Mechanism and Catalytic Site Atlas

Introduction

The mechanism uses the very common Ser/His/Asp triad. Histidine deprotonates serine, which initiates a nucleophilic attack on the carboxylate carbon of the substrate. The oxyanion collapses, eliminating a water molecule with concomitant deprotonation of histidine. Histidine then deprotonates the hydrogen peroxide substrate, which initiates a nucleophilic attack on the carbonyl carbon of the enzyme-substrate intermediate. The oxyanion collapses, eliminating serine with concomitant deprotonation of histidine. A chloride ion initiates a nucleophilic attack on the peroxointermediate in a substitution reaction, regenerating the carboxylic acid and producing the hypochlorous acid intermediate. The hypochlorous acid then halogenates the alkane.

The peroxoacid is protected from hydrolysis by the hydrophobic environment of the active site as chloride, bromide or iodide enter the active site via a tunnel. The halogen then attacks the distil oxygen of the peroxoacid reforming the organic acid and hypohalous acid. Hypohalous acid may leave the active site and react with a range of substrates in solution. There is some evidence to support binding of some small hydrophobic substrates to the active site to aid specific halogenation.

The reaction catalysed by the bacterial non-heme, metal-ion- and cofactor- independent haloperoxidases is not a true peroxidase reaction, but the perhydrolysis of an ester formed between the serine residue of the catalytic triad and the organic acid. In the presence of hydrogen peroxide this ester is perhydrolysed leading to the formation of the organic peracid which can then oxidise halide ions resulting in the formation of hypohalous acid which acts as the halogenating agent.