Oxygen uptake and evolution by iron porphyrin enzymes

Abstract

Three detailed mechanisms are considered for the catalatic decomposition of H₂O₂. It is shown that the first of these, akin to the earlier hypotheses for catalase action, cannot satisfy the magnetic, titrimetric, and kinetic evidence. The second mechanism involves oxidation of the Fe^III porphyrin to the equivalent of Fe^V. The electron deficiency is distributed over the ligands so that even in the most oxidized complex the iron is in the Fe^IV or possibly even the Fe^III state. In the third scheme it is suggested that the reduction step (in which O₂ is liberated) takes place at a carbon atom, while the site of the oxidation is the metal atom as commonly supposed. The liberation of O₂ from H₂O₂ can be catalysed by 6-coordinate ruthenium II complexes. In the catalytic cycle, the metal appears to be oxidized to Rdv, then reduced to Ru^II. Ethanol or ascorbic acid can substitute for H₂O₂ in the reduction. Evidence for H₂O₂ attack on the ligands is suggestive but not conclusive. A brief comment is made on the bonding of oxygen to haemoglobin and myoglobin. The accumulated evidence for the structures of catalase, peroxidase, and myoglobin complexes is utilized in a scheme for the uptake of oxygen by cytochrome oxidase.