Alcaligenes faecalis nitrite reductase

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.)...

Kakutani, T., Watanabe, H., Arima, K., and Beppu, T. (1981). Purification and properties of a copper-containing nitrite reductase from a denitrifying bacterium, Alcaligenes faecalis strain S-6. J. Biochem. (Tokyo) 89, 453-461. 

Murphy, M. E. P., Turley, S., and Adman, E. T., 1997, Structure of nitrite bound to copper-containing nitrite reductase from Alcaligenes faecalis, J. Biol. Chem. 272 28455928460. 

Figure 4 X-ray crystal structure of oxidized copper-containing nitrite reductase from Alcaligenes faecalis at 2.0 A resolution. Coordinates are from PDB entry 1AS7 ...

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 ...

Until now, pseudoazurin has only been found in bacteria, e.g., in the denitrifying bacteria Alcaligenes faecalis and Achromobacter cycloclastes. It is the electron donor of the green copper-protein nitrite reductase which catalyses the reduction of nitrite (NO2) to nitrogen monoxide (NO) [86-89]. The physiological electron donor of pseudoazurin is as yet unknown [70]. Pseudoazurin has a molecular mass of approximately 13.5 kD and a chain length of about 123 amino acid residues [88]. The additional amino acids, as compared with plastocyanin, form the C-terminal end of the protein (Fig. 14). The pseudoazurins have redox potentials of about 230 mV [90,91]. 

Structure (of nitrite reductase from Alcaligenes faecalis). The subunits of nitrite reductase contain two domains. Each domain consists of a /1-barrel structure, similar to that in the small blue proteins [272]. The type 1 copper center is embedded in one of these /1-barrels and is coordinated by the ligands His 95 (domain 1), Cys 136, His 145, and Met 150 (domain 2) [26]. The type 2 copper center is coordinated by a water molecule and three histidine ligands His 100 and His 135 (subunit x) as well as His 306 (subunit y). It is situated at the interface of two subunits [272], The water molecule is displaced from the type 2 copper ion upon nitrite binding (Fig. 34). Although both copper ions are coordinated by neighboring residues, they are located approximately 12.5 A apart, which prevents a direct electron transfer [272]. 

Most proteins with type 1 copper centers are blue, although the nitrite reductases from Achromobacter cycloclastes, Alcaligenes faecalis, and Pseudomonas aureofaciens, are green [26,27]. This is probably caused by a distortion of the type 1 copper center, although the interrelation of distortion and absorption properties of the copper centers have not yet been clarified [27]. 

Fig. 34. The surrounding structure of the type 1 and type 2 copper-binding centers of nitrite reductase from Alcaligenes faecalis. The structure is equivalent to that of Achromobacter. From Averill 1996 [26] with permission...

Nitrite reduction to nitric oxide is catalyzed by dissimilatory nitrite reductase. The enzymes purified from Alcaligenes faecalis (Iwasaki and Matsub-ara, 1971), Pseudomonas aeruginosa (Walker and Nicholas, 1961), andMj-crococcus denitrificans (Newton, 1969) have been shown to contain c d-type cytochrome. The nitrite reductase from Achromobacter cycloclastes does not... 

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