Haloperoxidases have been the only halogenating enzymes know for more than 35 years. They produce in general free hypohalous acid which acts as the halogenating agent and leads thus to a product formation almost identical to chemical halogenation reactions using electrophilic halogen species. With the detection of FADH2-dependent halogenases a type of enzymes was found that shows high substrate specificity and catalyzes regioselective halogenation reactions. FADH2-dependent halogenases are involved in many biosynthetic pathways and catalyze the halogenation of aromatic and aliphatic substrates activated for electrophilic attack. These enzymes also produce hypohalous acids. However, in contrast to haloperoxidases, the hypohalous acid generated cannot leave the active site but can only react with substrates at the active site resulting in selective halogenation reactions. FADH2-dependent halogenases are a twocomponent system with a flavin reductase as the second component. They show similarity to flavin-dependent monooxygenases in forming an enzyme-bound flavin hydroperoxide intermediate. This flavin hydroperoxide reacts with a halide to form hypohalous acid which may then react in a selective reaction with the organic substrate. For halogenation of unactivated carbon atoms, non-heme iron, α- ketoglutarate- and O2-dependent halogenases use a radical mechanism. A substrate radical intermediate which is formed by abstraction of a H. abstracts a halide radical from the non-heme iron coordination complex resulting in the formation of a halogenated methyl group. While in vitro application of both types of halogenases is problematic, due to issues with cofactor recycling, in vivo use which overcomes this problem seems to have a very high potential.
Keywords: Haloperoxidase, FADH2-dependent, Halogenase, Non-heme iron, Radical mechanism, Hypohalous acid, Regioselectivity