Nitrite reductases active site

Hsu SCN, Chang YL, Chuang W-J, et al. Copper(I) nitro complex with an anionic [HB(3,5-Me2Pz)3] ligand a synthetic model for the copper nitrite reductase active site. Inorg Chem. 2012 51 9297-9308. 

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. 

Fig. 6.9 The catalysts for denitrification. Nitrate is reduced by a molybdenum enzyme while nitrite and oxides of nitrogen are reduced today mainly by copper enzymes. However, there are alternatives, probably earlier iron enzymes. The electron transfer bct complex is common to that in oxidative phosphorylation and similar to the bf complex of photosynthesis, while cytochrome c2 is to be compared with cytochrome c of oxidative phosphorylation. These four processes are linked in energy capture via proton (H+) gradients see Figure 6.8(a) and (b) and the lower parts of Fig. 6.9 which show separately the active site of the all iron NO-reductase, and the active site of cytochrome oxidase (02 reductase).

Urata, K., and Satoh, T. (1991). Enzyme localization and orientation of the active site of dissimilatory nitrite reductase from Bacillus firmus. Arch. Microbiol. 156, 24-27. 

As the focus of this review is on copper-dioxygen chemistry, we shall briefly summarize major aspects of the active site chemistry of those proteins involved in 02 processing. The active site structure and chemistry of hemocyanin (He, 02 carrier) and tyrosinase (Tyr, monooxygenase) will be emphasized, since the chemical studies described herein are most relevant to their function. The major classes of these proteins and their origins, primary functions, and leading references are provided in Table 1. Other classes of copper proteins not included here are blue electron carriers [13], copper-thiolate proteins (metallothioneines) [17], and NO reductases (e.g., nitrite [NIR] [18] or nitrous oxide [19]). 

The latter inhibition is reversed by light. Urea inactivation-reaclivation studies showed parallel loss and recovery of nitrite and hyroxylamine reductase activities, and nitrite was shown to inhibit hydroxylamine reduction. These results have suggested that the enzyme has a common binding site for nitrite and hydroxylamine. The absorption spectra of the A. fischeri enzyme (oxidized, reduced, and reduced plus nitrite or hydroxylamine) are shown in Fig. 39. 

Achromobacter fischeri, nitrite reductase, physical properties, 277-279 Active site, lipoamide dehydrogenase, 105 Acyl hydrazides, catalase and, 379 Acyltransferase activity, glyceraldehyde-... 

Many species of bacteria also have an assimilatory nitrite reductase which is located in the cytoplasm. There is relatively little known about such enzymes but the electron donor is throught to be NADPH and the active site again has siroheme (Cole, 1988). The assimilatory nitrite reductases of both plants and bacteria use nitrite that is provided as the product of the assimilatory nitrate reductases. Nitrate is a very common natural N source for plant and bacterial growth. 

In various species of bacteria several different types of non-assimilatory nitrite reductases are found. Escherichia coli has a cytoplasmic NAD(P)H-dependent enzyme whose role seems to be detoxification of nitrite. This type of enzyme, coded for by the nirB gene, also contains siroheme as the redox active catalytic center (Cole, 1988). Additionally in E. coli, and expressed under different conditions to the cytoplasmic enzyme, is a periplasmic nitrite reductase that catalyses formation of ammonia from nitrite (Cole, 1988). This enzyme has five c-type (Figure 1) hemes per polypeptide chain one of these hemes, the catalytic site, has the unique CXXCK sequence as its attachment site (Einsle et al., 1999). Electrons reach this type of nitrite reductase, which is fairly widely distributed amongst the microbial world, from the cytoplasmic membrane electron transfer chain. The exact electron donor partner from such chains for this type of nitrite reductase is unknown (Berks et al., 1995). 

Figure 5 X-ray crystal structures of the Type 2 copper active site of nitrite reductase from Alcaligenes faecalis (a) oxidized and (b) nitrite-soaked. Panel (c) is a rotated view of (b) and shows the hydrogen-bonding network. Coordinates are from PDB entries 1AS7 and 1AS6 ...

E. M. Maes, "Structural Characterization of Iron and Copper Active Sites by Resonance Raman Spectroscopy Nitrophorin, Nitrite Reductase, and Iron-Sulfur Proteins, PhD. Dissertation, University of Houston, 2000. 

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