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Nitric oxide (NO) has a very important physiological function, and it is the unique small diffusible signaling molecule.When NO molecules bind to heme irons in α subunits in hemoglobin (Hb), they have two coordinate forms for Fe2+: one is 5-coordinate, and the other is 6-coordinate.However, there is only 6-coordinate for Fe2+ when NO molecules bind to heme irons in β subunits.When the amount of NO is at a minimal concentration, NO molecules mainly bind to α subunits.The results show that NO molecules do not transfer from heme irons of nitrosylhemoglobin (HbNO) to the thiol groups of Cysteine residues β93 (Cysβ93) to form s-nitrosohemoglobin (Hb-SNO) in the presence of minimal NO in hemoglobin solution.The presence of minimal NO in hemoglobin solution does not decrease the transportation of oxygen, but it does improve its transport ability.It is still under further research whether this mechanism is underlying in the therapy for the disease of cardiovascular system.