How nature reduces molecular oxygen to water conserving energy at the same time

Molecular oxygen appeared in the atmosphere about three billion years ago. Nature developed two membrane integrated enzymatic systems which reduce oxygen to water and use the energy of this reaction to produce biologically important energy carriers. These enzymes are the haem-copper terminal oxidases, e.g. cytochrome c oxidase, and the bd oxidases. The atomic structures of representative members of both enzyme families were determined. These evolutionary unrelated enzymes apparently use the same mechanisms to conserve energy and to prevent the formation of toxic reactive oxygen species.