Proton and electron transfer in bacterial reaction centers

The bacterial reaction center couples light-induced electron transfer to pr oton pumping across the membrane by reactions of a quinone molecule Q(B) th at binds two electrons and two protons at the active site. This article rev iews recent experimental work on the mechanism of the proton-coupled electr on transfer and the pathways for proton transfer to the QB site. The mechan ism of the first electron transfer, k(AB)((1)), Q(A)(-)Q(B) --> Q(A)Q(B)(-) , was shown to be rate limited by conformational gating. The mechanism of t he second electron transfer, k(AB)((2)), was shown to involve rapid reversi ble proton transfer to the semiquinone followed by rate-limiting electron t ransfer, H++Q(A)(-)Q(B)(-) double left right arrow Q(A)Q(B)H --> Q(A)(Q(B)H )(-). The pathways for transfer of the first and second protons were elucid ated by high-resolution X-ray crystallography as well as kinetic studies sh owing changes in the rate of proton transfer due to site directed mutations and metal ion binding. (C) 2000 Elsevier Science B.V. All rights reserved.