Oxidases and Oxygenases

In most cells, at least 90% of the molecular oxygen consumed is used in oxidative phosphorylation. The remaining O 2 is used in a wide variety of specialized metabolic reactions. At least 200 known enzymes use O 2 as a substrate. Because O 2 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 H 2O 2. 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 O 2 into the oxidized products. Dioxygenases are uncommon enzymes that incorporate both atoms of O 2 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 O 2 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 + BH 2 +O 2 <=> A-OH + B + H 2O

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