In most cells, at least 90% of the molecular oxygen consumed is used in oxidative phosphorylation. The remaining O2 is used in a wide variety of specialized metabolic reactions. At least 200 known enzymes use O2 as a substrate. Because O2 is rather unreactive, virtually all of these 200 enzymes use a metal ion to enhance the reactivity of oxygen, just as cytochrome oxidase does.

Oxidases - Oxidases are enzymes that catalyze the oxidation of a substrate without incorporating oxygen into the product. A two-electron oxidation is usually involved, so the oxygen is converted to H2O2. Most oxidases utilize either a metal or a flavin coenzyme. D-amino acid oxidases catalyze the following reaction:

Oxygenases - Oxygenases are enzymes that incorporate oxygen atoms from O2 into the oxidized products. Dioxygenases are uncommon enzymes that incorporate both atoms of O2 into one substrate. An example is tryptophan 2,3-dioxygenase, which catalyzes the reaction below:

Monooxygenases are much more common than dioxygenases. They incorporate one atom from O2 into a product and reduce the other atom to water. A monooxygenase has one substrate that accepts oxygen and another that furnishes the two H atoms that reduce the other oxygen to water. Because two substrates are oxidized, enzymes of this type are also called mixed-function oxidases. The general reaction catalyzed by monooxygenases is the following:

AH + BH2 +O2 <=> A-OH + B + H2O

The substrate AH usually becomes hydroxylated by this class of ezymes, so they are also called hydroxylases. For example, this type of enzyme is used to hydroxylate steroids.

See also: Cytochrome P450, Reactive Oxygen, Oxygen Metabolism and Human Disease