Purple CuA center

Purple Cua centers display an intense purple color with strong electronic absorption bands around 480 (e ax--5,000 M cm ) and 530 nm (e ax--4,000 M cm l (Table 3).25-28,98-ioi Another moderately strong absorption around 800 nm (e ax" F600M cm ) has also been observed. They are found in cytochrome c oxidase a terminal oxidase in the... 

Table 3 Electronic absorption data for selected purple Cua centers. 

Despite a poor amino acid sequence homology, the mononuclear cupredoxin copper and purple Cua centers share strong structural homology of the cupredoxin fold as indicated by amino acid sequence alignment. This similarity has been confirmed by the availability of the crystal structures of proteins containing the purple Cua sites in native N2OR, soluble domain... 

Table 5 Metal-ligand bond distances of purple Cua centers (A). 

In addition to the intense colors, the blue copper, green copper, and purple Cua centers also exhibit unusually narrow copper hyperfine constants (24 < 80 x 10 " cm ). It was proposed that... 

A third type of copper center, first recognized in cytochrome c oxidase (see Fig. 18-10) is called CuA or purple CuA. Each copper ion is bonded to an imidazole and two cysteines serve as bridging ligands. The two copper ions are about 0.24 nm apart, and the two Cu2+ ions together can accept a single electron from an external donor such as cytochrome c or azurin to give a half-reduced form.521a/b... 

One interesting feature of cupredoxins is that members of its family display a variety of intense and beautiful colors, from blue (e.g., plastocyanin and azurin), to green (e.g., plantacyanin and some nitrite reductases), to red (e.g., nitrosocyanin), to yellow (e.g., some model cupredoxin proteins and compounds), and to purple (e.g., Cua center from cytochrome c oxidase and nitrous oxide reductase). This rainbow of colors makes cupredoxins both fun to work with and challenging to study. The structure and function of each of the cupredoxin and structurally related centers will be described below and the origin of the color will be explained. 

The purple color of the Cua center originates from two S-to-Cu CT transitions of approximately equal intensity. In a dinuclear center like Cua, the valence d orbitals split into pairs with phases that are bonding f) and antibonding (f ) with respect to metal-metal interactions (see Figure 7). The energy separation of the two orbitals, related to the electronic coupling matrix element between the two copper ions see Section 8.4.8.3.3), is determined by... 

Figure 10 Alteration of the genetic code for incorporation of non-natural amino acids, (a) In nonsense suppression, the stop codon UAG is decoded by a non-natural tRNA with the anticodon CUA. In vivo decoding of the UAG codon by this tRNA is in competition with termination of protein synthesis by release factor 1 (RFl). Purified in vitro translation systems allow omission of RF1 from the reaction mixture, (b) A new codon-anticodon pair can be created using four-base codons such as GGGU. Crystal structures of these codon-anticodon complexes in the ribosomal decoding center revealed that the C in the third anticodon position interacts with both the third and fourth codon position (purple line) while the extra A in the anticodon loop does not contact the codon.(c) Non-natural base pairs also allow creation of new codon-anticodon pairs. Shown here is the interaction of the base Y with either base X or (hydrogen bonds are indicated by red dashes).

The other copper-only binuclear centre to be considered is the CuA or purple copper complex. It is part of the terminal oxidase in mitochondrial respiration, cytochrome c oxidase (COX). Its EPR signature, a seven-line spectrum, has since long been known to be different from the classes type 1 to 3 and arises from two copper ions in a 1.5 valence (or mixed valence) state, first proposed from EPR-analysis of a similar center in nitrous oxide (N20) reductase. There is a close correspondence between the blue and purple states of copper since each of the two copper ions in CuA can be considered as being structurally related to the mononuclear blue site coordination. 

Reduction of N20 to N2 by bacteria (Eq. 18-30, step d) is catalyzed by the copper-containing nitrous oxide reductase. The purple enzyme is a dimer of 66-kDa subunits, each containing four atoms of Cu.353 It has spectroscopic properties similar to those of cytochome c oxidase and a dinuclear copper-thiolate center similar to that of CuA in cytochrome c oxidase (p. 1030). 

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