The bacterial reaction center couples light-induced electron transfer to proton pumping across the membrane by reactions of a quinone molecule Q(B) that binds two electrons and two protons at the active site. This article reviews recent experimental work on the mechanism of the proton-coupled electron transfer and the pathways for proton transfer to the Q(B) site. The mechanism of the first electron transfer, k((1))(AB), Q(-)(A)Q(B)-->Q(A)Q(-)(B), was shown to be rate limited by conformational gating. The mechanism of the second electron transfer, k((2))(AB), was shown to involve rapid reversible proton transfer to the semiquinone followed by rate-limiting electron transfer, H(+)+Q(-)(A)Q(-)(B) ifQ(-)(A)Q(B)H-->Q(A)(Q(B)H)(-). The pathways for transfer of the first and second protons were elucidated by high-resolution X-ray crystallography as well as kinetic studies showing changes in the rate of proton transfer due to site directed mutations and metal ion binding.

The mechanism of the electron transfer reaction, Q A ؊ ⅐ Q B 3 Q A Q B ؊ ⅐ , was studied in isolated reaction centers from the photosynthetic bacterium Rhodobacter sphaeroides by replacing the native Q 10 in the Q A binding site with quinones having different redox potentials. These substitutions are expected to change the intrinsic electron transfer rate by changing the redox free energy (i.e., driving force) for electron transfer without affecting other events that may be associated with the electron transfer (e.g., protein dynamics or protonation). The electron transfer from Q A ؊ ⅐ to Q B was measured by three independent methods: a functional assay involving cytochrome c 2 to measure the rate of Q A ؊ ⅐ oxidation, optical kinetic spectroscopy to measure changes in semiquinone absorption, and kinetic near-IR spectroscopy to measure electrochromic shifts that occur in response to electron transfer. The results show that the rate of the observed electron transfer from Q A ؊ ⅐ to Q B does not change as the redox free energy for electron transfer is varied over a range of 150 meV. The strong temperature dependence of the observed rate rules out the possibility that the reaction is activationless. We conclude, therefore, that the independence of the observed rate on the driving force for electron transfer is due to conformational gating, that is, the rate limiting step is a conformational change required before electron transfer. This change is proposed to be the movement, controlled kinetically either by protein dynamics or intermolecular interactions, of Q B by Ϸ5 Å as observed in the x-ray studies of Stowell et al.

The mechanism of the proton-coupled electron transfer reaction, Q A -Q B -

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 Q B (the secondary quinone acceptor). A unique pathway for proton transfer to the Q B 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-