Flavin-Mediated Hydroxylation Reactions

Flavins react with oxygen and can form stable flavin peroxides in an aprotic solvent or buried in a protein. It is this hydroperoxide or peroxide that is the oxygenating agent in flavoproteins. This property is used in nature to carry out aromatic hydroxylations, halogenations, Baeyer-Villiger oxygenations, hydroxylation of xenobiotics and some metabolites, as well as light emission from luciferase. Several groups of enzymes seem to have evolved hydroxylating properties independently of each other. One group consists of the two-component flavin-dependent hydroxylases that sue many of the same principles as the single component hydroxylases, although they also have some special requirements. After a brief introduction to the reactivity of flavins with oxygen, we examine p-hydroxybenzoate hydroxylase as the paradigm for the chemistry and protein functions exhibited by these enzymes. We then discuss unique features of each group of enzymes and the exciting prospects for future research.