Critical redox and allosteric aspects of nitric oxide interactions with hemoglobin

Nitric oxide (NO) is an important signaling molecule. Relatively long-lived NO adducts at the heme and SH groups of hemoglobin (Hb) could enable NO to carry out long-range signaling functions. In spite of significant advances, there remain as yet unresolved issues regarding the possible role of Hb in moderating NO-signaling events that affect blood pressure regulation. In this review, we summarize recent reports concerning the redox and allosteric aspects of NO/Hb interactions that have advanced our understanding of the physiological significance of NO binding to heme groups (forming NO-Hb) and of reactions promoting formation of S-nitrosated Hb (SNO-Hb). Allosteric mechanisms modify the bioactivity of NO/Hb complexes by altering the lifetime of NO-Hb and the properties of SNO-Hb. Redox reactions are significant because of the complex chemistry possible for NO and its oxidation products. Reactions at ferrous and ferric heme sites have differing consequences and affinities for interactions with NO. Moreover, redox changes at heme groups affect reactivity of SH groups and vice versa. In spite of low levels of NO-Hb and SNO-Hb found in vivo, recent findings do not rule out participation of NO-Hb or SNO-Hb in NO-dependent signaling reactions.