monooxygenases

Monooxygenases are enzymes that catalyze the incorporation of a single atom of molecular oxygen into an organic substrate while the remaining oxygen atom is reduced to water. They are a subset of oxidoreductases that use reducing equivalents such as NADPH or NADH to drive oxygen activation. In contrast to dioxygenases, which insert both atoms of O2 into substrates, monooxygenases introduce only one oxygen atom into the substrate and release the other as water.

The major families include heme-thiolate cytochrome P450 monooxygenases, flavin-containing monooxygenases (FMOs), and copper-dependent monooxygenases. Cytochrome P450 enzymes are a large and diverse superfamily that oxidize aliphatic and aromatic compounds, enabling detoxification, metabolism of pharmaceuticals, and biosynthesis of natural products. Flavin-containing monooxygenases use a flavin cofactor that is reduced by NADPH and forms a C4a-hydroperoxyflavin intermediate that delivers an oxygen atom to substrates such as amines, sulfurs, and heteroatoms. Copper-dependent monooxygenases, including methane monooxygenase and ammonia monooxygenase, activate O2 at copper centers to hydroxylate hydrocarbons or oxidize ammonia, respectively. Some monooxygenases act on specific substrates like p-hydroxybenzoate hydroxylase, a flavin-dependent enzyme.

Most monooxygenases follow a general stoichiometry RH + O2 + reduced cofactor + H+ → ROH + H2O + oxidized cofactor. Mechanistically, the active oxidant is a high-valent metal-oxo species, such as Compound I in P450s or a hydroperoxyflavin in FMOs.

They are found across life and are central to drug metabolism, pollutant degradation, and biosynthetic pathways, making them valuable in biocatalysis and biotechnology.