Halogenases

Halogenases are enzymes that catalyze the incorporation of halogen atoms—fluorine, chlorine, bromine, or iodine—into organic molecules. They are found in bacteria, fungi, and plants and play important roles in the biosynthesis of natural products and in xenobiotic metabolism. By introducing halogens, halogenases can alter the reactivity, polarity, and biological activity of substrates.

Two major mechanistic classes are commonly recognized. Haloperoxidases, which include heme-containing and vanadium-dependent enzymes, use hydrogen peroxide to generate HOX species from halide ions and effect electrophilic halogenation of substrates. They tend to have broad substrate scope but relatively limited regioselectivity. Classic examples are chloroperoxidase from Caldariomyces fumago and related fungal or marine enzymes.

Flavin-dependent halogenases and related non-heme enzymes catalyze more selective transformations. Flavin-dependent halogenases (FDHs) employ FADH2 and a flavin reductase to generate a locally active hypohalous acid within the enzyme’s active site, allowing site- and substrate-selective halogenation. Tryptophan halogenases such as RebH and PrnA chlorinate tryptophan at defined positions, and several FDHs can accept related substrates or catalyze bromination. Many FDHs function in natural-product biosynthetic gene clusters and require a dedicated reductase.

Fe(II)/α-ketoglutarate-dependent halogenases represent a third class that halogenates aliphatic carbons, often on carrier-protein-bound substrates. SyrB2 is a well-characterized example that chlorinates a specific carbon in the substrate during assembly of a natural product.

Applications of halogenases include enabling site-specific halogenation in complex molecules under mild conditions, providing versatile handles for downstream chemistry, and expanding the repertoire of biocatalytic synthesis. Limitations include cofactor requirements, oxygen sensitivity, and, for some enzymes, narrow substrate scope.