Ferric microperoxidase-11 catalyzes peroxynitrite isomerization

Abstract

Microperoxidase-11 (MP11) is an undecapeptide derived from horse heart cytochrome c offering the possibility to study the reactivity of the heme group relatively unshielded by the protein. Here, the peroxynitrite isomerization to NO3− catalyzed by ferric MP11 (MP11–Fe(III)) is reported. Data were obtained between pH3.6 and 8.1, at 20.0°C. The value of the second-order rate constant (kon) for peroxynitrite isomerization to NO3− by MP11–Fe(III) decreases from (1.1±0.1)×105M−1s−1, at pH3.6, to (6.1±0.6)×103M−1s−1, at pH8.1. The pH dependence of kon (pKa=6.9) suggests that peroxynitrous acid reacts preferentially with MP11–Fe(III). The MP11–Fe(III)-catalyzed isomerization of peroxynitrite to NO3− has been ascribed to the reactive penta-coordinated heme–Fe atom of MP11–Fe(III). In fact, cyanide binding to the sixth coordination position of the heme–Fe atom inhibits the MP11–Fe(III)-catalyzed isomerization of peroxynitrite to NO3−. The values of the first-order rate constant (k0) for isomerization of peroxynitrite to NO3− in the presence of the MP11–Fe(III)–CN complex are superimposable to those obtained in the absence of MP–Fe(III). Values of kon for peroxynitrite isomerization to NO3− by MP11–Fe(III) overlap those obtained for penta-coordinated cardiolipin–cytochrome c complex and for carboxymethylated cytochrome c in absence and presence of cardiolipin. Present results highlight the role of the heme–Fe(III) co-ordination state in the modulation of cytochrome c reactivity.