A Comparison between the Photoactivation Kinetics of Human and Bovine Rhodopsins

Abstract

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 I 480 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 I 380 intermediate in human rhodopsin than in the bovine protein.

An experimental comparison of human and bovine rhodopsin provides insight into the molecular basis of retinal disease

“…However, emerging evidence highlights the importance of comparative studies to address the variation in rhodopsin function , even among mammals . This trend also extends to human rhodopsin, where despite 93.4% sequence identity between the two pigments , several aspects of function differ from bovine rhodopsin, such as λ MAX , and kinetics of rhodopsin intermediates following light activation . This study shows that human rhodopsin and bovine rhodopsin also have significantly different rates of thermal decay and light‐activated retinal release.…”

“…This study highlights important differences between the function of bovine and human rhodopsin, along with hypotheses relating to the major structural elements that may be involved in these differences. We continue to build on recent work aiming to better characterize human rhodopsin , most notably how it differs from bovine rhodopsin. This is particularly critical given mounting evidence for significant functional variation among mammalian rhodopsins .…”

“…In order to properly interpret studies of bovine rhodopsin that concern inherited retinopathies most relevant to humans, such as RP, it is important to have a clear understanding of the functional differences between bovine and human rhodopsin. Several aspects of human rhodopsin function have been shown to differ from bovine rhodopsin, such as λ MAX and the kinetics of light activation , suggesting that functional discrepancy exists between the two visual pigments despite minimal sequence variation. Moreover, investigating the trajectory of functional differences between human and bovine rhodopsin can be better understood through examining ancestrally reconstructed mammalian rhodopsins , which may shed light on the relationship between rhodopsin function and amino acid variation among present day pigments.…”

Protein Sequence and Membrane Lipid Roles in the Activation Kinetics of Bovine and Human Rhodopsins

“…The photoactivation mechanisms of bovine and human rhodopsins were shown to follow the same kinetic model (Fig. 1) at 15 C, although they exhibit differences in their actual reaction rates (5,25). Human rhodopsin activation favors a path involving more of the deprotonated Schiff base intermediates than the bovine protein.…”

“…Human rhodopsin activation favors a path involving more of the deprotonated Schiff base intermediates than the bovine protein. Specifically, the equilibrium at the early Schiff base deprotonation steps are forward shifted in comparison to the bovine rhodopsin (25,26).…”