Phytocyanins stellacyanin

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). 

The third class consists of proteins that are composed of one or more BCB domains fused to a sequence domain(s) characteristic of evolutionarily unrelated protein families. Such a mosaic domain organization has been found in the phytocyanin protein family, stellacyanins, uclacyanins, and the recently characterized dicyanins (Section V) in blood coagulation factor VIII (Section VIII) and in nitrous oxide reductase (Section IX). 

It is also important to note that plants house a large group of phytocyanin-related proteins (22 and 24 in Arabidopsis and rice, respectively) that have extensive sequence identity and identical domain architecture to the stellacyanins and uclacyanins but lack the copper-hgating amino acid residues that are crucial for the formation of the type 1 copper site. These copper-free proteins, known as early noduhns, share a common ancestor with metazoan ephrins, which are involved in multiple aspects of embryonal development." " ... 

At this writing, the three-dimensional sttuctures of eight different naturally occurring type 1 copper proteins are known. These include the cupredoxins plastocyanin at 1.33 A resolution (pdb code 1 PTC), azurin at 1.8 A (pdb code 2AZA), pseudoazurin at 1.55 A (pdb code IPAZ), amicyanin at 1.3 A (pdb code lAAC), auracyanin at 1.55 A (pdb code IQHQ), rusticyanin at 1.9 A (pdb code IRCY), and the phytocyanins cucumber basic protein at 1.8 A (pdb code2CBP), and stellacyanin at 1.6 A (pdb code IJER) Atomic coordinates for these and all other single-domain type 1 copper proteins are available from the Research Collaboratory for Structural Bioinformatics (RCSB) Protein Data Bank (PDB) and can be accessed online at www.rcsb.org/pdb/. 

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... 

The physiological functions of the phytocyanins are also currently unknown. This family of proteins consists of stellacyanin (20 kD) [97, 101], umecyanin (14 kD), the basic blue protein (from cucumber = plantacyanin) (8 kD), and cusacyanin (molecular masses see [17]). 

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]. 

Stellacyanins, a subclass of phytocyanins occurring exclusively in plants, have a glutamine as axial ligand instead of methionine. The coordination of the copper can be described as trigonal pyramidal with a short bond distance of 2.2 A from the copper to the OEl atom of the axial glutamine ligand. " This is the shortest axial bond distance observed in cupredoxins so far. 

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. 

Stellacyanins are different from other phytocyanins and cupredoxins by the nature of the axial ligand coordinated to the copper the axial ligand is a conserved glutamine instead of methionine. They also exhibit lower reduction potentials (180-280 mV vs. NHE) than other cupredoxins, and undergo redox reactions with small inorganic complexes or at electrodes at unusually fast rates.The stellacyanin subfamily includes stellacyanins from lacquer tree Rhus Vernicifera and cucumber Cucumis sativus, mavicyanin from green zucchini, and umecyanin from horseradish root. 

---