A Comparison between the Photoactivation Kinetics of Human and Bovine Rhodopsins

Rhodopsin is a G-protein-coupled receptor important for vertebrate vision under dim light conditions. Many studies of the activation mechanism of bovine rhodopsin have been conducted, but there have been relatively few investigations of the human protein. A recent study of the late photointermediates of bovine rhodopsin studies at 15 °C and pH 7.3, 8.0, and 8.7 revealed a rather complex activation mechanism involving two metarhodopsin I₄₈₀ and metarhodopsin II intermediates. Human rhodopsin was studied under these same conditions using time-resolved optical absorption spectroscopy with measurements from 10 μs to 200 ms after photolysis. The results show that the two proteins follow the same photoactivation mechanism, although their kinetics differ significantly. The comparison of bovine and human rhodopsins shows that the initial Schiff base deprotonation equilibrium is more forward shifted in human rhodopsin, and more of the reaction flows through the metarhodopsin I₃₈₀ intermediate in human rhodopsin than in the bovine protein.