A study of crystallographic complexes obtained from vanadium haloperoxidase enzymes can determine key amino acids, leucine, and phenylalanine, that change their structural orientation to allow either chloride or bromide to bind to the enzymes active site, thereby determining halide specificity. This observation of bound halogens to these enzymes opens applications for specific biocatalytic halogenation of interest to pharmaceutical industries.

Abstract

Vanadium haloperoxidases have been studied to understand their mechanism and halide specificity. Crystal structures of vanadium bromoperoxidase enzyme complexes from Corallina piluifera, with vanadate and bromide and with phosphate and chloride, show significant displacement of loop residues 336–338 upon halogen binding. This shows a “closed position” of Leu337 locking the bromide ion in a hydrophobic environment favoring the vanadium peroxide reaction with the halogen by retaining the resulting hypobromite in the enzyme active site. The bound cofactor exists as a mixture of free vanadate and histidine553-vanadate adduct. A mutant enzyme Arg397Trp also has been crystallized with bound phosphate and its structure determined with and without the bound bromide ion. The precise positions of the bromine have been determined using its anomalous signal. The bromide binding site in the mutant enzyme is displaced by 2.5 Å resulting in a mixed population of the “open” and the “closed” forms of Leu337. This allows additional chloroperoxidase activity due to re-positioning of the halogen ion 0.6 Å closer to the vanadate ion. These studies support the application of vanadium haloperoxidase enzymes for selective halogenation of important drug molecules.