Plastocyanin

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Plastocyanin is an important copper-containing protein involved in electron-transfer. The protein is monomeric, with a molecular weight around 10,500 Da.

Contents 1 Function 2 Structure 3 Reactions 4 References

[edit] Function

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.

[edit] Structure

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 (2.07 Å) than Cu-S₂ (2.82 Å). 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 a highly conserved tyrosine. Additionally, algal plastocyanins contain similar acidic residues but are shaped differently from those of plant plastocyanins. In cyanobacteria, the distribution of charged residues on the surface is different from eukaryotic plastocyanins and variations among different bacterial species is large. Although the acidic patches are not conserved in bacteria the hydrophobic patch is always present. These hydrophobic and acidic patches are the believed to be the recognition/binding sites for the other proteins involved in electron transfer.

[edit] Reactions

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⁺ → Cu²⁺Pc + e^-

[edit] References

1. Freeman, H. C., Guss, J. M. Plastocyanin. Handbook of Metalloproteins (2001), 2 1153-1169.

2. Lippard, S. J., Berg, J. M. Blue Copper Proteins. Principles of Bioinorganic Chemistry (1994) 237-242.

3. Sato, K., Kohzuma, T., and Dennison, C. Active Site Structure and Electron-Transfer Reactivity of Plastocyanins. J. Am. Chem. Soc. (2003) 2101-2112 Entrez PubMed 12590538