Microperoxidase 8 as a Powerful Tool for Biological Applications

Microperoxidase 8 is a heme octapeptide obtained by hydrolytic digestion of cytochrome which contains the heme prosthetic group together with the amino acid residues 14 to 21 of horse cytochrome c, including His 18, whose imidazole group acts as the fifth axial ligand of the iron. It is a very good model for the coordination properties and the reactivity of hemoproteins like peroxidases and cytochromes P450. Indeed, it is first able to bind a wide variety of ligands on the 6th axial coordination position of the iron. Second, it is able to perform peroxidase-like reactions, such as the nitration of phenol by H2O2/NO2-, the oxidation of N-hydroxyguanidines with formation of nitrogen oxides. Third, it is also able to catalyze monooxygenase-like reactions, such as the N-dealkylation of aromatic amines and the O-dealkylation of aromatic ethers, the para-hydroxylation of aniline, the monooxygenation of polycyclic aromatic compounds and the S-oxidation of sulfides. In the later case no selectivity was observed with MP8 alone, whereas when MP8 was associated with a monoclonal antibody raised against it, the oxidation was stereoselective with a 45% enantiomeric excess in favor of the R isomer. This association constituted a new generation of artificial hemoproteins based on a monoclonal antibody, which we named "hemoabzyme" and which appeared as a promising biocatalyst for selective oxidation reactions. Finally, a new biomedical application has recently been found for MP8 as a biosensor for molecules of biological interest. For this, several strategies were envisioned to immobilize MP8 at the surface of an electrode, in such a way that the 6th coordination position of the iron may be accessible to ligands. Detection of the target molecule will then be realized upon measurement of the variations of the redox potential and of the current intensity induced by its binding on the iron of MP8.