Each inhibitor alone allows reduced b H and b L to accumulate, indicating that each inhibits the reoxidation of these hemes. These inhibitions are also incomplete, and reduced b L is oxidized through the leaks allowed by these inhibitors at least 10 times faster than reduced b H . Energization of SMP by the addition of ATP affords reduction of b L as well. Reverse electron transfer to b H and b L is inhibited partially by myxothiazol, much more by antimycin. These reagents also partially reduce cytochrome b in SMP from a ubiquinone-deficient yeast mutant.
Dashed arrows are hypothetical proton-translocation steps.
Our interest in this enzyme is related mainly to its mechanism of energy transduction and proton translocation, for the following reasons. In reverse transhydrogenation, the enzyme utilizes substrate binding energy for proton pumping. Therefore, with regard to the mechanism of energy transduction, the transhydrogenase works according to the same principles as the ATP synthase complex of mitochondria and bacteria, the proton and cation ATPases, and possibly certain redox-linked proton pumps. However, the relatively simple structure of the transhydrogenase recommends it as a model for study of the utilization of binding energy for vectorial translocation of protons and other cations. Our recent studies have shown the following.