Enzymatic chlorination and bromination

Our knowledge about the enzymes catalyzing the incorporation of halide ions during the biosynthesis of halometabolites has increased tremendously during the last 15 years. Between 1960 and 1995, haloperoxidases were the only halogenating enzymes known. However, absolute proof for the connection of haloperoxidases to the biosynthesis of halometabolites is still missing. In 1997, FADH(2)-dependent halogenases were identified as the type of halogenating enzymes responsible for the incorporation of chloride and bromide atoms into aromatic and aliphatic compounds activated for electrophilic attack. FADH(2)-dependent halogenases are two-component systems consisting of a flavin reductase providing the FADH(2) required by the halogenase. Elucidation of the three-dimensional structure of FADH(2)-dependent halogenases led to the understanding of the reaction mechanism, which involves the formation of hypohalous acids. Unactivated carbon atoms were found to be halogenated by nonheme iron, α-ketoglutarate- and O(2)-dependent halogenases. The reaction mechanism of this type of halogenase was shown to involve the formation of a substrate radical. These two types of halogenating enzymes, together with the much less common fluorinases, are the major types of halogenating enzymes. However, the existence of other types of halogenating enzymes, yet not detected, cannot be completely ruled out. Here, we describe the detection, purification, characterization, and reaction mechanisms of flavin-dependent halogenases and of nonheme iron, α-ketoglutarate- and O(2)-dependent halogenases.