Characterization of a novel microperoxidase from Marinobacter hydrocarbonoclasticus by electrospray ionization tandem mass spectrometry.

Microperoxidases are small heme-peptides obtained by proteolytic digestion of cytochrome c, exhibiting peroxidase activity. They consist of a short- or medium-length polypeptide chain, covalently linked to an iron protoporphyrin IX moiety via two thioether bonds involving Cys residues at the c-porphyrin A and B pyrrole rings. These small molecules are interesting for a wide range of possible applications. We have structurally characterized, by means of electrospray ionization (ESI) mass and tandem mass spectrometric experiments, a novel microperoxidase called MMP-5 (Marinobacter MicroPeroxidase-5), obtained by proteolytic digestion of cytochrome c552, a monoheminic electron-transfer protein isolated from Marinobacter hydrocarbonoclasticus. This microperoxidase, which still maintains the functional peptide moieties for peroxidase activity, is devoid of the two amino acids intercalating the Cys residues linked to the c-porphyrin, thus increasing its water solubility. Once submitted to the ESI source potential, MMP-5 showed an interesting tendency for the reduction of the iron protoporphyrin substructure. This behaviour was clearly evidenced by the mass shift exhibited by the reduced form.