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

Ferricyanide-mediated oxidation of ferrous oxygenated and carbonylated myoglobin (Mb(II)–O2 and Mb(II)–CO, respectively) is limited by O2 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)–O2 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) × 106 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 O2 and CO) NO is a ligand for both Mb(II) and Mb(III), Mb(II)–NO can be oxidized without requiring NO dissociation.