Chloroperoxidase

Chloroperoxidase is an example of a haloperoxidase, or enzyme that catalyzes the “halogenation of organic compounds in the presence of halide ions and peroxides such as H₂O₂” (3.)  These enzymes “are named after the most electronegative halide they are able to oxidize and classified according to their cofactor dependence as heme-type, vanadium-dependent and metal-free haloperoxidases” (3.) The metal cofactors present in both the heme-type and vanadium-dependent are suspected to aid in the catalysis of the reactions by increasing the affinity for peroxide sunstrates. Chloroperoxidases have also been shown to be inhibited by a sodium salt azide (NaN3), which binds in the active site and hinders the electron transport or flow of electrons in the enzyme (4.)

 The particular structure shown on this site is a vanadium-dependent chloroperoxidase isolated from the fungus Curvularia inaequalis. The vanadium ion is present in its highest oxidation state (5⁺) as hydrogen vanadate (HVO₄^2-) and is coordinated, with 3 non-residue oxygens, to the protien in a trigonal bipyramidal fashion. The product of the reaction catalyzed by this enzyme is HOCl, which is a strong oxidizing agent that may give insight into the function of protein.  The particular species of fungus from which the enzyme was derived “belongs to the group of dematiaceous hyphomycetes, which are pathogenic toward plants and/or are saprophytes, and it has been suggested that the enzyme and its product are used in the mechanism to oxidize plant cell walls to facilitate penetration of the fungus into the host” (4.)

The mechanism of this enzyme is only speculate at this time, based on the known structural data.  The first step is binding of peroxide into the active site, followed by the binding of a chloride ion to the activated peroxide.  Three residues near the peroxide-binding site “deserve special interest, the hydrophobic side chains of Trp350 and Phe397 as well as the imidazole ring of His404.  Both hydrophobic side chains provide a hydrophobic environment, which seems to be necessary to stabilize the chloride binding,” whereas the His404 appears to play a critical role as an acid-base group during catalysis (4.)  Without a definite picture of what the peroxide intermediate that is formed looks like, an actual understanding of the rest of the mechanism is hard to propose, but conjecture gives rise to the thought that the next step would be a nucleophilic attack by the chloride on the intermediate, since other peroxidases work in this fashion.  “Whatever the exact mechanism, a short lived HOCl intermediate is formed by the enzyme, and it is likely that His404 acts as the proton donor” (4.)

Schematic drawing of chloroperoxidase that presents a clearer picture of how the protein is arranged.

Jmol image of the crystallized structure of vanadium-dependent chloroperoxidase (1VNC.)

Close up of the active site of chloroperoxidase. After you check the box you see the three important amino acid residues involved in the reaction. Trp350 and His 404 are both part of helices(colored magenta) while Phe397 is part of a loop (colored white.)