Modulation of Energy Conversion Through Manipulation of the Retinal Thermal Equilibrium by an Aromatic Residue in the Seven-Transmembrane Receptor Bacteriorhodopsin

Light-driven proton pumps, a number of the microbial rhodopsin family seven-transmembrane receptors, distribute widely among Archaea, Eubacteria, and Eukaryota for harvesting and converting light energy in a wider spectral range. The common feature of those proteins is that the retinal chromophore maintains a cis-trans thermal equilibrium in the dark-adapted state. Absorption of a photon causes photo isomerization of the chromophore from the all-trans to the 13-cis, 15-anti configuration and triggers a series of structural rearrangements in the protein that initiates the vectorial translocation of a proton out of the cell. However, how the cis-trans thermal equilibrium impacts on the proton translocation photocycle, and further impacts on the ATP formation is still poorly understood. Here, as a novel study to address this challenge, Y185F mutant, an aromatic residue at the retinal binding pocket of bacteriorhodopsin, is identified to shift the cis-trans isomerization thermal equilibrium to a cis dominated state and causes a weakening of the M state, loss of the O state and elongation of the proton pumping cycle, and further decreases the ATP formation rate. Different interactions of Tyrosine 185 with the retinal chromophore are observed in the dark-adapted state and the M state, indicating that Tyrosine 185 may serve as a rotamer switch to maintain the proton translocation kinetics and further the energy conversion in bacteriorhodopsin.Reference: Ding, X., et al. 2016. Mediation mechanism of tyrosine 185 on the retinal isomerization equilibrium and the proton release channel in the seven-transmembrane receptor bacteriorhodopsin. Biochim. Biophys. Acta 1857:1786-1795.