Plastocyanin is a copper-containing protein involved in electron-transfer. The protein is monomeric, with a molecular weight around 10,500 Daltons, and 99 amino acids in most vascular plants. It is a member of the plastocyanin family of copper-binding proteins.
In photosynthesis, plastocyanin functions as an electron transfer agent between cytochrome f of the cytochrome b6f complex from photosystem II and P700+ from photosystem I. Cytochrome b6f complex and P700+ are both membrane-bound proteins with exposed residues on the lumen-side of the thylakoid membrane of chloroplasts. Cytochrome f acts as an electron donor while P700+ accepts electrons from reduced plastocyanin.
Although the molecular surface of plastocyanins differs for plants, algae, and cyanobacteria, the structure of the copper binding site is generally conserved. The copper binding site is described as a ‘distorted trigonal pyramidal’. The trigonal plane of the pyramidal base is composed of two nitrogen atoms (N1 and N2) from separate histidines and a sulfur (S1) from a cysteine. Sulfur (S2) from an axial methionine forms the apex. The ‘distortion’ occurs in the bond lengths between the copper and sulfur ligands. The Cu-S1 contact is shorter (207 picometers) than Cu-S2 (282 pm). The elongated Cu-S2 bonding destabilises the CuII form and increases the redox potential of the protein. The blue colour (597 nm peak absorption) is due to the Cu-S1 bond where Spπ to Cudx2-y2 charge transfer occurs.
In the reduced form of plastocyanin, His-87 will become protonated with a pKa of 4.4. Protonation prevents it acting as a ligand and the copper site geometry becomes trigonal planar.
While the molecular surface of the protein near the copper binding site varies slightly, all plastocyanins have a hydrophobic surface surrounding the exposed histidine of the copper binding site. In plant plastocyanins, acidic residues are located on either side of the highly conserved tyrosine-83. Algal plastocyanins, and those from vascular plants in the family Apiaceae, contain similar acidic residues but are shaped differently from those of plant plastocyanins—they lack residues 57 and 58. In cyanobacteria, the distribution of charged residues on the surface is different from eukaryotic plastocyanins and variations among different bacterial species is large. Many cyanobacterial plastocyanins have 107 amino acids. Although the acidic patches are not conserved in bacteria the hydrophobic patch is always present. These hydrophobic and acidic patches are believed to be the recognition/binding sites for the other proteins involved in electron transfer.
Plastocyanin (Cu2+Pc) is reduced by cytochrome f according to the following reaction:
- Cu2+Pc + e− → Cu+Pc
After dissociation, Cu+Pc diffuses through the lumen until recognition/binding occurs with P700+. P700+ oxidizes Cu+Pc according to the following reaction:
- Cu+Pc → Cu2+Pc + e−
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