Heme-NO complex

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. 

Fig. 8. Mossbauer spectra of a sample containing 9 mM [OEP PeCl] and 30 mM Al-methylimidazole in jyjV-dimethylacetamide, into which NO gas was bubbled for 5 minutes, taken at 4.2 K in a magnetic field of (a) 5.34 T parallel and (b) 20 mT perpendicular to the 7-beam. The dotted line corresponds to the spectrum of the low-spin ferric heme complex [OEP Fe(NMelm)2]+Cl (39% relative contribution) and the dashed line to the heme-NO complex [OEP Fe(NMelm)(NO)]+Cl (61% relative contribution). Reproduced with permission from Ref. (86).

The NO in this structure appears to be bound equally in two orientations, a component that is linear (Fe-N distance of 1.6 A and Fe-N-0 angle of 170°, Fig. 17), and a second component that is bent (Fe-N 2.0 A, Fe-N-0 110°). To account for this, the NO molecule has been refined as a mixture of both orientations. The roughly linear orientation is indicative of a ferric (Fe 0 NO complex (2, 65), indicating that the NP4-NO structure represents the first ferric heme-NO complex for any protein. However, the bent orientation is similar in geometry to a ferrous heme-NO complex (2, 65). The bend directs the NO toward a... 

Heme—NO complexes are the best known and most thoroughly studied of the biological metal-nitrosyl complexes. So many heme enzymes have been... 

A 10-step kinetic model has been developed (Santolini et al., 2001). Crystal structures of xyNOS show that a Tyr-409 indol nitrogen atom forms a strong hydrogen bond with the heme thiolate (Crane et al., 1988 Raman et al.1998 Fishmann et al., 1999). The Try-409 mutation suggests that the heme potential controls the NOS reactions (Adak et al. 2001). Suppression of this hydrogen bond through the mutation lowers the reduction potential of the heme, inhibits heme reduction and accelerates oxidation of the Fe(II) heme-NO complex. The Arg binding increases the reduction potential of the NOS heme. 

Couture, M., Adak, S., Stuehr, D.J., Rousseau, D. L. (2001) Regulation of the properties of the heme-NO complexes in nitric-oxide synthase by hydrogen bonding to the proximal cysteine, 7. Biol. Chem. 276, 38280-38288. 

Fig. 14 Mossbauer spectra of the di heme-NO complex in T. denitrificans cytochrome cdi in the presence of nitrite at pH 7.6. The data were recorded at 4.2 K with a magnetic Held of 50 mT applied parallel (A) or perpendicular (B) to the y-rays. The brackets indicate the positions of the quadrupole doublets of the reduced c heme which had been removed from the raw data. The solid lines are theoretical simulations. Reprinted from the reference [149], with the permission of John Wiley and Sons, copyright 1987...

In beef cubes thermally processed in water, weak radicals were detectable at room temperature, but were not strong enough to identify or quantitate. At 77K (liquid nitrogen temperature) signals from a nitroso>heme complex with two principal components appeared (Figure 1). The set of three sharp, narrow lines in the center of the spectra is typical of pentacoordinate Fe heme-NO complexes (13,14), while the broad downfield signal with moderate to no fine structure arises from electron interactions with tire heme apoprotein (14 -16). 

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