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

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Abstract
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 MI-conformation, are taken into account in the law of mass action by a higher value for the association constant of Ca2+ with MI than with MII. Thus the main features of the experimental curves of ΔMII-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 ΔMII 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.