Identification of the proton pathway in bacterial reaction centers: decrease of proton transfer rate by mutation of surface histidines at H126 and H128 and chemical rescue by imidazole identifies the initial proton donors.

The pathway for proton transfer to QB was studied in the reaction center (RC) from Rhodobacter sphaeroides. The binding of Zn2+ or Cd2+ to the RC surface at His-H126, His-H128, and Asp-H124 inhibits the rate of proton transfer to QB, suggesting that the His may be important for proton transfer [Paddock, M. L., Graige, M. S., Feher, G. and Okamura, M. Y. (1999) Proc. Natl. Acad. Sci. U.S.A. 96, 6183−6188]. To assess directly the role of the histidines, mutant RCs were constructed in which either one or both His were replaced with Ala. In the single His mutant RCs, no significant effects were observed. In contrast, in the double mutant RC at pH 8.5, the observed rates of proton uptake associated with both the first and the second proton-coupled electron-transfer reactions kAB(1) [QA-•QBGlu- + H+ → QA-•QBGluH → QAQB-•GluH] and kAB(2) [QA-•QB-• + H+ → QA-•(QBH)• → QA(QBH)-], were found to be slowed by factors of ∼10 and ∼4, respectively. Evidence that the observed changes in the double mutant RC are due to a ...