Flavin halogenation

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. ... 

To understand the carbanion mechanism in flavocytochrome 62 it is useful to first consider work carried out on related flavoenzymes. An investigation into o-amino acid oxidase by Walsh et al. 107), revealed that pyruvate was produced as a by-product of the oxidation of )8-chloroalanine to chloropyruvate. This observation was interpreted as evidence for a mechanism in which the initial step was C -H abstraction to form a carbanion intermediate. This intermediate would then be oxidized to form chloropyruvate or would undergo halogen elimination to form an enamine with subsequent ketonization to yield pyruvate. The analogous reaction of lactate oxidase with jS-chlorolactate gave similar results 108) and it was proposed that these flavoenzymes worked by a common mechanism. Further evidence consistent with these proposals was obtained by inactivation studies of flavin oxidases with acetylenic substrates, wherein the carbanion intermediate can lead to an allenic carbanion, which can then form a stable covalent adduct with the flavin group 109). Finally, it was noted that preformed nitroalkane carbanions, such as ethane nitronate, acted as substrates of D-amino acid oxidase 110). Thus three lines of experimental evidence were consistent with a carbanion mechanism in flavoenzymes such as D-amino acid oxidase. 

In recent years, the enzymes responsible for the introduction of halogen in biological systems have been identified. One group of vanadium bromoperoxidases is found in marine algae [69-71]. 7-Chlorination of tryptophan is involved in the biosynthesis of materials such as pyrrolnitrin and rebeccamycin. The enzyme is a flavin-dependent monooxygenase that generates hypochlorite ion. The structure of the enzyme is such that the OCl ion is conducted to the indole binding site over a distance of about 10 A [72, 73]. 

Figure 1.58 Halogenation reactions catalyzed by flavin-dependent halogenase. (a) Formation...

A further typical feature is the high reactivity of 8a-halogenated flavins, which can be compared to that of p-nitrobenzyl-halogenides. The facile replacement of bromine at position 8a by nucleophile functions of amino acids was the key step in the synthesis of the modified coenzymes which will be detailed below. Similarly halogen or -N2 at position 8 can be replaced by a variety of nucleophiles for example by amines, alcohols or mercaptide 76,152,177). 

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