Stellacyanin

Stellacyanin’s spectroscopic properties help us differentiate it from plastocyanin, which is another monocopper blue protein found in plants. [2] It is a 20kDa protein whose structure is made up of beta strands forming two beta sheets to form a Greek key beta barrel with variable alpha helical structure. The copper binding domain of the protein is located at the amino-terminal end, while the carboxyl-terminal end is rich in hydroxyproline and serine residues, typical of proteins associated with cell walls of plants. In addition, it is also heavily glycosylated.[1][3][4] The copper is tetrahedrally coordinated by a cysteine, 2 histidines, and a glutamine residue. The glutamine residue takes place of a methionine ligand typically found in other blue copper proteins.[1] In addition, electron transfer rates for stellacyanin are faster than for other type I copper proteins suggesting stellacyanin is more solvent accessible at the active site.[1] The exact function of stellacyanin is unknown. However, given the fact that type I copper proteins are involved in electron transfer and stellacyanin appears to be associated with the plant cell wall, it is suggested that it is involved in oxidative cross-linking reactions to build polymeric material making up the cell wall.[1][3] Cell wall structural glycoproteins contain hydroxyproline and serine-rich sequence domains which are found in stellacyanins. [2]

Stellacyanins are characterized by their uniquely low redox potentials, as low as +180 mv and reach up to +280 mV. Other blue copper protein redox potentials start around +310 mV and reach up to +680 mV.[2] A mutant cucumber stellacyanin was created by replacing the glutamine axial ligand (a ligand which all other blue proteins contain) with a methionine (Q99M) and purified. This was achieved through Polymerase Chain Reaction (PCR). The mutated stellacyanin calculated redox potential was +420 mV, which much higher than the redox potential of the stellacyanin found in nature (without methionine) at +260 mV. Stellacyanins are most involved in redox reactions of plants that take place during a defense response, and formation of lignin. [2]