Blue copper proteins phytocyanins

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). 

Solvent is usually excluded from the blue copper site, which is buried 6 A inside the protein, having only the His ligand from the copperbinding loop exposed to the surface. The phytocyanins, stellacyanin and plantacyanin (cucumber basic protein), are exceptions, in which both His ligands are solvent exposed and the copper ion is only 3 A beneath the protein surface. This situation makes the copper center in this family of blue copper proteins more accessible to low-molecular-weight solutes (see Section V). 

Crystal structures of three phytocyanins are currently available. Two are for plantacyanins, from cucumber (also known as cucumber basic protein) (Guss et al., 1988, 1996) and from spinach (Einsle et al., 2000), and one is for the recombinant BCB domain of cucumber stella-cyanin (Hart et al., 1996). The three proteins display folding topology identical to one another, suggesting that phytocyanins fold into a uniform structure, which can be designated as a phytocyanin fold. As a historical note, the crystallization of the cucumber basic protein and its preliminary crystallographic data were reported in 1977, before any structure of a blue copper protein was available (Colman et al., 1977). However, the structure was solved in 1988 only by application of the then newly... 

Cys residues of the 4-5 and 7-8 loops. The presence of this disulfide bond is one of the factors that distinguishes phytocyanins from other blue copper proteins. 

Nersissian AM, Immoos C, Hill MG, Hart PJ, Williams G, Herrmann RG, Valentine JS (1998) Uclacyanins, stellacyanins, and plantacyanins are distinct subfamilies of phytocyanins plant-specific mononuclear blue copper proteins. Protein Sci 7 1915-1929... 

Phytocyanins are a family of blue copper proteins found exclusively in plants.They are further classified into three subfamilies plantacyanins (PNC), stellacyanins (STC), and uclacya-nins (UCC)." Spectroscopic properties of uclacyanins are closer to those of blue copper centers and thus they were covered in Section 8.4.2.3. The function of phytocyanins has not been determined conclusively. 

With the exception of the plantacyanins, phytocyanins are chimeric proteins in their mature forms. They are composed of two structurally distinct sequence motifs, a 100-109-amino acid blue copper domain followed by a domain that varies in length between 30-220 amino acids, lacks any obvious consensus sequence, but resembles heavily glycosylated arabinogalactan proteins (AGP). Where such AGP domains are present, they are followed by a hydrophobic sequence predicted to provide a signal for the attachment of a GPI moiety, which anchors the protein to the cell surface (Figure 2). This cell-surface attachment of phytocyanins... 

The phytocyanins differ from the other small blue proteins in that the normal methionine ligand of the copper ion is replaced by a glutamate residue [24]. The phytocyanins as well as auracyanin are also characterized by their high extent of glycosylation which, in some cases, doubles the weight of the protein [68,71-73],... 

The type 1 copper center of the phytocyanins differs from that of the other small blue proteins in that the methionine ligand is replaced by a glutamine [24]. The redox potential of stellacyanin from Rhus vernicifera is 184 mV [68]. 

The central copper ion of the auracyanins is probably coordinated by two histidines, one cysteine, and a methionine residue. The auracyanins are unique among the small blue proteins in that they possess a methionine and a glutamine residue (see phytocyanins) which both could act as the fourth ligand coordinating the central copper ion. This copper center is surrounded by a hydro-phobic environment similar to that of the other small blue proteins [68]. Amino acid sequence comparisons place auracyanin in a phylogenetic tree at approximately equal distances from azurin and plastocyanin [68,92]. 

---