Nitrite reductases

A portion of the DNA sequence of nit-6, the Neurospora gene encoding the enzyme nitrite reductase. (JaviesD. Cotandene, University of Virginia ... 

H2O = 0.380 V. Indeed, there are only three such 6e reductions known in the whole of biology, the other two being nitrite reductase (N02 -I- 7H -h 6e ---------). NH3 -h 2H2O)... 

In 1973, the first naturally occurring isobacteriochlorin, iron-containing siroheme, was isolated1 from a sulfite reductase of Escherichia coli. Later it was also discovered in sulfite and nitrite reductases of numerous bacteria and plants.2 Iron-free sirohydrochlorins (also called factor II) were discovered in vitamin B12 producing bacteria.3-4 Together with factor III. a sirohydrochlorin methylated in the 20-position, the reduced forms of factor II and factor III were identified as biosynthetic intermediates in the biosynthesis of vitamin B12.5... 

In order to check whether the occurrence of the Rieske-type sequence motif is unique for the assimilatory nitrite reductase from Bacillus subtilis, the sequences of other assimilatory nitrite reductases were searched for the presence of the four putative ligands of Rieske-type clusters. A well-conserved sequence pattern... 

DeMoll-Decker H, JM Macy (1993) The periplasmic nitrite reductase of Thauera selenatis may catalyze the reduction of selenite to elementary selenium. Arch Microbiol 160 241-247. 

To provide a model for nitrite reductases72 Karlin and co-workers characterized a nitrite-bound complex (226) (r = 0.05)214 In an endeavor to model nitrite reductase activity, Tanaka and co-workers prepared a few mononuclear complexes (227) (r = 0.74)215 (228) (r = 0.82),216 (229) (r = 0.97),217 (230) (r = 0.16),217 (231) (r = 0.07),217 and (232) (r = 0.43 and r = 0.53)217 and studied the electrochemical reduction of N02A As a part of their activity on modeling heme-copper terminal oxidases, Holm and co-workers prepared complex (233) (r = 0.96).218 Using a sterically hindered tris(pyridylmethyl)amine, Canary et al. prepared a complex (234) (r=1.00), studied its redox behavior, and discussed various factors that may contribute to the difference (higher potential for the new complex) in the redox potential of a Cu Cu1 couple between substituted and unsubstituted ligands.2 9... 

Figure 2.7 (a) A front view of the nitrite reductase dimer with the five haems in each monomer in white, a bound Ca2+ ion in grey and Lys-133, which coordinates the active site iron of haem 1, in yellow, (b) The haem arrangement. The overall orientation corresponds to (a), with the active site located at haem 1. Reprinted with permission from Einsle et al., 1999. Copyright (1999), Macmillan Magazines Limited. 

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

The first interest in the electroreduction of N02 or NO catalyzed by metal complexes is to model the activity of nitrite reductase enzymes.327 There is also an extensive growth in studies related to the development of metal complex-based electrochemical sensors for NO determination in biological and environmental samples 328 329 Nitrate disproportionates to nitric oxide and nitrate in aqueous solution. 

Figure 6. Structural relationships between ascorbate oxidase, ceruloplasmin, nitrite reductase, and blood clotting factor VIII.

It is interesting to speculate why nitrite reductase has its type I coppers in domains 1, whereas in hCP the mononuclear copper binding sites are retained in the domains 2,4, and 6 where they are comparatively buried in the protein. One possible reason can be related to the difference in functions of the two proteins. NR has to interact with a relatively large pseudo-azurin macromolecule in order for electron transfer to take place,... 

Several copper enzymes will be discussed in detail in subsequent sections of this chapter. Information about major classes of copper enzymes, most of which will not be discussed, is collected in Table 5.1 as adapted from Chapter 14 of reference 49. Table 1 of reference 4 describes additional copper proteins such as the blue copper electron transfer proteins stellacyanin, amicyanin, auracyanin, rusticyanin, and so on. Nitrite reductase contains both normal and blue copper enzymes and facilitates the important biological reaction NO) — NO. Solomon s Chemical Reviews article4 contains extensive information on ligand field theory in relation to ground-state electronic properties of copper complexes and the application of... 

The NO/NO+ and NO/NO- self-exchange rates are quite slow (42). Therefore, the kinetics of nitric oxide electron transfer reactions are strongly affected by transition metal complexes, particularly by those that are labile and redox active which can serve to promote these reactions. Although iron is the most important metal target for nitric oxide in mammalian biology, other metal centers might also react with NO. For example, both cobalt (in the form of cobalamin) (43,44) and copper (in the form of different types of copper proteins) (45) have been identified as potential NO targets. In addition, a substantial fraction of the bacterial nitrite reductases (which catalyze reduction of NO2 to NO) are copper enzymes (46). The interactions of NO with such metal centers continue to be rich for further exploration. 

Zhang, Z., Naughton, D., Winyard, P. G., Benjamin, N., Blake, D. R., Symons, M. C. R., Generation of nitric oxide by a nitrite reductase activity of xanthine oxidase a potential pathway for nitric oxide formation in the absence of nitric oxide synthase activity. Biochem. Biophys. Res. Commun. 249 (1998), p. 767—772... 

Figure 14.1 (Left) X-ray structure of plastocyanin from poplar leaves as a ribbon diagram with the metal ion and its ligands highlighted, PDB code 1PLC (right) Type 1 Cu site in Cu(II)-nitrite reductase from Alcaligenes faecalis, PDB code 1AS6. (From Messerschmidt et al., 2001. Reproduced with permission from John Wiley Sons., Inc.)...

While nitrite reductases in many bacteria are haem proteins, some are copper containing homotrimers, which bind three Type I and three Type II copper centres. The Type I copper... 

Two-component system for nitrate Nitrate Nitrite reductase [EC 1.6.6.4] ... 

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