In biochemistry, an oxidase is an oxidoreductase (any enzyme that catalyzes a redox reaction) that uses dioxygen (O<sub>2</sub>) as the electron acceptor. In reactions involving donation of a hydrogen atom, oxygen is reduced to water (H<sub>2</sub>O) or hydrogen peroxide (H<sub>2</sub>O<sub>2</sub>). Some oxidation reactions, such as those involving monoamine oxidase or xanthine oxidase, typically do not involve free molecular oxygen.
The oxidases are a subclass of the oxidoreductases. The use of dioxygen is the only unifying feature; in the EC classification, these enzymes are scattered in many categories.
An important example is EC 7.1.1.9 cytochrome c oxidase, the key enzyme that allows the body to employ oxygen in the generation of energy and the final component of the electron transfer chain. Other examples are:
In microbiology, the oxidase test is used as a phenotypic characteristic for the identification of bacterial strains; it determines whether a given bacterium produces cytochrome oxidases (and therefore utilizes oxygen with an electron transfer chain).
The test is used to determine whether a bacterium is an aerobe or anaerobe. However a bacterium that is oxidase negative is not necessarily anaerobic, it may just indicate the bacterium does not possess cytochrome c oxidase.