Nitrosyls intermediate

In the process, the iron is reduced to the ferrous form. Ferric cytochrome c is reduced by nitric oxide through a nitrosyl intermediate to produce ferrous cytochrome c and nitrite (Orii and Shimada, 1978). The nitrosyl cytochrome c absorbs at 560 nm, which is slightly higher than the 550-nm peak observed for reduced cytochrome c. Nitric oxide may be an interference in the assay of superoxide from cultured cells by the cytochrome c method. When nitric oxide reacts with cytochrome c, there is an initial decrease in absorbance at 550 nm as the nitrosyl complex is formed followed by a rise in absorbance as the complex decomposes to nitrite and reduced cytochrome c. This is a potential artifact in studies measuring the release of superoxide from cultured endothelial cells or other cells that make nitric oxide. 

Hulse, C. L., and Averill, B. A. (1989). Evidence for a copper-nitrosyl intermediate in denitrification by the copper-containing nitrite reductase of Achromobacter cycloclastes. J. Am. Chem. Soc. Ill, 2322-2323. 

Green plants, algae, fungi, cyanobacteria and bacteria that assimilate nitrate also produce assimilatory nitrite reductases, which catalyze the six-electron reduction of nitrite to ammonia (equation 89). The formation of heme-nitrosyl intermediates has been detected in several cases,1515 while hydroxylamine is commonly thought to be an intermediate. Added hydroxylamine is rapidly reduced to ammonia. However, no intermediates are released, and ammonia is the only product... 

This chapter focuses on the chemistry ofbiomimetic copper nitrosyl complexes relevant to the NO-copper interactions in proteins that are central players in dissimilatory nitrogen oxide reduction (denitrification). The current state of knowledge of NO-copper interactions in nitrite reductase, a key denitrifying enzyme, is briefly surveyed the syntheses, structures, and reactivity of copper nitrosyl model complexes prepared to date are presented and the insight these model studies provide into the mechanisms of denitrification and the structures of other copper protein nitrosyl intermediates are discussed. Emphasis is placed on analysis of the geometric features, electronic structures, and biomimetic reactivity with NO or NOf of the only structurally characterized copper nitrosyls, a dicopper(II) complex bridged by NO and a mononuclear tris(pyrazolyl)hydroborate complex having a Cu(I)-NO formulation. 

One proposed mechanism for CuNiR, which parallels catalysis by the well-characterized heme NiRs, involves the formation of a Cu -NO+ intermediate formed by protonation of bound nitrite followed by the abstraction of an oxygen atom to form OH . Although the Cu -NO intermediate has not been observed directly, the formation of N2O, a minor product of nitrite reduction by AfNiR, has been attributed to the reaction of a Cu nitrosyl intermediate with NO to form N2O. Consistent with this, large amounts of N20 were formed during turnover in the presence of N-labeled nitrite and N-labeled NO. 

The structure of the AfNiRH255 N mutant with both nitrite and H2O bound to the active site Cu has led to the proposal that a transient in catalysis may be a Cu OH NO species prior to this step. Protonation of the OH at the active site by His255 via the bridging water and release of NO regenerates the active site (Figure 6A). In a variant of this reaction scheme, a proton from Asp is transferred to nitrite to form a Cu NOOH intermediate. The subsequent internal electron transfer and protonation of this species by His255 yields NO and Cu H2O regenerating the type 2 center for further catalysis. Neither of these proposed mechanisms involves the putative nitrosyl intermediate. ... 

Barley, Meyer and coworkers showed that Fe TMPyP and Fe TSPP were effective electrocatalysts for the reduction of NOj to N2O, NH3, and NH20H . Current efficiencies as high as 97% were obtained for the formation of ammonia in some cases, Table 7.4. Under acidic conditions where nitrite disproportionates into mainly NO, a nitrosyl intermediate was implicated during... 

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