In enzymology, a malate oxidase () is an enzyme that catalyzes the chemical reaction (S)-malate + O2 oxaloacetate + H2O2 Thus, the two substrates of this enzyme are (S)-malate and O2, whereas its two products are oxaloacetate and H2O2. This enzyme belongs to the family of oxidoreductases, specifically those acting on the CH-OH group of donor with oxygen as acceptor. The systematic name of this enzyme class is (S)-malate:oxygen oxidoreductase. Other names in common use include FAD-dependent malate oxidase, malic oxidase, and malic dehydrogenase II. This enzyme participates in pyruvate metabolism. It employs one cofactor, FAD. The enzyme is commonly localized on the inner surface of the cytoplasmic membrane although another family member (malate dehydrogenase 2 (NAD)) is found in the mitochondrial matrix. Mechanisms Malate oxidase belongs to the family of malate dehydrogenases (EC 1.1.1.37) (MDH) that reversibly catalyze the oxidation of malate to oxaloacetate by means of the reduction of a cofactor. The most common isozymes of malate dehydrogenase use NAD+ or NADP+ as a cofactor to accept electrons and protons. However, the main difference of malate oxidase is that it normally employs FAD as redox partner as alternative. Contrary to pyridine based NAD+/NADP+, FAD comprises a quinone moiety, which is reduced by the forward reaction. FAD is thereby converted to FADH2. In this case, malate oxidase is qualified as malate dehydrogenase (quinone). In mutant strains of Escherichia coli lacking the activity of NAD-dependent malate dehydrogenase, malate oxidase is expressed. It is suggested that products of malate dehydrogenase could be responsible for repression of malate oxidase. This would confirm the existence of a family of structurally different malate dehydrogenases. Malate oxidase is induced only in cells, which completely lack the activity of NAD-specific malate dehydrogenase. Irradiation of cytoplasm membranes of Mycobacterium smegmatis with ultraviolet light (360 nm) for 10 minutes resulted in about a 50% loss of malate oxidase activity. The addition of vitamin K, containing a functional naphthoquinone ring, restores the oxidation activity of malate oxidase. The quinone functionality of vitamin K can hence act as an alternative for FAD. Biological Reference However, instead of using NAD+, NADP+ or FAD as cofactors, malate oxidase can also shift to oxygen as oxidant and proton acceptor. (S)-malate + O2 ⇌ oxaloacetate + H2O2 Although seemingly unlikely because of its reactive oxidative character, hydrogen peroxide is found in biological systems including the human body. It signals oxidative stress from wounds to