University of Arkansas, USA
Title: Photoinduced electron transfer in cytochrome bc1: Kinetics of ubiquinone transfer from the Qo site to the Qi site, and evidence for communication between the monomers in the dimer.
Francis Millett received his B.S in Chemistry from the University of Wisconsin in 1965, his Ph.D. in Chemical Physics from Columbia University in 1970, and was an NIH Postdoctoral Fellow at California Institute of Technology from 1970-1972. He joined the faculty of the University of Arkansas in 1972, and is now a Distinguished Professor. He developed, together with Bill Durham, the ruthenium photoreduction method which made it possible to measure the kinetics of key steps in electron transfer during mitochondrial oxidative phosphorylation. He has directed collaborative, multidisciplinary research which combines rapid kinetics methods, site-directed mutagenesis, X-ray crystallography, and NMR to investigate protein structure-function relationships.
The electron transfer reactions within wild-type Rhodobacter sphaeroides cytochrome bc1 (cyt bc1) were studied using a ruthenium dimer to rapidly photooxidize cyt c1. It was found that when cyt bH was initially reduced before the reaction, photooxidation of cyt c1 led to bifurcated reduction of both the iron-sulfur protein and cyt bL by QH2 in the Qo site, followed by re-oxidation of two equivalents of cyt bL and cyt bH. It was proposed that the newly formed ubiquinone diffused through the hydrophobic cavity linking the Qo site of the reactive monomer A to the Qi site of the other monomer B, leading to oxidation of cyt bH in monomer B followed by oxidation of cyt bL in monomer A by cross-monomer electron transfer. Addition of one equivalent of the Qi site inhibitor antimycin to the cyt bc1 dimer had very little effect on any of the electron transfer reactions, while addition of a second equivalent completely inhibited re-oxidation of cyt bL and cyt bH. It was also found that addition of one equivalent of the Qo site inhibitor stigmatellin to the cyt bc1 dimer completely inhibited all electron transfer reactions in both monomers of the dimer. These experiments are consistent with a half-of-the-sites mechanism in which only one monomer of the dimer is active at a time, implying monomer-monomer interactions. The rapid electron transfer reaction from the ISP to cyt c1 was found to be greatly decreased by viscosity, indicating a multi-step diffusional mechanism as the iron-sulfur protein rotates from the b state to the c1 state.