Proton-coupled electron transfer at the Q o-site of the bc1 complex controls the rate of ubihydroquinone oxidation

Abstract

The rate-limiting reaction of the bc 1 complex from Rhodobacter sphaeroides is transfer of the first electron from ubihydroquinone (quinol, QH 2) to the [2Fe–2S] cluster of the Rieske iron–sulfur protein (ISP) at the Q o-site. Formation of the ES-complex requires participation of two substrates (S), QH 2 and ISP ox. From the variation of rate with [S], the binding constants for both substrates involved in formation of the complex can be estimated. The configuration of the ES-complex likely involves the dissociated form of the oxidized ISP (ISP ox) docked at the b-interface on cyt b, in a complex in which N ε of His-161 (bovine sequence) forms a H-bond with the quinol OH. A coupled proton and electron transfer occurs along this H-bond. This brief review discusses the information available on the nature of this reaction from kinetic, structural and mutagenesis studies. The rate is much slower than expected from the distance involved, likely because it is controlled by the low probability of finding the proton in the configuration required for electron transfer. A simplified treatment of the activation barrier is developed in terms of a probability function determined by the Brønsted relationship, and a Marcus treatment of the electron transfer step. Incorporation of this relationship into a computer model allows exploration of the energy landscape. A set of parameters including reasonable values for activation energy, reorganization energy, distances between reactants, and driving forces, all consistent with experimental data, explains why the rate is slow, and accounts for the altered kinetics in mutant strains in which the driving force and energy profile are modified by changes in E m and/or p K of ISP or heme b L.