Pseudoazurin copper site

Fig. 4. (a) Copper site in pseudoazurin. (b) Ribbon drawing of the pseudoazurin backbone. (c and d) Schematic of pseudoazurin topology. 

The first class is cupredoxins—single-domain blue copper proteins composed of only one BCB domain. These proteins include plastocyanin, azurin, pseudoazurin, amicyanin, auracyanins, rusticyanin, halocyanin, and sulfocyanin (see Section IV). Plantacyanin of the phytocyanin family (Section V), subunit II of the cytochrome c oxidase, and the recently characterized nitrosocyanin also fall into this class. The last two are single BCB domain polypeptides closely related structurally to cupredoxins, but harboring, respectively, a binuclear copper site known as CuA and a novel type of copper-binding site called red (see Sections IX and X). 

The type I copper sites function as electron transfer centers in the blue copper proteins and in multicopper enzymes, particularly oxidases (33). They are characterized by their intense blue color, their unusually small A values, and their very positive redox potentials (Table II). X-ray crystal structures of several blue copper proteins have been determined, notably plastocyanin (34), azurin (35), cucumber basic blue protein (36), and pseudoazurin (37). The active site structures show marked similarities but also distinct differences (Fig. 8). 

The physiological electron donors for NIR are either the cupredoxins pseudoazurin or azurin depending on the organism. Pseudoazurin, for example, reduces the type-1 copper and subsequently the electron is transferred to the type-2 copper site, which is also the binding site for nitrite. Nitrite is bound to the type-2 copper site as demonstrated by electron nucleus double resonance studies on NIR from Achromohacter xylosoxidans and in a crystal structure of the complex between nitrite and NIR from A. cycloclastes The crystal structure of the complex shows that nitrite binds asymmetrically with the oxygens toward the copper. 

Blue copper proteins, 36 323, 377-378, see also Azurin Plastocyanin active site protonations, 36 396-398 charge, 36 398-401 classification, 36 378-379 comparison with rubredoxin, 36 404 coordinated amino acid spacing, 36 399 cucumber basic protein, 36 390 electron transfer routes, 36 403-404 electron transport, 36 378 EXAFS studies, 36 390-391 functional role, 36 382-383 occurrence, 36 379-382 properties, 36 380 pseudoazurin, 36 389-390 reduction potentials, 36 393-396 self-exchange rate constants, 36 401-403 UV-VIS spectra, 36 391-393 Blue species... 

The first crystal structure information on a blue copper protein, for poplar plastocyanin in the Cu(II) state, was published in 1978 (2, 3). Since then, the Cu(I) state and related apo and Hg(II) substituted forms (5, 6), the green algal plastocyanin from Enteromorpha prolifera [Cu(II)] (7), azurin from Alcaligenes denitrificans [Cu(II) and Cu(D] (8, 9), azurin from Pseudomonas aeruginosa [Cu(II)] (10, 11), as well as pseudoazurin from Alcaligenes faecalis S-6 (12), and the cucumber basic protein, both in the Cu(II) state, have been published (13), making this one of the best-documented class of proteins. In addition, information as to three-dimensional structure in solution has been obtained from two-dimensional NMR studies on French bean and Scenedesmus obliquus plastocyanins (14,15). This review is concerned in the main with the active site chemistry. Other recent reviews are listed (16-20). 

A final point concerns the variability in number of amino acids separating the residues coordinating the Cu active site, and the possible controlling influence this might have. Available information is summarized in Table VI. In the case of azurin and pseudoazurin, there is the additional influence of the peptide C=0 on coordination at the Cu. All of the blue copper proteins have similar features, with coordination of the copper to one His from the first part of the sequence, and three other residues from the latter part. At present there appears to be no... 

The T1 center serves as the electron uptake site from azurin or pseudoazurin (74). Binding and reduction of substrate nitrite takes place at the T2 site. Hence, as in the blue oxidases discussed above, the internal T1 T2 ET in this copper... 

The metal-binding site structures of pseudoazurin and auracyanin are similar to that of other cupredoxins, with one cysteine and two histidines forming a trigonal plane and a weak axial methionine to complete the distorted tetrahedral geometry. The Cu-S (Met) distance (2.76 A) in pseudoazurin is shorter than those of blue copper centers in plastocyanin and azurin, and is still longer than those in plantacyanin and NiR (see Table 4). 

---