Covalently modified microperoxidases as heme-peptide models for peroxidases

Microperoxidase-8 (MP8) and microperoxidase-9 (MP9) have been covalently modified by attachment of proline-containing residues to the amino terminal peptide chain in order to obtain new peroxidase model systems. The catalytic activities of these derivatives in the oxidation of p-cresol by hydrogen peroxide have been compared to that of MP8. The presence of steric hindrance above the heme reduces the formation rate of the catalytically active species, while the reactivity is increased when the amino group of a proline residue is close to the iron. The modification of the catalyst affects the rate of degradation processes undergone by the heme group during catalysis. A bulky aromatic group on the distal side decreases the stability of the complex because it reduces the mobility of a phenoxy radical species formed during catalysis, while the presence of proline residues increases the number of turnovers of the heme catalysts before degradation. The complex Pro2-MP8 obtained by addition of two proline residues to MP8 exhibits the best catalytic performance in terms of activity and chemical stability.