The different roles of ubiquinone-10 (UQ10) at the primary and secondary quinone (QAand QB) binding sites ofRho-dobacter sphaeroidesR26 reaction centres are governed by the protein microenvironment. The 4C¼O carbonyl group of QAis unusually strongly hydrogen-bonded, in contrast to QB. This asymmetric binding seems to determine their dif-ferent functions. The asymmetric hydrogen-bonding at QA can be caused intrinsically by distortion of the methoxy groups or extrinsically by binding to specific amino-acid side groups. .
