Structural Perspective on Enzymatic Halogenation

Abstract

Simple halogen substituents frequently afford key structural features that account for the potency and selectivity of natural products, including antibiotics and hormones. For example, when a single chlorine atom on the antibiotic vancomycin is replaced by hydrogen, the resulting antibacterial activity decreases by up to 70% (HarrisC. M.KannanR.KopeckaH.HarrisT. M.HarrisC. M.KannanR.KopeckaH.HarrisT. M.J. Am. Chem. Soc.198510766526658). This Account analyzes how structure underlies mechanism in halogenases, the molecular machines designed by nature to incorporate halogens into diverse substrates.Traditional synthetic methods of integrating halogens into complex molecules are often complicated by a lack of specificity and regioselectivity. Nature, however, has developed a variety of elegant mechanisms for halogenating specific substrates with both regio- and stereoselectivity. An improved understanding of the biological routes toward halogenation could lead to the development of novel synthetic methods for the creation of new compounds with enhanced functions. Already, researchers have co-opted a fluorinase from the microorganism Streptomyces cattleya to produce 18F-labeled molecules for use in positron emission tomography (PET) (DengH.CobbS. L.GeeA. D.LockhartA.MartarelloL.McGlincheyR. P.O’HaganD.OnegaM.DengH.CobbS. L.GeeA. D.LockhartA.MartarelloL.McGlincheyR. P.O’HaganD.OnegaM.Chem. Commun.2006652654). Therefore, the discovery and characterization of naturally occurring enzymatic halogenation mechanisms has become an active area of research.The catalogue of known halogenating enzymes has expanded from the familiar haloperoxidases to include oxygen-dependent enzymes and fluorinases. Recently, the discovery of a nucleophilic halogenase that catalyzes chlorinations has expanded the repertoire of biological halogenation chemistry (DongC.HuangF.DengH.SchaffrathC.SpencerJ. B.O’HaganD.NaismithJ. H.DongC.HuangF.DengH.SchaffrathC.SpencerJ. B.O’HaganD.NaismithJ. H.14765200Nature2004427561565). Structural characterization has provided a basis toward a mechanistic understanding of the specificity and chemistry of these enzymes. In particular, the latest crystallographic snapshots of active site architecture and halide binding sites have provided key insights into enzyme catalysis.Herein is a summary of the five classes of halogenases, focusing on the three most recently discovered: flavin-dependent halogenases, non-heme iron-dependent halogenases, and nucleophilic halogenases. Further, the potential roles of halide-binding sites in determining halide selectivity are discussed, as well as whether or not binding-site composition is always a seminal factor for selectivity. Expanding our understanding of the basic chemical principles that dictate the activity of the halogenases will advance both biology and chemistry. A thorough mechanistic analysis will elucidate the biological principles that dictate specificity, and the application of those principles to new synthetic techniques will expand the utility of halogenations in s...

Vanadium haloperoxidases: From the discovery 30 years ago to X-ray crystallographic and V K-edge absorption spectroscopic studies

“…Indeed a slight bromoperoxidase activity was obtained in two enzyme mutants, one mutated inside the active site (Ser358, Figure 4A) and the other mutated on a tyrosine residue located outside the active site. The current hypothesis [7,8,55] explains the change of catalytic properties and halide specificity by modifications of H-bond coordination and redox potential of the VO 4 moiety, rather than selective halide binding.…”

“…After halide oxidation, the electron oxidized halogen intermediate "X + ", which is a mixture of different halogen species such as OX -, X 3 -or X 2 , may rapidly halogenate organic substrates in a non-enzymatic reaction (Scheme 1). In the absence of organic substrate, a second equivalent of H 2 O 2 is oxidized to form a single oxygen and X -[for reviews 1,7,8,10,44].…”

“…Haloperoxidases have been classified in two groups, according to the nature of their cofactor, vanadium or heme (iron-containing porphyrin) [1,[7][8][9][10]. Metal-free haloperoxidases have also been described in some bacteria [11].…”

The Vanadium Iodoperoxidase from the Marine Flavobacteriaceae Species Zobellia galactanivorans Reveals Novel Molecular and Evolutionary Features of Halide Specificity in the Vanadium Haloperoxidase Enzyme Family

“…In the active center of fungal VCPO and algal VBPO, the vanadate cofactor is covalently bound to the imidazole ring of a conserved histidine residue and finely coordinated by neighboring amino acid side chains through hydrogen bonds (1,4,(14)(15)(16). The first step of the catalytic cycle is the coordination of hydrogen peroxide to vanadate, leading to the formation of a stable peroxovanadate intermediate as shown by X-ray diffraction and X-ray absorption spectroscopy (17)(18)(19).…”

Transformation of Biomass into Commodity Chemicals Using Enzymes or Cells

“…413 Halogenating enzymes have been divided into hydrogen peroxide-requiring haloperoxidases (hemedependent or vanadium-dependent), O 2 -dependent halogenases (FADH 2 -dependent or nonheme iron-dependent), and nucleophilic halogenases using chloride, for example. 414 Unfortunately, the widely studied haloperoxidases catalyze the formation of hypohalous acid (HOCl or HOBr) but have no control on the regioselectivity of the subsequent halogenation, which occurs outside the enzyme. Chlorination of acetic acid using chloroperoxidase led to dichloroacetic rather than monochloroacetic acid.…”