Blue copper protein

Plastocyanin is a copper-containing protein involved in electron-transfer. The protein is a monomer, with a molecular weight around 10,500 Daltons, and 99 amino acids in most vascular plants. It is a member of the .

In photosynthesis, plastocyanin functions as an electron transfer agent between of the from photosystem II and P700+ from photosystem I. 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.

Plastocyanin was the first of the blue copper proteins to be characterised by X-ray crystallography. The tertiary structure is a beta-barrel — common in proteins which bind to other proteins.

Although the molecular surface of plastocyanins differs for plants, algae, and cyanobacteria, the structure of the copper binding site is generally conserved. The geometry of 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 histidine residues and a sulfur atom (S₁) from a cysteine residue. A second sulfur atom (S₂) from an axial methionine residue forms the apex. The ‘distortion’ occurs in the bond lengths between the copper atom and sulfur ligands. The Cu-S₁ contact is much shorter (207 picometers) than Cu-S₂ (282 pm). The elongated Cu-S₂ bonding destabilises the Cu^II form and increases the redox potential of the protein. The blue colour (597 nm peak absorption) is due to the Cu-S₁ bond where S_pπ to Cu_dx²-y² charge transfer occurs.

In the reduced form of plastocyanin, His-87 will become protonated with a pK_a of 4.4. Protonation prevents it acting as a ligand and the copper site geometry becomes trigonal planar.

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Wikipedia