G protein-coupled receptor rhodopsin serves as a light detector for vertebrate vision in dim-light environments. It contains 11-cis retinal, which links to the protein via a protonated Schiff base (PSB). Absorbing a photon, retinal undergoes cis-to-trans isomerization to trigger a visual signal. To serve as a dim-light photoreceptor, rhodopsin was evolutionarily optimized for high sensitivity. The optimization required minimization of dark noise, which originates from thermal isomerization without involving a photon. Thus, characterization of rhodopsin's thermal reactions is key to molecular function and mechanism of dim-light vision. We studied rhodopsin's thermal reactions and found that they include both thermal isomerization and hydrolysis of the PSB. Both reactions have unusually high activation energies and pre-exponential factors, which cannot be explained using transition-state theory and collision theory. We propose that these unusual kinetic parameters are related to an extensive hydrogen-bonding network in the transmembrane region, which stabilizes rhodopsin for dim-light vision.

Thermal stability of rhodopsin is key to vertebrate dim-light vision

244th National Meeting (2012)

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