Identificationof Ubiquinol Binding Motifs at theQo-Site of the Cytochrome bc1Complex.

Enzymes of the bc1complex family powerthe biosphere through their central role in respiration and photosynthesis.These enzymes couple the oxidation of quinol molecules by cytochrome cto the transfer of protons across the membrane, to generatea proton-motive force that drives ATP synthesis. Key for the functionof the bc1complex is the initial redoxprocess that involves a bifurcated electron transfer in which thetwo electrons from a quinol substrate are passed to different electronacceptors in the bc1complex. The electrontransfer is coupled to proton transfer. The overall mechanism of quinoloxidation by the bc1complex is well enoughcharacterized to allow exploration at the atomistic level, but detailsare still highly controversial. The controversy stems from the uncertainbinding motifs of quinol at the so-called Qoactive site of the bc1complex.Here we employ a combination of classical all atom molecular dynamicsand quantum chemical calculations to reveal the binding modes of quinolat the Qo-site of the bc1complex from Rhodobacter capsulatus. The calculations suggest a novel configuration of amino acid residuesresponsible for quinol binding and support a mechanism for proton-coupledelectron transfer from quinol to iron–sulfur cluster througha bridging hydrogen bond from histidine that stabilizes the reactioncomplex. [ABSTRACT FROM AUTHOR]