Nitrous reductase

Nitric oxide reductase (P) Nitrous oxide reductase (P) Ascorbate oxidase (P) Cytochrome oxidase (PM) Copper ATPase pumps (PM)... 

Nitrite reductase Nitrous oxide reductase Metallothionein ACE-1 (MAC)... 

Figure 3.13 Structure of the Cuz centre in nitrous oxide reductase. The central sulfide interacts with all four copper atoms. (From Rees, 2002. Copyright 2002 Annual Reviews.)...

Figure 14.13 Structure of the subunits of the homodimer nitrous oxide reductase (red and blue) and of the Cuz site. (From Chen et al., 2004. Reproduced with permission from John Wiley Sons., Inc.)...

Nitrite reductases and nitrous oxide reductases are relatively newly found copper-containing proteins involved in bacterial denitrification. N2O reductase may bear a relationship to cytochrome oxidase and, indeed, parallels it somewhat in function, being the terminal electron acceptor in its pathway. 

Nitrous oxide (N2O) reductases carry out the terminal reduction step in the denitrification pathway. One has been isolated from Pseudomonas... 

Nitrosyidisulfonic acid, reaction mechanisms, 22 129, 130 Nitrous acid, 33 103 decomposition, rate constants, 22 157 as oxidizing agent, 22 133 reaction mechanisms, 22 143-156 electrophilic nitrosations, 22 144-152 with inorganic species, 22 148, 149 nitrite oxidation by metals, 22 152-154 oxidation by halogens, 22 154, 155 in solution, 22 143, 144 reduction by metals, 22 155, 156 Nitrous oxide reductase, 40 368 Nitroxyl, reaction mechanisms, 22 138 Nitrozation, pentaamminecobalt(III) complexes, 34 181... 

Nitric oxide and iron nitrosyl complexes have been observed in the reduction of nitrite by bacterial nitrite reductases, which contain iron chlorin or iron isobac-terichlorin [151]. A specific nitric oxide reductase also exists to convert NO to nitrous oxide [9]. Iron complexes of chlorins, isobacteriochlorins, and porphyrins, as well as ruthenium and osmium polypyridines, and cobalt and nickel... 

Although the pathway of Eq. (1) is now based on much evidence (Section 111) and is unambiguous in the case of at least one bacterium [Pseudomonas stutzeri strain Zobell (f. sp. P. perfectomarina)], there have been alternative hypothesis. One hypothesis, advanced by the Hollocher group (Garber and Hollocher, 1981 St. John and Hollocher, 1977), considered NO as a likely intermediate, but one that remained at least partly enzyme-bound and was not entirely free to diffuse. This view was based on the outcome of certain kinetic and isotope experiments which can be summarized as follows. When denitrifying bacteria were challenged simultaneously with [ N]nitrite and ordinary NO, the cells reduced both compounds concomitantly to N2 (or to N2O in the presence of acetylene which is a specific inhibitor (Balderston et al., 1976 Yoshinari and Knowles, 1976) of nitrous oxide reductase). In the process, little NO was generally detected in the gas phase pool of NO and there was relatively little isotopically mixed N2O formed. That is, most of the N and N reduced to NjO appeared as N2O... 

Bell, L. C., and Ferguson, S. J. (1991). Nitric and nitrous oxide reductases are active under aerobic conditions in cells of Thiosphaera pantotropha. Biochem. J. 273, 423-427. 

Coyle, C. L., Zumft, W. G., Kroneck, P. M. H., Kiimer, H., and Jakob, W. (1985). Nitrous oxide reductase from denitrifying Pseudomonas perfectomarina. Purification and properties of a novel multicopper enzyme. Eur. J. Biochem 153, 459-467. 

Jones, A. M., Hollocher, T. C., and Knowles, R. (1992). Nitrous oxide reductase of Flexibacter canadensis A unique membrane-bound enzyme. FEMS Microbiol. Lett. 92, 205-210. 

Jiingst, A., Braun, C., and Zumft, W. G. (1991a). Close linkage in Pseudomonas stutzeri of the structural genes for respiratory nitrite reductase and nitrous oxide reductase, and other essential genes for denitrification. Mol. Gen. Genet. 225, 241-248. 

Snyder, S. W., and Hollocher, T. C. (1987). Purification and some characteristics of nitrous oxide reductase from Paracoccus denitrificans. J. Biol. Chem. 262, 6515-6525. 

Wharton, D. C., and Weintraub, S. T. (1980). Identification of nitric oxide and nitrous oxide as products of nitrate reduction by Pseudomonas cytochrome oxidase (nitrite reductase). Biochem. Biophys. Res. Commun. 97, 236-242. 

The other copper-only binuclear centre to be considered is the CuA or purple copper complex. It is part of the terminal oxidase in mitochondrial respiration, cytochrome c oxidase (COX). Its EPR signature, a seven-line spectrum, has since long been known to be different from the classes type 1 to 3 and arises from two copper ions in a 1.5 valence (or mixed valence) state, first proposed from EPR-analysis of a similar center in nitrous oxide (N20) reductase. There is a close correspondence between the blue and purple states of copper since each of the two copper ions in CuA can be considered as being structurally related to the mononuclear blue site coordination. 

The expression and characterization of a recombinant subunit II of the archaebacterial terminal oxidase complex in Sulfolobus acidocaldarius was achieved. The binuclear CuA centre was shown to be correctly inserted. A protonation of one of the coordinating histidines was suggested from the pH-profile.109 The subunit is part of a supercomplex SoxM which also has been isolated in a catalytically competent form for the first time.110 Nitrous oxide reductase (NOR) was prepared from Hyphomicrobium denitrificans and charac-... 

A new representative of a multicopper cluster in a protein is Cuz in nitrous oxide reductase. As was discussed above this enzyme contains a binuclear CuA centre as in COX. While the latter in addition has CuB in the form of a copper-heme group, N20 reductase has Cuz which is the site of dinitrogen formation from the substrate N20. Recently a central inorganic sulfide has been found as a ligand to copper and multiple forms of Cuz were detected in the enzyme from Paracoccus pantotrophus.134 More recently a tetranuclear copper cluster with X-S bridges was proposed as structure for Cuz..135... 

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