Ferricyanide-mediated oxidation of ferrous nitrosylated sperm whale myoglobin involves the formation of the ferric nitrosylated intermediate

Abstract

Ferricyanide-mediated oxidation of ferrous oxygenated and carbonylated myoglobin (Mb(II)–O 2 and Mb(II)–CO, respectively) is limited by O 2 and CO dissociation, respectively, then the transient deoxygenated derivative (Mb(II)) is rapidly oxidized. Here, kinetics of ferricyanide-mediated oxidation of ferrous nitrosylated sperm whale myoblobin (Mb(II)–NO) is reported. Unlike for Mb(II)–O 2 and Mb(II)–CO, ferricyanide reacts with Mb(II)–NO forming first a transient ferric nitrosylated species (Mb(III)–NO), followed by the NO dissociation from Mb(III)–NO. Values of the second-order rate constant for ferricyanide-mediated oxidation of Mb(II)–NO ( i.e., for the formation of the transient Mb(III)–NO species) and of the first-order rate constant for NO dissociation from Mb(III)–NO ( i.e., for Mb(III) formation) are (1.3 ± 0.2) × 10 6 M −1 s −1 and 7.6 ± 1.3 s −1, respectively, at pH 8.3 and 20.0 °C. Since NO dissociation from Mb(II)–NO is very slow, and (unlike O 2 and CO) NO is a ligand for both Mb(II) and Mb(III), Mb(II)–NO can be oxidized without requiring NO dissociation.