Heme nitric oxide/oxygen binding

In summary, there is still much to understand about the nitrite reduction reaction. The crystal structures have shown how nitrite can bind to the di heme iron and protons can be provided to one of its oxygen atoms from two histidine residues. However, as yet no rapid reaction study has detected the release of product nitric oxide rather than the formation of the inhibitory dead-end ferrous di heme-NO complex. It is also not clear why the rate of interheme electron transfer is so slow over 11 A when nitrite or nitric oxide is the ligand to the d heme. 

Martin E, Berka V, Bogatenkova E, Mnrad E, Tsai AL. Ligand selectivity of solnble guanylyl cyclase - Effect of the hydrogen-bonding tyrosine in the distal heme pocket on binding of oxygen, nitric oxide, and carbon monoxide. J. Biol. Chem. 2006 281 27836-27845. 

Abu-Soud, H., Rousseau, DL, and Stuehr, DJ. 1996. Nitric oxide binding to the heme of neuronal nitric-oxide synthase links its activity to changes in oxygen tension. J. Biol. Chem. 271 32515-32518. 

Hemoglobin also transports carbon dioxide and nitric oxide (NO), a molecule with important regulatory tasks in the body. The poisonous molecule carbon monoxide (CO) binds stronger to the heme group than oxygen, carbon dioxide, and NO, and this explains why CO is dangerous. 

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