Oxygenases and oxidases belong to the E.C. 1 class of enzymes. Oxygenases catalyze the incorporation of oxygen into a substrate. Dioxygenases incorporate both oxygen atoms into the substrate, whilst monooxygenases incorporate a single oxygen atom as a hydroxyl group into the substrate and the second oxygen atom is reduced to water. Oxidases catalyze the transfer of two electrons from a donor to oxygen, producing hydrogen peroxide. Usually a flavin co-enzyme or metal co-factor is required.