Plastocyanin is a single strand protein, mw 10,500, that plays an integral part in photosynthesis. This protein has a copper center, consisting of one copper atom surrounded by two nitrogen bonds and two sulfur bonds. This tetrahedral geometry is unusual for copper compounds in nature, but seems to minimize energies of oxidation and reduction for the copper atom at high pH. At low pH (<5), however, a second, redox inactive configuration of plastocyanin becomes prevalent. In this configuration the outermost nitrogen bond becomes weaker and longer, creating the more common trigonal planar copper geometry. P700 is ideally suited to remove electrons from plastocyanin at low pH. In fact, P700 works best at low pH when most other electron acceptors can no longer remove electrons from Cu(I) plastocyanin.
Highlighting FeaturesShow copper center and its nearest neighbors (2N and 2 S) Show amino acids surrounding copper center (2 histidines, methionine, and cysteine) Show all as strands Show as strands with Cu and nearest neighbors
At left is an image of the single strand protein Cu(II)-plastocyanin from spinach. These results were obtained by purification of the plastocyanin from plant extract, followed by vapor diffusion against a buffer solution to maintain pH at 4.4. Crystals were then analyzed by X-ray diffraction.
Electron access to the active site is by a short, hydrophobic ‘exit’ and a longer hydrophilic tunnel “entrance”. The tunnel is ideally suited to accept electrons from the thalykoid membrane protein called cytochrome F because of oppositely charged patches on the surfaces of the two proteins that counteract the energy that the electron has to have to get through the tunnel. This electron is donated to one of the sulfur groups (Cys) neighboring the copper, and wavelengths of 600 nm excite the electron enough for P700 to remove electrons from the short exit of the protein and eventually create chemical energy in the form of NADPH. When copper is deficient, this protein is also deficient which causes interveinal chlorosis in young leaves.
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Church, WB, JM Guss, JJ Potter, and HC Freeman. 1986. The crystal structure of mercury-substituted poplar plastocyanin at 1.9-Å resolution. J. Biol. Chem. 261:234-237.
Colman, PM, HC Freeman, JM Guss, M Murata, VA Norris, JAM Ramshaw and MP Venkantappa. 1978. X-ray crystal structure analysis of plastocyanin at 2.7Å resolution. Nature 272:319-324.
Guss, JM, PR Harrowell, M Murata, VA Norris, and HC Feeman. 1986. Crystal structure analyses of reduced (CuI) poplar plastocyanin at six pH values. J. Mol. Biol. 192:361-387.
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This page constructed by DD Levine as a class project for “Mineral Nutrition of Plants”, SoilSci/Botany/Horticulture 626, under the direction of Drs. P. Barak and E. Spalding, and was contributed to the and was contributed to the Virtual Museum of Minerals and Molecules