Hydrogen Bonding to the Substrate Is Not Required for Rieske Iron-Sulfur Protein Docking to the Quinol Oxidation Site of Complex III

Complex III or the cytochrome (cyt) bc₁ complex constitutes an integral part of the respiratory chain of most aerobic organisms and of the photosynthetic apparatus of anoxygenic purple bacteria. The function of cyt bc₁ is to couple the reaction of electron transfer from ubiquinol to cytochrome c to proton pumping across the membrane. Mechanistically, the electron transfer reaction requires docking of its Rieske iron-sulfur protein (ISP) subunit to the quinol oxidation site (Q_P) of the complex. Formation of an H-bond between the ISP and the bound substrate was proposed to mediate the docking. Here we show that the binding of oxazolidinedione-type inhibitors famoxadone, jg144, and fenamidone induces docking of the ISP to the Q_P site in the absence of the H-bond formation both in mitochondrial and bacterial cyt bc₁ complexes, demonstrating that ISP docking is independent of the proposed direct ISP-inhibitor interaction. The binding of oxazolidinedione-type inhibitors to cyt bc₁ of different species reveals a toxophore that appears to interact optimally with residues in the Q_P site. The effect of modifications or additions to the toxophore on the binding to cyt bc₁ from different species could not be predicted from structure-based sequence alignments, as demonstrated by the altered binding mode of famoxadone to bacterial cyt bc₁.