Effect of Iron–Nitric Oxide Complexes on the Reactivity of Hemoglobin Cysteines

Abstract

Human erythrocyte hemoglobin (Hb) has two reactive cysteines located on the surface of β-subunits. These cysteines play an important role in the adjustment of Hb functions. It is known that they are involved in the transport of intracellular nitric oxide (NO), redox signaling, and the regulation of dimeric-tetrameric Hb equilibrium. It is shown in this work that the incorporation of Cys-93β as ligands in iron-NO complexes is another way to regulate of SH group reactivity. Such complexes stabilize the SH group as a thiolate anion (R–S–), the reactivity of which is significantly higher than that of the protonated form of thiol (Cys-SH). This is why the thiols included in the complexes show increased activity in relation to electrophilic agents, such as ThioGlo1. Conversely, as part of the complexes, thiols are protected from oxidation by tert-butyl hydroperoxide. The incorporation of SH groups into complexes of iron and NO can be considered a means of thiol protection from irreversible oxidation upon oxidative stress.