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(-)(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.

Functional role of Coenzyme Q in the energy coupling of NADH‐CoQ oxidoreductase (Complex I): Stabilization of the semiquinone state with the application of inside‐positive membrane potential to proteoliposomes

“…Many of the membrane-bound protein complexes in the respiratory and photosynthetic systems are known to react with quinone. X-ray crystallographic information clearly confirmed the general functional roles of quinones, for example, at the Q o and Q i sites of mitochondrial complex III [4,8] as well as at the Q A and Q B sites of the bacterial photosynthetic reaction center [23,36]. Various research groups detected proteinbound forms of quinone species by their semiquinone signals using EPR spectroscopy.…”

Wide-dynamic-range kinetic investigations of deep proton tunnelling in proteins

“…A better understanding of this phenomenon is sought because it underpins so many fundamental life processes [13][14][15][16][17][18] . One particularly important process involves the delivery of protons to the interior of proteins so that key biochemical reactions can proceed [19][20][21] . In the case of the oxidative phosphorylation complexes, such as cytochrome oxidase, and in the photosynthesis complex, cytochrome b 6 f, vectorial proton pumping is also involved.…”

“…Thus, the experiments presented here, which are the first temperature dependent measurements of tunneling in a biological proton wire and cover a much larger temperature range than previous studies 2,3,5,9 , can help us to better understand this important process. Because the deep tunneling rate is so exquisitely sensitive to tunnel-distance variations, it has the potential to play an important role in controlling transport along protein wires or in enzymatic reactions 5,19,20 .…”

Induced conformational changes upon Cd 2+ binding at photosynthetic reaction centers

“…The coupled ET͞PT reactions involving Q A/B in bRC proceed in two phases, as shown by the following reaction schemes (reviewed in refs. 1 The PT to Glu-L212 near Q B is a prerequisite for the first ET event belonging to kinetic phase 1 (Eq. 1b).…”