Nitrite, reduction mechanism

A number of studies have highlighted the possibility of the use of monometallic Pd catalyst for effective nitrate/nitrite reduction if a reducible support is selected. Epron et proposed a reaction mechanism for Pd/Ce02, (Fig. 1) in which oxygen vacancies at the Pd-support interface were active sites for the reduction of nitrate. Although the selectivity to nitrogen was poor, the... 

Weeg-Aerssens, E., Tiedje, J. M., and Averill, B. A. (1988). Evidence from isotope labeling studies for a sequential mechanism for dissimilatory nitrite reduction. J. Am. Chem. Soc. 110, 6851-6856. 

Cytochrome cd nitrite reductase from Paracoceus pantotrophus has a different mechanism, with two identical subunits, each with domains containing a c-type cytochrome heme and a dj-type cytochrome heme. Electrons from external donors enter through the c heme the d heme is the site of nitrite reduction to NO and oxygen reduction to water. One of the puzzles of the mechanism is how the NO can escape from... 

Direct reaction of NO with enzymes has been shown for cytochrome c oxidase (cyt c oxidase). The reaction of NO with the binuclear metal centre of cyt c oxidase apparently leads to the formation of nitrite at the active site [123] the mechanism of which was described as the opposite of nitrite reduction to NO by non-haem nitrite reductases [124]. The inhibition was caused by the binding of NO to the reduced copper centre of the enzyme rather than the expected reaction with Fe. ... 

NO adds reversibly to reduced cobalamin, Cbl(II).156 It does not react directly with aquacobalamin(III), (0blni(H2O)), but it does add to Cbl,n(N02 ) and Cblm(NO).175 Acid hydrolysis of the dinitroso species releases nitrite, and binding of nitrite to Cblln(H20) generates Cbln,(. 02 ). This sequence thus affords a nitrite-catalyzed mechanism for NO substitution at Cblln(H20). The reaction of NO with Com porphyrins is quite complex.176 In the first step, NO displaces an axial water ligand to form a weakly bound mono NO complex this mono NO complex reacts with a second molecule of NO to form nitrite and a reduced Co-NO complex. This latter process is called reductive nitrosylation. Manganese(II) porphyrins bind NO very rapidly.177 Stability constants have been measured for the formation of mono and bis NO complexes of Cun(dithiocarbamate)2.157... 

Resonance Raman studies of CO and NO bound to PaNiR showed unusually high stretching frequencies compared with other hemoproteins due to the different electronic properties of d heme." Nitrite binds to heme d in both ferrous and ferric oxidation states but has a higher affinity for ferrous Fe with Aid values of approximately IpM and 2.5mM, respectively. Extensive evidence for the formation of an electrophilic heme-nitrosyl species derived from nitrite (formulated as Fe -NO" " or Fe" -NO) based on isotope exchange with 0-labeled water into NO, nitrosyl transfer to N nucleophiles, and FTIR studies of the interaction of NO with oxidized PsNiR." In the latter study a band at l,910cm was assigned to a ferric NO heme complex. This species is considered as the key intermediate in the mechanism for nitrite reduction." ... 

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 electrochemical behaviour of H2O in molten LiNOs-KNOs eutectic has been elucidated.Contrary to previous reports, electroreduction of H2O is coupled with nitrite reduction, probably involving an autocatalytic mechanism and an adsorbed intermediate. 

Dissimilatory nitrite reductase of denitrifying bacteria is usually a soluble enzyme and it has been difficult to ascribe a phosphorylative function associated with the conversion of nitrite to nitric oxide. However, the demonstration by Wood (1978) that the terminal reductase in nitrite respiration is located in the periplasm implies that electrons generated in the cytoplasm must traverse the cytoplasmic membrane to the periplasmic nitrite reduction site. This location would require proton pumping, thus facilitating phosphorylation by the chemiosmotic mechanism. 

Reaction mechanism Based on the observation of reaction intermediates in the crystal structure and on quantum chemical calculations Einsle et al. [148] propose an outline of the first detailed reaction mechanism of the cytochrome c Nir from W. succinogenes. Nitrite reduction starts with a het-erolytic cleavage of the weak N-O bond, which is facilitated by a pronounced backbonding interaction between nitrite and the reduced active site iron. The protons come firom a highly conserved histidine and tyrosine. Elimination of one of both amino acids results in a significant reduced activity. Subsequently, two rapid one-electron reductions lead to a FeNO form and, by protonation, to a HNO adduct. A further two-electron two-proton step leads to hydroxylamine. The iron in the hydroxylamine complex is in the Fe(III) state [149], which is unusual compared to synthetic iron-hydroxylamine complexes where the iron is mainly in the Fe(II) state. Finally, it readily loses water to give the product, ammonia. This presumably dissociates firom the Fe(III) form of the active site, whose re-reduction closes the reaction cycle. 

