Oxygen binding and activation with hemoproteins: model approach using iron-porphyrin complexes

Abstract

Using myoglobin reconstituted with synthetic hemes, the struc— ture—oxygen binding relationship is investigated. Oxygen affinity is controlled by (1) the basicity of heme iron and (2) site specific inter— action of the peripheral substituents of heme with globin side chain. The effect of hydrophobic cavity size on the oxygen binding is examined by using the hydrocarbon chain—bridged metalloporphyrin—imidazole complexes (metal: Fe(II), Co(II)). The stability of oxy form of this model com— plexes changes with the fitness of the cavity. As oxygenase, L—tryptophan pyrrolase is taken up. Fe(II)porphyrin is shown to be the best model of its active site. This model complex effectively catalyzes the oxygenation of 3—substituted indoles to form products corresponding to formylkynurenine (the oxygenation product of L—tryptophan) with 40—50% conversion of the substrate. ESR analysis of the oxygenation system indicates cooperative electron transfer (COET) occurs from the substituted indole anion to the oxygen in the ternary system: indole anion Fe(II)porphyrin O2 This type of electron transfer is a new concept that should occur in any strong donor Fe(II)porphyrin acceptor system. The COET process for L' ''Fe(II)porphyrin. . '02 complex is discussed with the ionization potential value of L as a criterion.