PARENT SESSION

Symposium S1A Proton-coupled electon transport and ATPase
Monday August 30th, 2004 10:20 AM-12:20 PM Room 511D
Chair: Colin Wraight
Co-Chair: Wolfgang Junge

Proton-coupled electron transfer during hydroquinone oxidation in bc complexes and in a biomimetic system. David Kramer¹, Jonathon Cape^*,1, Michael Bowman³, ¹ Institute of Biological Chemistry, Pullman, WA, USA³ WR Wiley Environmental Molecular Sciences Laboratory, Richland, WA, USA

ABSTRACT- While the structure and overall kinetics of cyt bc₁ and b₆f complexes are well understood, there are key open questions about the basic chemistry of ubiquinol oxidation at its Q_o site, which is thought to involve at least one critical proton coupled electron transfer (PCET) reaction. One difficulty in studying the Q_o site is that the intermediates of the reaction have not been identified, leading to many competing models. Here we report on a biomimetic system in which quinols are H-bonded to a ruthenium benzimidazolate complex in organic solvents to mimic the pre-catalytic state in the enzyme, and laser pulses initiate the oxidative process. The driving force and other properties of the system are readily and systematically varied to test models for PCET reactions, and because measurements can be made under truly pre-steady-state conditions, chemical intermediates can be identified. We observed that the oxidation of quinol occurs via a n=1 process, resulting in the formation of a transient, EPR-detectable neutral semiquinone species. In the absence of secondary bases, the quinol OH proton is accepted by the benzimidazolate ligand upon oxidation. These results are consistent with a PCET reaction in the biomimetic system. Thermal activation of the reaction with ubiquinol, but not other quinols, yields E_act(H) > E_act(D), a highly unusual situation that is mirrored by measurements in the intact enzyme during steady state oxidation of ubiquinol. Further comparisons of the driving force and isotope dependencies of normal and side reactions lead us to conclude that similar reactions occur in both the native (yeast cyt bc1 complex) and biomemtic systems, and we conclude that exotic simultaneous double electron transfer reactions, as have recently been proposed for the cyt bc1 complex, are very unlikely. In the biomimetic system, adding triethylamine (TEA) as an exogenous base results in the formation of a semiquinone anion, i.e. the second OH proton is accepted by TEA, and we suggest a model where a similar reaction gates electron transfer within the Q_o pocket.

KEY WORDS: Q-cycle, b₆f, proton coupled electron transfer, bc1 complex