Cyanide binding to ferrous and ferric microperoxidase-11.

Abstract

Microperoxidase-11 (MP11) is an undecapeptide derived from horse heart cytochrome c (cytc). MP11 is characterized by a covalently linked solvent-exposed heme group, the heme-Fe atom being axially coordinated by a histidyl residue. Here, the reactions of ferrous and ferric MP11 (MP11-Fe(II) and MP11-Fe(III), respectively) with cyanide have been investigated from the kinetic and thermodynamic viewpoints, at pH 7.0 and 20.0°C. Values of the second-order rate constant for cyanide binding to MP11-Fe(II) and MP11-Fe(III) are 4.5M(-1)s(-1) and 8.9×10(3) M(-1)s(-1), respectively. Values of the first-order rate constant for cyanide dissociation from ligated MP11-Fe(II) and MP11-Fe(III) are 1.8×10(-1)s(-1) and 1.5×10(-3)s(-1), respectively. Values of the dissociation equilibrium constant for cyanide binding to MP11-Fe(II) and MP11-Fe(III) are 3.7×10(-2) and 1.7×10(-7)M, respectively, matching very well with those calculated from kinetic parameters so that no intermediate species seem to be involved in the ligand-binding process. The pH-dependence of cyanide binding to MP11-Fe(III) indicates that CN(-) is the only binding species. Present results have been analyzed in parallel with those of several heme-proteins, suggesting that (1) the ligand accessibility to the metal center and cyanide ionization may modulate the formation of heme-Fe-cyanide complexes, and (2) the general polarity of the heme pocket and/or hydrogen bonding of the heme-bound ligand may affect cyanide exit from the protein matrix. Microperoxidase-11 (MP11) is an undecapeptide derived from horse heart cytochrome c. Penta-coordinated MP11 displays a very high reactivity towards cyanide, whereas the reactivity of hexa-coordinated horse heart cytochrome c is very low.