ИСТИНА

A retinal protein from psychrotrophic bacterium Exiguobacterium sibiricum (ESR) functions as a light-driven proton pump. The presence of a lysine instead of usual carboxylic residue at the position corresponding to intramolecular proton donor for the Schiff base is the unique structural feature of ESR. Previously we have studied the photocycle and proton transport in this protein and its mutants and showed that presence of Lys96 greatly accelerates reprotonation of the Schiff base. In this study we examined kinetics of light-induced transmembrane electrical potential difference generation in proteoliposomes reconstituted with ESR, using the direct electrometric method at different pH. The total electrogenic response of ESRis comparable with that produced by bacteriorhodopsin but individual components and their pH dependence are substantially different. The fastest components (~3 and 50 µs) correspond to the Schiff base deprotonation, transfer of a proton to the primary acceptor Asp85 and formation of the M intermediate. The major electrogenic phase related to proton transfer from the bulk to Lys96 and the Schiff base exhibits pH dependence. It occurs with τ=0.5 ms at pH 6.6 and splits into two components, 0.25 and 4 ms, at pH 8.5. The latter correlates with the Schiff base reprotonation, whereas the former was attributed to proton transfer from the bulk to the donor site. The final electrogenic phase with the time constant of 15 ms corresponds to deprotonation of the proton acceptor and recovery of the initial state. At pH 5.0, the amplitude of  decreases 10 fold, reflecting a decreased yield and rate of proton transfer, apparently from protonation of the acceptor in the initial state. The observed features of potential generation correlate with three-dimentional structure and mechanism of proton transfer in ESR.