Enzymatic halogenation catalyzed via a catalytic triad and by oxidoreductases

During the search for haloperoxidases in bacteria we detected a type of enzymes that catalyzed the peroxide-dependent halogenation of organic substrates. However, in contrast to already known haloperoxidases, these enzymes do not contain a prosthetic group or metal ions nor any other cofactor. Biochemical and molecular genetic studies revealed that they contain a catalytic triad consisting of a serine, a histidine, and an aspartate. The reaction they catalyze is actually the perhydrolysis of an acetic acid serine ester leading to the formation of peracetic acid. As a strong oxidizing agent the enzymatically formed peracetic acid can oxidize halide ions, resulting in the formation of hypohalous acid which then acts as the actual halogenating agent. Since hypohalous acid is also formed by the heme- and vanadium-containing haloperoxidases, enzymatic halogenation catalyzed by haloperoxidases and perhydrolases in general lacks substrate specificity and regioselectivity. However, detailed studies on the biosynthesis of several halometabolites led to the detection of a novel type of halogenases. These enzymes consist of a two-component system and require NADH and FAD for activity. Whereas the gene for one of the components is part of the biosynthetic cluster of the halometabolite, the second component is an enzyme which is also present in bacteria from which no halometabolites have ever been isolated, like Escherichia coli. In contrast to haloperoxidases and perhydrolases the newly detected NADH/FAD-dependent halogenases are substrate-specific and regioselective and might provide ideal tools for specific halogenation reactions.