Nitrite oxidase

Nitrite oxidase Bacterial unknown LMo( )h Unknown Unknown 292,293... 

Although molybdenum and tungsten enzymes carry the name of a single substrate, they are often not as selective as this nomenclature suggests. Many of the enzymes process more than one substrate, both in vivo and in vitro. Several enzymes can function as both oxidases and reductases, for example, xanthine oxidases not only oxidize purines but can deoxygenate amine N-oxides [82]. There are also sets of enzymes that catalyze the same reaction but in opposite directions. These enzymes include aldehyde and formate oxidases/carboxylic acid reductase [31,75] and nitrate reductase/nitrite oxidase [83-87]. These complementary enzymes have considerable sequence homology, and the direction of the preferred catalytic reaction depends on the electrochemical reduction potentials of the redox partners that have evolved to couple the reactions to cellular redox systems and metabolic requirements. 

Tanaka Y, Fukumori Y, Yamanaka T (1982) The complete amino acid sequence of Nitrobacter agilis cytochrome c-550. Biochim Biophys Acta 707 14-20 Tanaka Y, Fukumori Y, Yamanaka T (1983) Purification of cytochrome o,c, from Nitrobacter agilis and characterization of nitrite oxidase system of the bacterium. Arch Microbiol 135 265-271... 

Cell-free Nitrobacter extracts were shown by Aleem and Alexander (1958) to be able to oxidize nitrite to nitrate. Subsequently, Aleem and Nason (1959) reported that the nitrite-oxidizing activity resided in a cytochrome-containing particle designated as nitrite oxidase. Nason (1962) states that their data implicated the action of the nitrite oxidase system to involve the enzymatic transfer of electrons from nitrite to molecular oxygen via cytochrome c- and cytochrome oxidase-like components. Iron is specifically required in the oxidation process. 

Nitrite oxidase and superoxide dismutase activities of MnTMPyP... 

The final step in the nitrification process is the oxidation of nitrite to nitrate by the enzyme nitrite oxidase (NoX) or, more properly nitrite dehydrogenase, via the reaction ... 

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... 

Others oxotransferases) (2 pyranopterins bonded to Mo) (8-10 members) Nitrate reduction dissimilatory terminal respiratory oxidase Pyridoxal oxidase Xanthine dehydrogenases Pyrogallol transhydrolase Nitrate to nitrite... 

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).

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

Zhang, Z., Naughton, D. P., Blake, D. R., Benjamin, N., Stevens, C. R., Winyard, P. G., Symons, M. C. R., Harrison, R., Human xanthine oxidase converts nitrite ions into nitric oxide. Biochem. Soc. Trans. 25 (1997), p.524S... 

T. A., Eisenthal, R., Harrison, R., Reduction of organic nitrites to nitric oxide catalysed by xanthine oxidase possible role in metabolism of nitrovasodilators. Biochem. Biophys. Res. Commun. 270 (2000), p. 880-885... 

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