Figure 26 Proposed mechanism of nitrite reduction to NO by cdj. (A B) reductive activation of resting (inactive enzyme) and tyrosine displacement (not shown) (B C) nitrite binding (C D) protonation of nitrite complex (D E) cleavage of N-0 bond and elimination of H2O (E F) intramolecular iron oxidation (valence isomerization) (F G H C and F G I C) redox reactions involving heme c and heme cl nitrosyl complex followed by rapid dissociation of NO ( ) enzyme gets trapped in a "dead end" species in the absence of reducing substrate or nitrite. Adapted with permission from Ref (21). Copyright 2014 American Chemical Society.

Recently, a combination of closed-shell singlet, open-shell broken symmetry singlet and spin triplet DFT calculations was used to probe the reaction coordinate for nitrite reduction catalyzed by mARC. Regarding an oxyl radical transfer mechanism (Figure 2.36), the computed reaction barrier (AH was found to be 14 kcal moF. This reaction is approximately thermoneutral since Mo(v) dioxo species are very unstable toward proton-ation. However, protonation of the equatorial oxo is expected to lead to an overall increase in the exergonic nature of this reaction (Figure 2.36). However, if the 0 oxygen of N02 that is coordinated to the Mo(iv) ion is... 

Two major pathways have been shown to exist in nitrite reduction [274]. In the first pathway, nitrite is reduced to NO, while in the second there is a direct conversion of nitrite to NH3 or NH4" ". Two classes of nitrite reductase (NIR), namely the cytochromes cd [274], and the copper nitrite reductase [274], have been identified for the first pathway and two classes of enzyme, namely the siroheme nitrite reductase and cytochrome c nitrite reductase, have been proposed to follow the second pathway. The mechanism of these four enzymes has been recently reviewed [274], and only a brief summary of the electron-transfer reactions of cytochrome cd nitrite reductase will be given here. The initial step in the conversion of NO2 to NO involves a binding of the nitrite ion to the metal of the reduced heme. This first step is followed by the uptake of two protons and the loss of one water molecule to yield an electrophilic ferrous pe +-NO+ species, also formulated as a pe +-NO" complex. The dissociation of NO from this species produces the ferric heme d, which is in turn reduced back to its original state by heme c. Why the eri2yme does not reduce the nitrosyl species, Fe -]s[0 or Fe -NO to its Fe -NO form, prior to dissociation of NO in the heme, has been discussed in the literature [274], and may... 

Nitrate reduction. According to Kaspar (1982), P. acidipropionici, P. freudenreichii, P. jensenii, P. shermanii and P. thoenii can reduce nitrate to nitrite and further to N2O. Formation of N2O from nitrite in prokaryotes may represent a mechanism of detoxification rather than transformation of energy. N2O was not further reduced oxygen inhibited nitrate reduction by P. acidipropionici and P. thoenii. The enzymes of nitrate and nitrite reduction were either constitutive or derepressed in anaerobiosis only P. pentosaceum contained a constitutive nitrate reductase. Nitrate stimulated the synthesis of nitrate reductase in P. acidipropionici, specific growth rates and biomass yields were increased by the addition of nitrate. Nitrite at a concentration of 10 mM was not inhibitory. 

Karlin P (1966) The content of B-group vitamins in kefir and its enrichment by adding Propionibacterium shermanii. Prikl Biokhim Mikrobiol 2 386-391 Kaspar HF (1982) Nitrite reduction to nitrous oxide by propionibacteria detoxification mechanism. Arch Microbiol 133 126-130... 

Maekawa S, Matsui T, Hirao K, Shigeta Y. THeoretical study on reaction mechanisms of nitrite reduction by copper nitrite complexes toward understanding and controlling possible mechanisms of copper nitrite reductase. J Phys Chem B. 2015 119 5392-5403. 

Bacteria have been Implicated in the formation of N-nitroso compounds under a wide variety of conditions representing both vitro and vivo situations Mechanisms of participation and/or catalysis Include a) decrease of the pH of the system, b) reduction of nitrate to nitrite, c) adsorption of amine onto the cell surface or cytoplasmic membrane, d) actual enzymatic formation. The literature of the field will be reviewed and experimental evidence which tests the above mechanisms will be presented ... 

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