Structure and evolution of cytochrome oxidase

The structural features of cytochrome oxidases are reviewed in light of their evolution. The substrate specificity (quinol vs. cytochromec) is reflected in the presence of a unique copper centre (Cu _A ) in cytochromec oxidases. In several lines of evolution, quinol oxidases have independently lost this copper. Also, the most primitive cytochromec oxidases do not contain this copper, and electron entry takes place viac-type haems. These enzymes, exemplified by the rhizobial FixN complex, probably remind the first oxidases. They are related to the denitrification enzyme nitric oxide reductase.