Nitrite reductase function

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,... 

With M. gryphiswaldense, Schuler and Bauerlein (1996) recorded an Fe uptake rate from Fe " citrate of 0.86 nmol min mg dry weight and suggested that the major portion of Fe is taken up in an energy-dependent process possibly by a reductive step (Schuler, 1999). Fukumori et al. (1997) proposed that the dissimilatory nitrite reductase of M. magnetotacticum may function as an Fe" oxidizing enzyme. Later, Fuko-mori (2000) suggested an Fe "quinate complex as the source of Fe which is subsequently reduced in the cell in a microaerobic environment at about neutral pH by the iron reductase NADH (an assimilatory enzyme). 

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. 

Deletion of the regulatory nirQ gene results simultaneously in a loss of the ability to reduce both nitrite and NO in P. stutzeri (Braun and Zumft, 1992). Nitrite reductase is synthesized and is active in vitro but nitric oxide reductase is synthesized in an inactive form. Although the exact function of the nirQ gene product is unknown, the gene encodes a protein homologous with the NtrC family of transcriptional activators (Zumft, 1993). 

Glockner, A. B., Jiingst, A., and Zumft, W. G. (1993). Copper-containing nitrite reductase from Pseudomonas aureofaciens is functional in a mutationally cytochrome cd,-free background (NirS ) of Pseudomonas stutzeri. Arch. Microbiol. 160, 18-26. 

Iwasaki, N., Noji, S., and Shidara, S. (1975). Achromobacter cycloclastes nitrite reductase. The function of copper, amino acid composition, and ESR spectra. J. Biochem. (Tokyo) 78, 355-361. 

Most cytochromes have only one heme group per polypeptide chain,112 but the 115-residue cytochrome c3 from the sulfate-reducing bacterium Desulfovibrio binds four hemes (Fig. 16-8C).104,113 115 Each one seems to have a different redox potential in the -0.20 to -0.38 V range.114 Another c-type cytochrome, also from Desulfovibrio, contains six hemes in a much larger 66-kDa protein and functions as a nitrite reductase.116... 

Campbell, W.H. Kinghom, J.R. (1990). Functional domains of assimilatory nitrate reductases and nitrite reductases. Trends in Biochemistry 15, 315-19. 

The constrained nature of the copper center in BCB domains reduces its reorganization energy, which is considered an important feature for their function in long-range electron transfer processes. They are capable of tunneling electrons, usually over 10- to 12-A distances, intramolecu-larly within the same protein (in the case of multicopper oxidases and nitrite reductases) or intermolecularly between a donor and an acceptor protein (in the case of cupredoxins) in a thermodynamically favorable environment. 

Surprisingly, its biological redox partners remain largely unknown. It has been implicated in anaerobic nitrite respiration and it has been shown that azurin can donate electrons to nitrite reductase, a function that is proposed to be carried out by another cupredoxin, pseudoazurin (see Section IV, E). On the other hand, azurin is not an inducible protein and denitrifying bacteria express azurin constitutively under aerobic conditions. 

Although nitrite reductases do not belong to the cupredoxin family, they are discussed together with pseudoazurin because of their close functional relationships). 

There are several types of nitrite reduetion reaetion in biology and the use of the eommon name initrite reductasei for the enzymes eatalysing these reactions causes endless confusion. Thus we begin this article by outlining the different types and functions of the nitrite reductases before focusing on the structure/function relationships for one type of enzyme, cytochrome cdi nitrite reductase. 

Cheesman, M., Ferguson, S. J., Moir, J. W. B., Richardson, D. J., Zumft, W. G., and Thomson, A. J., 1997, Two enzymes with a common function but different heme ligands. The optical and magnetic properties of the heme groups in the oxidised forms of nitrite reductase, cytochrome cdi, from Pseudomonas stutzeri and Thiosphaera pantotropha. Biochemistry 36 16267916276. 

The copper-containing nitrite reductase from A. cycloclastes may also have evolved from this ancestral oxidase. Nitrite reductase is a two-domain protein that functions as a trimeric molecule. During its evolution from the ancestral copper oxidase, a gene inversion must have occurred, so that domain 2 of the ancestral oxidase is now domain 1 of nitrite reductase. Domain 1 of the ancestral oxidase lost its type-1 copper but has become domain 2 in nitrite reductase after the gene inversion. 

Braker, G., Zhou, J., Wu, L., Devol, A. H., and Tiedje, J. M. (2000). Nitrite reductase genes (mVlCand nirS) as functional markers to investigate diversity of denitrifying bacteria in Pacific Northwest marine sediment communities. Appl. Environ. Microbiol. 66, 2096—2104. 

In all photoautotrophs, reduction of NOj" to NH4 is achieved in two distinct enzymatic steps (Campbell, 2001). First, assimilatory nitrate reductase (NR) catalyzes the two electron reduction from NOj" to NO2. NR is a large soluble cytoplasmic enzyme with FAD (flavin adinine dinucleotide), an iron-containing cytochrome and molybdopterin prosthetic groups, and requires NADH and/or NADPH as an electron donor (Guerrero et al, 1981). Functional NR is in the form of a homodimer and therefore requires two atoms of iron per enzyme. Following transport into the chloroplast, NO2 undergoes a 6 e reduction to NH4 via assimilatory nitrite reductase (NiR). NiR, a soluble chloroplastic enzyme, contains five iron atoms per active enzyme molecule, and requires photosynthetically reduced ferredoxin as an electron donor (Guerrero et al., 1981). 

Still another nitrite reductase, cytochrome c NIR, contains five heme groups, only one of which functions as the active site. A combination of calculations and crystallographic studies has suggested a mechanism in which nitrite replaces a water molecule on one side of the Fe(II) heme (a lysine N is on the opposite side), one of the oxygens of N02 is protonated, and the N — 0 bond is broken with loss of H2O, leaving a linear Fe(III)—NO species with a low-spin Fe(III). Addition of two electrons and H" " leads to Fe HNO, which is then reduced to Fe H2NOH. Yet another electron and another allow release of H2O and formation of an Fe NH3 complex. Release of ammonia and a final electron and water addition complete the cycle. Overall, six electrons and seven hydrogen ions react with the nitrite ... 

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