The fully oxidized form of the cytochrome bd quinol oxidase from E. coli does not participate in the catalytic cycle: Direct evidence from rapid kinetics studies

Cytochrome bd catalyzes the two-electron oxidation of either ubiquinol or menaquinol and the four-electron reduction of O₂ to H₂O. In the current work, the rates of reduction of the fully oxidized and oxoferryl forms of the enzyme by the 2-electron donor ubiquinol-1 and single electron donor N,N,N′,N′-tetramethyl-p-phenylendiamine (TMPD) have been examined by stopped-flow techniques. Reduction of the all-ferric form of the enzyme is 1000-fold slower than required for a step in the catalytic cycle, whereas the observed rates of reduction of the oxoferryl and singly-reduced forms of the cytochrome are consistent with the catalytic turnover. The data support models of the catalytic cycle which do not include the fully oxidized form of the enzyme as an intermediate.