Oxygenases are enzymes, which are involved in the transfer of oxygen molecules to a substrate. The reaction oxidises the substrate by transporting oxygen from molecular oxygen (oxygen found in the air) to the substrate. Oxygenases are part of the oxidoreductases family and their EC numbers are EC1.13 or EC1.14. Oxidoreductases facilitate the transfer of an electron from a donor or a reductant to an electron acceptor or oxidant.
Oxygenases comprise constitutive and inducible isozymes HO-1 and HO-2. They are regarded as an important intracellular source of carbon monoxide and iron. Heme oxygenase (HO-1) has a helical structure, which contains glycines in the distal helix that are designed to facilitate substrate binding and increase flexibility.
There are two types of oxygenases; Monooxygenases: these enzymes are also known as mixed function oxidase, and they transfer one atom of oxygen to the substrate, reducing the other oxygen atom to water, and Dioxygenases: also known as oxygen transferases, these enzymes use both molecular oxygen atoms.
Monooxygenases, which are sometimes called hydroxylases, catalyse the transfer of one oxygen atom to a substrate. In this case, the second atom is reduced to water. To facilitate the reaction, a reductant, for example, NADH, must be present. The reductant used determines the group of hydroxylases.
One of the most important monooxygenases is cytochrome P450 oxidase, which is involved in the breakdown of several chemicals within the body. Cytochrome P450, also known as CYPs, are a group of enzymes, which contain heme as a cofactor. In mammals, these enzymes play an integral role in the production and synthesis of hormones and in the oxidation of steroids, fatty acids and xenobiotics (chemicals that are not naturally produced or expected to be found within the organism). In plants, these enzymes are instrumental in the biosynthesis of fatty acids, protective chemicals. Cytochrome P450 enzymes can also metabolise potentially harmful compounds within the liver. Examples include drugs.
Dioxygenases are classed as oxidoreductase enzymes, which oxidise a substrate without reducing one of the oxygen atoms. Unlike monooxygenases, dioxygenases catalyse the reaction without reducing an oxygen atom or producing water. In the majority of cases, dioxygenases incorporate dioxygen into a sole substrate, rather than a number of substrates. Several cofactors can be used to bring about the reaction, but the most common is iron. Tryptophan-2,3-dioxygenase is an example of a dioxygenase. This enzyme is involved in catalysing tryptophan.
Oxygenases are not the same as oxidases. Oxidase is a more general term used to describe enzymes that catalyse the transfer of an electron to a substrate. Oxygenases are a smaller, more specific group of enzymes that take oxygen atoms and transfer them to a substrate.