COVALENT COMPLEX OF MICROPEROXIDASE WITH A 21-RESIDUE SYNTHETIC PEPTIDE AS A MAQUETTE FOR LOW-MOLECULAR-MASS REDOX PROTEINS

Here we report the structural and functional characterization of a cov alent complex (MKP) obtained by cross-linking microperoxidase (Mp), th e haem-undecapeptide obtained by the peptic digestion of cytochrome c, with a 21-residue synthetic peptide (P21) analogous to the S-peptide of the RNase A. The covalent complex has been prepared by introducing a disulphide bond between Cys-1 of P21 and Lys-13 of Mp, previously mo dified with a thiol-containing reagent. On formation of the complex (w hich is a monomer), the helical content of P21 increases significantly . The results obtained indicate that His-13 of P21 coordinates to the sixth co-ordination position of the haem iron, thus leading to the for mation of a complex characterized by an equilibrium between an 'open' and a 'closed' structure, as confirmed by molecular dynamics simulatio ns. Under acidic pH conditions, where His-13 of P21 is loosely bound t o the haem iron ('open' conformation), MKP displays appreciable, quasi reversible electrochemical activity; in contrast, at neutral pH ('clos ed' conformation) electrochemical behaviour is negligible, indicating that P21 interferes with the electron-transfer properties typical of M p. On the whole, MKP is a suitable starting material for building a mi niature haem system, with interesting potential for application to bio sensor technology.