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Francis Millett

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.

Biography

Biography: Francis Millett

Abstract

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.    

References:

  1. Janzon, Julia; Yuan, Quan; Malatesta, Francesco; Hellwig, Petra; Ludwig, Bernd; Durham, Bill; Millett, Francis.  “Probing the Paracoccus denitrificans cytochrome c1-Cytochrome  c552 interaction by mutagenesis and fast kinetics,” Biochemistry (2008) 47(49), 12974-12984.
  2. Mills, D.Z.; Xu, Shujuan; Geren, L.; Hiser, C.; Qin, L.; Sharpe, M.A.; McCracken, J.; Durham, B.; Millett, F.; Ferguson-Miller, S.  Proton-Dependent Electron Transfer from CuA to Heme a and Altered EPR Spectra in Mutants Close to Heme a of Cytochrome Oxidase.  Biochemistry (2008), 47(44), 11499-11509.
  3. Castellani, M.; Havens, J.; Kleinschroth, T.; Millett, F.; Durham, B.; Malatesta, F.; Ludwig, B.  “The acidic domain of cytochrome c(1) in Paracoccus denitrificans, analogous to the acidic subunits in eukaryotic bc(1) conmplexes, is not involved in the electron transfer reaction to its native substrate cytochrome c(552).  Biochim Biophys Acta (2011), 1807:1383-1389
  4.  Havens, J.; Castellani, M.; Kleinschroth, T.; Ludwig, B.; Durham, B.; Millett, F.  Photoinitiated electron transfer within the paracoccus denitrificans cytochrome bc(1) complex:  Mobility of the iron-sulfur protein is modulated by the occupant of the Q(o) site.  Biochemistry (2011) Dec 6;50(48):10462-10472. 
  5.  Durham B, Millett F.  “Design of photoactive ruthenium complexes to study electron transfer and proton pumping in cytochrome oxidase.”   Biochim Biophys Acta. (2012) Apr;1817(4):567-74.