Kinetics and Mechanism of Reduction of Metmyoglobins by Dithionite. Role of the Heme Propionates

Abstract The kinetics of the reduction by dithionite ions of metmyoglobins reconstituted with porphinatoiron(III) complexes containing the protoporphyrin IX dimethyl ester, the deuteroporphyrin IX dimethyl ester, and octaethylporphyrin were studied at 25°C and pH 6.3–9.2 in aqueous solution. Esterification of the heme propionate groups accelerated the reduction rate by SO2\divdot as well as the case of octaethylporphyrin (which does not contain the propionate groups); it also affected the redox potential of metmyoglobin, but much less so the rate of the self-exchange reaction. The reaction mechanisms are discussed.