[A molecular model of cooperative binding of Ca2+ with rhodopsin molecules in photoreceptor membranes]

Kinetics of calcium binding by photoreceptor membranes of cattle retina in concentration Ca2+ 0.5 and 1.0.10(-5) M in 5 mM tris-HCl buffer, pH 7.4 at 37 degrees C has been studied. Such kinetics is of oscillating nature. Analysis of calcium binding process curves by photoreceptor membranes allow to conclude, that crystalline areas of rhodopsin (receptor domains) can be formed in the structure of photoreceptor membranes. Conformation states and structure of rhodopsin molecules Ca-binding sites in receptor domains depend on the presence of Ca2+ in the medium. The structure of rhodopsin molecules Ca-binding sites in receptor domain formed in the presence of Ca2+ in the medium was proposed. According to the Hodgkin and Huxley conception concerning the properties of Na(+)- and K(+)-channels, the receptor domain with such a structure of rhodopsin molecules Ca-binding sites can represent the conjugate system of Na(+)- and K(+)-channels. Molecular mechanisms of photoreceptor and nerve cells excitation was also proposed.