The effect of Ca²⁺ on the metarhodopsin I–II transition

Summary

The model for the effect of Ca²⁺ on the Meta I–II transition (cf. Part I) is formulated quantitatively and in detail. The clamping forces of Ca²⁺, which shift the equilibrium to the closed M_I-conformation, are taken into account in the law of mass action by a higher value for the association constant of Ca²⁺ with M_I than with M_II. Thus the main features of the experimental curves of ΔM_II-absorption-change as a function of Ca²⁺- and H⁺-concentration can be reproduced by a few simple association equilibria. The agreement can be improved by calculating with a conformative coupling between two rhodopsin molecules. In a further modification of the model also the observed increase of ΔM_II in the alkaline beyond pH 9 is reproduced. Finally, the models lead to an opening and closing mechanism for a Ca²⁺-permeable pore across the disc membrane, thus contributing to a hypothesis of visual excitation.