Nonoxidizable ubiquinol derivatives that the suitable for the study of the ubiquinol oxidation site in the cytochrome bc_1 complex

摘要

Recent X-ray crystallographic analyses of the mitochondrial cytochrome bc_1 complex show ubiquinone binding at the Q_i site, but attempts to show binding of ubiquinol or ubiquinone at the Q_o site have been unsuccessful, even though the binding of noncompetitive Q_o site inhibitors near the putative ubiquinol binding pocket is well established. We speculate that ubiquinol binds transiently to the Q_o site only when both heme b_L and the iron sulfur cluster are in the oxidized form, an experimental condition difficult to obtain since ubiquinol will be oxidized once bound to the site. Stable binding at the Q_o site might be achieved by a nonoxidizable ubiquinol-like compound. For this purpose, the isomers 2,3,4-trimethoxy-5-decyl-6-methyl-phenol (TMDMP) and 2,3,4-trimethoxy-5-methyl-6-decyl-phenol (TMMDP) were synthesized from 2,3-dimethoxy-5-methyl-6-decyl-1,4-benzoquinol (Q_0C_(10)) by controlled methylation and separated by TLC and HPLC. The structures of TMDMP and TMMDP were established by ~1H-~(13)C-two-dimensional NMR. Both are competitive inhibitors of the cytochrome bc_1 complex, with TMDMP being the stronger one. Preliminary results suggest that TMDMP binds tightly enough to make X-ray crystallography of inhibitor-bc_1 complex co-crystals feasible. The binding site of TMDMP does not overlap with the binding sites of stigmatellin, MOA-stilbene (MOAS), undecylhydroxydioxobenzothiazole (UHDBT) and myxothaizol.