The effect of Ca2+ on the metarhodopsin I-II transition. II. Model calculations and hypothesis on a molecular mechanism of visual excitation.
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 delta MII-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 delta MII 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.
D002118:Calcium / Q000378:metabolism / Q000494:pharmacology*; D002463:Cell Membrane Permeability; D006863:Hydrogen-Ion Concentration; D008954:Models, Biological; D010782:Photolysis; D010786:Photoreceptor Cells / Q000378:metabolism; D011522:Protons; D012168:Retinal Pigments / Q000378:metabolism*; D012243:Rhodopsin / Q000378:metabolism*; D013816:Thermodynamics
D011522:Protons; D012168:Retinal Pigments; D012243:Rhodopsin; D002118:Calcium