Nitrite reductase trimer

As mentioned earlier, the copper containing nitrite reductase is a trimer of identical subunits. In each subunit there is a type 1 copper which acts analogously to the c-type heme in cytochrome cd and thus is the point of entry of electron into the enzyme. The three eatalytic sites have type 2... 

Fig. 8. (a) Drawing of the trimer of nitrite reductase from Achromobacter cycloclastes. (b) Drawing of the interface between domain 1 (subunit A) and domain 2 of the adjacent symmetry-related molecule (subunit C) of nitrite reductase from A. cycloclastes. (c) Drawing of domain 1 and 3 of ascorbate oxidase. The type-1 copper is in domain 3 and the trinuclear copper center is between domain 1 and domain 3. The domains have an orientation similar to that of the corresponding domains of the nitrite reductase shown in b. The figure was produced by the RIBBON Program (S7). 

In the trimer of nitrite reductase a six-domain structure is realized, which is reminiscent of the six-domain structure of ceruloplasmin (112), which was deduced from the amino-acid sequence alignment with the other blue oxidases (101). However, the arrangement of the six gene segments in ceruloplasmin is not simply a triplication of an ancestral... 

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. 

Copper enzymes participate in two important reactions catalyzed by denitrifying bacteria. Nitrite reductases from species of Achromohactei and Alcaligene are trimeric proteins made up of 37-kDa subunits, each of which contains one type 1 (blue) copper and one type 2 (nonblue) copper. The first copper serves as an electron acceptor from a small blue pseudoazurin. LS44a second copper,... 

Nitrite reductase is a homotrimer in its native state [272]. Depending on the origin the chain lengths of the respective subunits vary between 340 and 379 amino acid residues. The trimer contains three type 1 and three type 2 copper centers. The type 1 copper centers are localized within the subunits, whereas the type 2 copper centers are coordinated by residues from two different subunits [272,273] (Fig. 33). 

Fig. 33. Type 1 and 2 copper centers of the trimer nitrite reductase. From Fenderson et al. 1991 [273] with permission...

The copper-containing nitrite reductases (Cu NiR s) also contain two distinct types of chromophore. The best characterized is that from Achromobacter cycloclastes, for which a 2.3 A resolution X-ray structure has been reported (12). The enzyme consists of an a3 trimer of 34.5 kDa subunits, each of which contains two copper atoms in distinct sites. One is an... 

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