ИСТИНА

Cytochrome bd oxidase from Escherichia coli couples the transfer of electrons from quinol to O2 to membrane potential generation. The enzyme comprises hemes b558 and b595 and heme d, where oxygen chemistry occurs via sequential formation of a few catalytic intermediates. The steady-state behavior of the isolated cytochrome bd has been examined by stopped-flow multiwavelength absorption spectroscopy. Under turnover conditions sustained by O2 and dithiothreitol-reduced ubiquinone, we found that the mostly populated catalytic intermediates are ferryl and oxy-ferrous species, with a minor fraction of the oxidase containing ferric heme d and possibly one electron on heme b558. These novel findings clearly differ from those obtained with mammalian cytochrome c oxidase, in which oxygen intermediates were not found to be populated at detectable levels under similar conditions. The results are discussed in the light of previously proposed models of the cytochrome bd catalytic cycle.