Dissociation of the proximal His-Fe bond upon NO binding to ferrous zebrafish nitrobindin

Nitrobindins (Nbs) are all-β-barrel heme-proteins present in prokaryotes and eukaryotes. Although the physiological role(s) of Nbs are still unclear, it has been postulated that they are involved in the NO/O₂ metabolism, which is particularly relevant in fishes for the oxygen supply. Here, the reactivity of ferrous Danio rerio Nb (Dr-Nb(II)) towards NO has been investigated from the spectroscopic and kinetic viewpoints and compared with those of Mycobacterium tuberculosis Nb, Arabidopsis thaliana Nb, Homo sapiens Nb, and Equus ferus caballus myoglobin. Between pH 5.5 and 9.1 at 22.0 °C, Dr-Nb(II) nitrosylation is a monophasic process; values of the second-order rate constant for Dr-Nb(II) nitrosylation and of the first-order rate constant for Dr-Nb(II)-NO denitrosylation are pH-independent ranging between 1.6 × 10⁶ M^-1 s^-1 and 2.3 × 10⁶ M^-1 s^-1 and between 5.3 × 10^-2 s^-1 and 8.2 × 10^-2 s^-1, respectively. Interestingly, both UV-Vis and EPR spectroscopies indicate that the heme-Fe(II) atom of Dr-Nb(II)-NO is five-coordinated. Kinetics of Dr-Nb(II) nitrosylation may reflect the ligand accessibility to the metal center, which is likely impaired by the crowded network of water molecules which shields the heme pocket from the bulk solvent. On the other hand, kinetics of Dr-Nb(II)-NO denitrosylation may reflect an easy pathway for the ligand escape into the outer solvent.