Proton pumping bo3 oxidase from Vitreoscilla

Membrane bound heme-copper oxidases reduce oxygen to water, using the released energy for vectorial proton transport across the membrane. Both reactions contribute to the generation of the proton motive force by electrogenic proton and electron transfer reactions across the membrane. These reactions are tightly coupled and precisely orchestrated to ensure energy conservation and suppression of Reactive Oxygen Species (ROS) production. Experimental discrimination between protons required for oxygen reduction and pumped protons is typically not possible. Therefore, a terminal oxidase pumping a different ion than protons would provide an elegant system to study mechanistic aspects of these intricate pumps. The group of Webster described a sodium pumping quinol oxidase in the bacterium Vitreoscilla, but no further studies have been described with this enzyme.
Our aim was thus to isolate, purify and analyse the properties of the Vitriocilla quinol oxidase to see, if it could be used a model system to elucidate the pumping mechanism using single turnover measurements with the flow-flash technique. We successfully cloned, heterologously expressed and purified the enzyme in E. coli. To our disappointment, we could neither observe sodium dependency of the enzymatic activity nor sodium pumping of the protein in inverted membrane vesicles, purified protein in detergent, or the enzyme reconstituted into proteoliposomes. Instead, we conducted several experiments that directly show proton pumping by the Vitreoscilla cytochrome bo, as compared with the related enzyme of E. coli.