Proton and electron transfer in bacterial reaction centers

Abstract

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 − AQ B→Q AQ − 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 − AQ − B⇔Q − AQ BH→Q A(Q BH) −. 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.