binding curve in the bromthymol blue titration, whereas the time course of the reaction was generally biphasic. It was inferred from the kinetic analyses that the reaction proceeds in two steps. The first bimolecular step is characterized by an increase in the apparent pKa of the bound dye, while the second unimolecular step by a red shift of the absorption band of the unionized dye. The dye binding to partially oxygenated hemocyanin was examined spectrophotometrically; the fractional change in the binding was found to be ahead of the increase in the average degree of O2 saturation. It was concluded that the structural changes in hemocyanin which lead to the increased dye affinity take place at an early stage of the ligand binding sequence.