Coordination Chemistry in Protein Cages: From Heme Proteins to Organometallo-enzymes

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Heme enzymes responsible for a variety of oxidation are believed to afford an O=Fe(IV) porphyrin cation radical intermediate as the common reactive species. We have studied detailed mechanisms of the intermediate formation by employing synthetic model complexes and successfully characterized peracid bound forms of Fe(III) prophyrins as a precursor of the intermediate. More importantly, the nature of the O—O bond cleavages in the peroxo-Fe(III) porphyrin, homolysis and heterolysis, was evaluated at the molecular level. We have designed a series of myoglobin mutants on the basis of these mechanistic aspects of the intermediate formation to introduce peroxidase, catalase, and P450 activities into myoglobin. Myoglobin mutants are able to catalyze asymmetric oxidation such as sulfoxidation, epoxidation, and hydroxylation. An iron storage protein, ferritin, was also used as a reaction vessel of metal catalyzed reactions including hydrogenation, Suzuki coupling reactions, and styrene polymerization. These results clearly demonstrate that we are able to design molecular catalysts by utilizing interior surface of protein cages.