Photochemistry of Visual Pigment in a G(q) Protein-Coupled Receptor (GPCR)-Insights from Structural and Spectral Tuning Studies on Squid Rhodopsin

摘要

Vertebrate visual pigment rhodopsin is a G protein-coupled receptor (GPCR) responsible for twilight vision. Upon activation by light, rhodopsin initiates the visual transduction cascade that leads to the transmission of an optic nerve signal to the brain.[1–3] In both flying and terrestrial animals such a signal is vital for foraging, navigation, and mate selection within their immediate environment.[4, 5] Both the vertebrate (bovine) and invertebrate (squid) rhodopsins contain a 11-cis-retinal chromophore bound covalently to a protonated Schiff base (PSBR) nitrogen atom of a lysine residue at site 296 and 305, respectively. In bovine rhodopsin, Glu113 is the main counterion and forms a hydrogen bond with the PSBR.[6–9] However, in squid rhodopsin the position corresponding to Glu113 is occupied by Tyr111, and together with Asn87 they serve as potential hydrogen-bonding partners for the PSBR. There is only one glutamate (Glu180) in the interior of squid rhodopsin and is situated at -5 - away from PSBR and corresponds to Glu181 in bovine rhodopsin. All other glutamates in squid rhodopsin are at the surface and more than 17 - away from PSBR.[10, 11]