Halogenating enzymes

More recently, a novel type of halogenating enzyme, named hydroperoxide halolyase, which generates halogenated aldehydes, has been described in the marine diatom Stephanopyxis turris (Wichard and Pohnert 2006). In other microalgae, halogenation of organic compounds was shown to mainly involve methyl halide transferases (Moore et al. 1996 Manley 2002), and no vHPO has yet been identified on genomic data obtained from diatoms (Scala et al. 2002 Armbrust et al. 2004). Clearly, these emissions are not directly associated with an oxidative burst. 

Rebeccamycin is an antitumor or antibiotic agent isolated from bacteria " and contains a maleimide indolocarbazole framework. The Trp units in this molecule are chlorinated at the C-7 position and a flavin-dependent halogenase was identified as the enzyme that carries out this chlorination (Scheme 10). " There are many other halogenating enzymes known in the literature and these enzymes are responsible for the syntheses of metabolites containing bromine, chlorine, and fluorine. ... 

DeBoer E, Plat H, Wever R (1987) Algal vanadium(V)-bromoperoxidase. A halogenating enzyme retaining full activity in apolar solvent systems. In Laane C, Tramper J, Lilly MD (eds) Biocatalysis in organic media. Elsevier, Amsterdam, p 317... 

Dong C, Kotzsch A, Dorward M, van Pee K-H, Naismith JH (2004) Crystallization and X-ray Diffraction of a Halogenating Enzyme, Tryptophan 7-Halogenase, from Pseudomonas fluorescens. Acta Cryst D60 1438... 

Classification of Halogenating Enzymes and their Reaction Mechanisms... 

The only type of halogenating enzymes known until 1997 were peroxidases and perhydrolases which catalyze the formation of carbon halogen bonds using halide ions, hydrogen peroxide and an organic substrate activated for electrophilic attack. 

Phenolsulfonephthalein (Phenol Red) is brominated to 3,3, 5,5 -tetrabromophe-nolsulfonephthalein (Bromophenol Blue) by many haloperoxidases11, 1S1. This reaction has been used for the detection of halogenating enzymes by different groups19, 471. 

So far no investigations on the stereospecificity of this type of halogenating enzymes have been reported. 

A completely purified halogenating enzyme has not been isolated from marine sources, but the existence of haloperoxidases has been securely established. Pedersen (1976) succeeded in demonstrating the existence of a brominating cind hydroxylating enzyme in the red alga Cystoclonium purpureum, and Manley (1977) has recently illustrated the in vitro bromina-tion of precursor molecules in cell-free extracts of the red alga Odonthalia floccosa. Both enzymes were found to catalyse the bromination of phenolic precursors such as p-hydroxybenzyl alcohol (55) to yield the halo derivatives as in 56. 

Figure 1.59 Formation of hypochlorous acid from FAD-4o -OOH (a) mechanism of tryptophan chlorination (b) catalyzed by tryptophan 7-halogenase and (c) catalyzed by halogenating enzyme RebH.

The individual enzymes are called chloro-, bromo-, and iodoperoxidase. The name reflects the smallest halide ion that they can oxidize, in correlation to the corresponding redox potential. Bearing in mind their unique position as halogenating enzymes and the large variety of structurally different halometabolites produced by them, it is not surprising that the majority of haloperoxidases are characterized by a low product selectivity and wide substrate tolerance. As a consequence, any asymmetric induction observed in haloperoxidase-catalyzed reactions is usually low. 

Examples of primers used in the amplification of genes coding for halogenating enzymes are given in Table 45.2. 

Butler, A. and Sandy, M. (2009) Mechanistic considerations of halogenating enzymes. Nature, 460, 848-854. 

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