Reaction of NO with Heme Proteins and Metals

Reaction of NO with Heme Proteins and Metals 

FIGURE 8 Model for the NO-mediated activation of guanylate cyclase via the liberation of the axial histidine ligand from coordination of NO to the heme iron. 

The above chemistry can be described as reductive nitrosation. That is, the reduction of the metal by bound NO results in bound NO formation. The NO complex can then nitrosate a nucleophile such as a thiol (to give a nitrosothiol) or H2O (to give nitrite). Significantly, the reduction of metal salts (nonheme bound) by NO has also been observed (Gwost and Caulton, 1973), and therefore this chemistry may not be limited to heme proteins. 

NO is an extremely good ligand for Fe hemes (see, e.g., Jameson and Ibers, 1994, and references therein). The bonding between NO and the reduced Fe heme center is the result of two types of orbital interactions There is both a donation of electrons from NO to the metal center and, due to an interaction between the d-orbitals on the metal and the antibonding orbitals on NO, a donation of electrons from the electron-rich metal center back to NO (this component of the bonding is referred to as backbonding). 

The geometry of the backbonding orbitals involved and the overall bonding scheme are depicted in Fig. 9. The backbonding phenomenon is general for other ligands, such as O2 and CO, as well. 

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VIII. Reaction of NO with Heme Proteins and Metals... 

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