We have used fluorescence quench titrations, EPR spectroscopy and steady-state kinetics to study the effects of site-directed mutants of FrdB, FrdC and FrdD on the proximal menaquinol (MQH2) binding site (QP)of Escherichia colifumarate reductase (FrdABCD) in cytoplasmic membrane preparations. Fluorescence quench (FQ) titrations with the fluorophore and MQH2 analog 2-n-heptyl-4-hydroxyquinoline-N-oxide (HOQNO) indi-cate that the QPsite is defined by residues from FrdB, FrdC and FrdD. In FQ titrations, wild-type FrdABCD binds HOQNO with an apparentKdof nm, and the following mutations significantly increase this value: FrdB-T205H (Kd¼39 nm);.
