... Q225 and K228 from FrdB; R28, E29, W86, L89 and A93 fromFrdC; and W14, F17, G18, H80, R81 and H84 fromFrdD [3,4,10]. Site-directed mutants of some of these res-idues have been generated and ... HOQNO binding measuredby both the FQ and EPR methods (Figs 2 and 3).Ha¨gerha¨ll and coworkers [20] demonstrated by potenti-ometric titration and EPR spectroscopy that a mena-semiquinone ... panels, FrdB and FrdA are above the MQ ⁄ HOQNO plane, and FrdC and FrdD are substantially below theMQ ⁄ HOQNO plane. Residues from FrdB,FrdC and FrdD have labels starting with ‘B-’,‘C-’, and ‘D-’,...