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The binding of oxygen to heme irons in hemoglobin promotes the binding of nitric oxide (NO) to cysteineβ93, formingS-nitrosohemoglobin. Deoxygenation is accompanied by an allosteric transition in S-nitrosohemoglobin [from the R (oxygenated) to the T (deoxygenated) structure] that releases the NO group. S-nitrosohemoglobin contracts blood vessels and decreases cerebral perfusion in the R structure and relaxes vessels to improve blood flow in the T structure. By thus sensing the physiological oxygen gradient in tissues, hemoglobin exploits conformation-associated changes in the position of cysteineβ93 SNO to bring local blood flow into line with oxygen requirements.