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 ^{{radical dot}}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 ^{{radical dot}}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 ^{{radical dot}}NO dissociation from Mb(II)-NO is very slow, and (unlike O₂ and CO) ^{{radical dot}}NO is a ligand for both Mb(II) and Mb(III), Mb(II)-NO can be oxidized without requiring ^{{radical dot}}NO dissociation.