J. Biol. Chem., Vol. 268, Issue 4, 2953-2959, Feb, 1993

Metabolism-based transformation of myoglobin to an oxidase by BrCCl3 and molecular modeling of the oxidase form

Y Osawa, JF Darbyshire, PJ Steinbach and BR Brooks
Laboratory of Chemical Pharmacology, National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, Maryland 20892.

The stoichiometric reductive debromination of BrCCl3 to a trichloromethyl radical by myoglobin caused the prosthetic heme to become covalently cross-linked to the protein moiety and transformed myoglobin from an oxygen storage protein to an oxidase. This was shown in experiments in which oxygen consumption was measured during redox cycling of the altered myoglobin in the presence of ascorbate or an enzymatic reducing system containing diaphorase and NADH. Redox cycling eventually led to loss of the protein-bound heme adduct and oxidase activity of myoglobin. We have used molecular modeling and the known structure of the protein-bound heme adduct to identify probable mechanisms for transformation of myoglobin to an oxidase. Based on these modeling studies, the most likely structure of the experimentally observed adduct involves ligation to the heme iron of the epsilon- nitrogen atom of histidine 97 and/or that of histidine 64. The model structures revealed access of solvent to the heme active site, which could facilitate oxygen reduction. The transformation of myoglobins and perhaps other hemoproteins to oxidases may have toxicological importance in causing the tissue damage resulting from exposure to various xenobiotics and endogenous chemicals as well as explaining how hemoproteins are inactivated during catalysis.