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The cytochrome  bo  3  quinol oxidase from  Vitreoscilla  ( vbo  3 ) catalyses oxidation of ubiquinol and reduction of O 2  to H 2 O. Data from earlier studies suggested that the free energy released in this reaction is used to pump sodium ions instead of protons across a membrane. Here, we have studied the functional properties of heterologously expressed  vbo  3  with a variety of methods. (i) Following oxygen consumption with a Clark-type electrode, we did not observe a measurable effect of Na +  on the oxidase activity of purified  vbo  3  solubilized in detergent or reconstituted in liposomes. (ii) Using fluorescent dyes, we find that  vbo  3  does not pump Na +  ions, but H +  across the membrane, and that H + -pumping is not influenced by the presence of Na + . (iii) Using an oxygen pulse method, it was found that 2 H + /e −  are ejected from proteoliposomes, in agreement with the values found for the H + -pumping  bo  3  oxidase of  Escherichia coli  ( ecbo  3 ). This coincides with the interpretation that 1 H + /e −  is pumped across the membrane and 1 H + /e −  is released during quinol oxidation. (iv) When the electron transfer kinetics of  vbo  3  upon reaction with oxygen were followed in single turnover experiments, a similar sequence of reaction steps was observed as reported for the  E. coli  enzyme and none of these reactions was notably affected by the presence of Na + . Overall the data show that  vbo  3  is a proton pumping terminal oxidase, behaving similarly to the  Escherichia coli bo  3  quinol oxidase.

The proton pumping bo oxidase from Vitreoscilla