Crystallographic Studies of Quinol Oxidation Site Inhibitors: A Modified Classification of Inhibitors for the Cytochrome bc1 Complex

Cytochrome bc1 is an integral membrane protein complex essential for cellular respiration and photosynthesis; it couples electron transfer from quinol to cytochrome c to proton translocation across the membrane. Specific bc1 inhibitors have not only played crucial roles in elucidating the mechanism of bc1 function but have also provided leads for the development of novel antibiotics. Crystal structures of bovine bc1 in complex with the specific Qo site inhibitors azoxystrobin, MOAS, myxothiazol, stigmatellin and 5-undecyl-6-hydroxy-4,7-dioxobenzothiazole were determined. Interactions, conformational changes and possible mechanisms of resistance, specific to each inhibitor, were defined. Residues and secondary structure elements that are capable of discriminating different classes of Qo site inhibitors were identified for the cytochrome b subunit. Directions in the displacement of the cd1 helix of cytochrome b subunit in response to various Qo site inhibitors were correlated to the binary conformational switch of the extrinsic domain of the iron–sulfur protein subunit. The new structural information, together with structures previously determined, provide a basis that, combined with biophysical and mutational data, suggest a modification to the existing classification of bc1 inhibitors. bc1 inhibitors are grouped into three classes: class P inhibitors bind to the Qo site, class N inhibitors bind to the Qi site and the class PN inhibitors target both sites. Class P contains two subgroups, Pm and Pf, that are distinct by their ability to induce mobile or fixed conformation of iron–sulfur protein.