Cupredoxins proteins

II. Cupredoxins Proteins That Bind Only Type I Copper. 148... 

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. 

Blue (or type 1 ) copper proteins (or cupredoxins) are important components of biological electron transfer processes in many organisms ranging from bacteria to animals, from fungi to plants.56 They are characterized by ... 

Another interesting blue protein is stellacyanin (FW = 20 000) from the Japanese lacquer tree Rhus vernicifera, in which, with respect to the other cupredoxins, glutamine replaces the methionine ligand.64 Stellacyanin also bears an overall positive charge (p/=9.9). It, therefore, gives a reversible Cu(II)/Cu(I) response at a glassy carbon electrode in aqueous solution (pH 7.6).61 The formal electrode potential of the Cu(II)/Cu(I) reduction (E01 = + 0.18 V vs. NHE) is the lowest among cupredoxins. 

Finally, we examine azurin, a blue protein (FW = 14 000) devoted to bacterial electron transport, the copper centre of which has a penta-coordinate trigonal bipyramidal geometry, at variance with all the other cupredoxins, Figure 39.73... 

The crystal structure of the pseudoazurin from Alcaligenes faecalis S-6 sometimes referred to as the blue protein (also as cupredoxin), has been reported to 2.0 A [74]. The protein folds in /3-sandwich which is described as being similar to plastocyanin and azurin. 

Cucumber basic blue protein (Cbp) is a protein without known function, also known as cusacyanin or plantacyanin. Its structure (Guss et al., 1988) completes the repertoire of cupredoxins with known structures. The topology of its folding is similar (Fig. 5) to those of plastocyanin and azurin, as might have been expected from sequence similarities and... 

Spectra, but, in general, leaves the copper site the most exposed of the four cupredoxins. The sequence of Cbp is quite similar to that of stella-cyanin. Stellacyanin is a plant protein, also of unknown function, having visible spectra characteristic of type I copper, but lacking the methionine ligand found in all other type I proteins. A disulfide bond has been suggested as a potential copper ligand in stellacyanin the Cbp has both a methionine and the disulfide, so that prior to the structure determina-... 

A recently characterized single-domain copper protein, auracyanin (Trost et al., 1988), is a dimeric protein which has a visible spectrum more like that of the A. faecalis cupredoxin (pseudoazurin, subgroup II see Table II) than that of either azurin or plastocyanin, but, because of its cysteine content and rhombic EPR, it has been put in the other class in Table II. 

In summary, then, comparison of the four known cupredoxin structures helps reveal the essence" of this kind of protein. Schematically (see Fig. 6), it can be seen that the jS-barrel structure is composed of three parts an amino-terminal portion, a middle portion, and a carboxy-terminal portion (a functional grouping which probably is not relevant to the folding of the polypeptide). One part of the amino-terminal loop... 

The nature of the copper in these proteins is not totally clear. Dooley et al (1988) reported that the Achromobacter protein may have two kinds of type I sites in a total of three copper sites per dimeric protein, while the A. faecalis protein was reported to be a tetrameric protein with both type I and type II coppers (KakutanielaZ., 1981). Interestingly, the Achromobacter protein is green. Both of these nitrite reductases accept electrons from a cupredoxin. 

Tab. 5 Potential values for copper(ll/l) proteins (principally cupredoxins) in aqueous solution (all values are presumed to be for 25 °C, p, 0.1 M)...

Hart PD et al (1996) A missing link in cupredoxins crystal structure of cucumber stellacyanin at 1.6 A resolution. Protein Sci 5 2175-2183 PDBID 1JER... 

Several papers utilizing 3 mm probe capabilities were also published that delved into the area of protein structures. Hepatocyte nuclear factor 4 was studied by a group lead by Williams.155 It is interesting to note that a 2.7 A X-ray study showed a fatty acid in the pocket of HNF4y. Gas chromatography coupled mass spectrometry (GC/MS) and 3 mm NMR studies of extracts from purified HNF4x led to the identification of mixtures of saturated and crs-monounsaturated Ci4 to Qg fatty acids. Bertini and co-workers156 utilized 3 mm probe capabilities in the determination of the solution structure of CopC, a cupredoxin-like protein involved in copper homeostasis. 

Fig. 2. Proteins that bind Cu(I). (a) Saccharomyces cerevisiae metallothionein (Cupl, pdb code laqr). Cupl binds up to seven Cu(I) ions (medium gray spheres) using 10 cysteine sulfur atoms (light spheres) in a polythiolate cluster (Peterson et al., 1996). All bonds shorter than 2.8 A are shown as dotted lines, (b) Cucumis sativus stellacyanin (pdb code Ijer). Both Cu(l) and Cu(ll) are bound by a pseudo-trigonal planar arrangement of (His)2Cys residues with an axial Gin ligand (Hart et al., 1996). In other cupredoxins such as plastocyanin, a Met residue is the axial ligand (Adman, 1991).

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 constrained nature of the copper center in BCB domains reduces its reorganization energy, which is considered an important feature for their function in long-range electron transfer processes. They are capable of tunneling electrons, usually over 10- to 12-A distances, intramolecu-larly within the same protein (in the case of multicopper oxidases and nitrite reductases) or intermolecularly between a donor and an acceptor protein (in the case of cupredoxins) in a thermodynamically favorable environment. 

With a few exceptions, cupredoxins are freely diffusible proteins. They accept and donate a single electron to their redox partners during which process the protein-bound copper oscillates between Cu(II) and Cu(I). The cupredoxin and its redox partners form a transient complex that will dissociate upon a successful electron transfer act. Therefore, the protein-protein interactions between a diffusible cupredoxin and its redox partner may not be as specific as one might expect. Indeed, the binding... 

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