Identification of the proton pathway in bacterial reaction centers: Inhibition of proton transfer by binding of Zn 21 or Cd 21 (bacterial photosynthesisyRhodobacter sphaeroidesymetal bindingyproton-coupled electron transfer)

The reaction center (RC) from Rhodobacter sphaeroides converts light into chemical energy through the light induced two-electron, two-proton reduction of a bound quinone molecule QB (the secondary quinone acceptor). A unique pathway for proton transfer to the QB site had so far not been determined. To study the molecular basis for proton transfer, we investigated the effects of exogenous metal ion binding on the kinetics of the proton-assisted electron trans- fer kAB (2) (QA 2• QB 2• 1 H 1 3 QA(QBH) 2 , where QA is the primary quinone acceptor). Zn 21 and Cd 21 bound stoichio- metrically to the RC (KD 10 2 -fold) and has become the rate-limiting step. The lack of an effect of the metal binding on the charge recombination reaction D 1• QAQB 2• 3 DQAQB suggests that the binding site is located far (>10 A) from QB. This hypoth- esis is confirmed by preliminary x-ray structure analysis. The large change in the rate of proton transfer caused by the stoichiometric binding of the metal ion shows that there is one dominant site of proton entry into the RC from which proton transfer to QB 2• occurs.