Sandbox 143

From Proteopedia

Plastocyanin from a Green Alga, Enteromorpha prolifera

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Overview

Plastocyanin is an important copper-containing protein involved in photosynthesis by all higher plants and some algae namely by Enteromorpha prolifera. Plastocyanin extracted from this alga, was the first algal blue copper protein characterized by X-ray crystallography and one of the best characterized electron transfer protein of the photosynthetic apparatus. This protein was intensively studied between 1981 and 1994 because of these particular spectroscopic and electronic properties. The high resolution structural analysis by molecular replacement in 1989 provided an accurate description of the structure of this protein. J. Mol. Biol. Plastocyanin is an electron donor localized in the intern membrane of thylakoïd in chloroplast. This is a monomeric protein with a single polypeptide chain of 98 amino acids and one copper atom. The molecular weight of plastocyanin is around 10,500 Daltons.

Structure

Residue distribution

Plastocyanin extracted from a Green Alga, Enteromorpha prolifera, has a β-sandwich structure as a slightly flattened cylinder with approximate dimensions 40 Å × 32 Å × 28 Å. Nature This β-sandwich is composed of two β-sheets (I and II) separated by a hydrophobic core. Turns on the two β-sheets occur between residues 42 to 45 and 47 to 50. Seven strands (1 to 4 and 6 to 8) of the polypeptide backbone have substantial β character and contribute to the β-sheets. The 5 strand has no β character and formed a helical segment. The symmetry of this molecule related on an van der Waals’ bond on the northern loop between strands 3 and 4 the side-chain of Pro36 which contacts Gln 68. This plastocyanin includes important “acidic patch” which is localized between residues 59 to 61 and 42 to 45. Eur.J. Biochem. This acidic patch is significant in electron transfer namely facilitating electrostatic recognition of her redox partner( see Role in Photosynthesis). This molecule has 111 solvent sites and 16 intermolecular hydrogen bonds.

Ligand

Ion copper in the oxidized state 2 + is localized at one end of the molecule, 6 Å below the surface. The copper atom is in the core of a hydrophobic patch composed of residues His-37, Cys-84, His-87 and Met-92. This copper binding site has a distorted trigonal pyramidal shape. The base of the pyramide is composed of one sulfur from a cysteine and two nitrogen atoms from two different histidins and the apex is formed by one sulfur from a methionin. The distortion occurs on the bond between the copper and the sulfur atom. Plastocyanin in the reduced form with ion copper in the form +1 has a different copper binding site shape. His-87 a residue of the hydrophobic patch will be protonated and the copper site has a trigonal planar structure.

Role in Photosynthesis

Context

In the sunlight reaction, in photosynthesis, plastocyanin is an important electron donor to the Photosystem I (P700).Thanks to its hydrophobic surface, plastocyanin is localised in the intern membrane of the thylakoid in chloroplasts. Its redox potential, about 370 mV, has allowed to determine the place of plastocyanin in the electron transport chain (between the photosystem II and the photosystem I): between the cytochrome b₆f complex and the photosystem I. Plastocyanin receive an electron from the cytochrome b₆f complex and give up its electron to the photosystem I Journal of Bioenergetics and Biomembranes .

Electron Transfer Mechanism

The copper atom bound to the plastocyanin is in the shape Cu²⁺. This shape of plastocyanin is reduced by the cytochrome b₆f according to the following reaction :

Cu²⁺Pc + e^- → Cu⁺Pc

The electron is given up by the cytochrome b₆f and transforms plastocyanin in shape Cu²⁺ into plastocyanin in shape Cu⁺. Then this plastocyanin diffuses through the lumen of thylakoid (remember its localisation in intern membrane of the thylakoid) until the recognition/binding occurs with the photosystem I. Photosystem I (P700) oxidizes Cu⁺Pc according to the following reaction :

Cu⁺Pc → Cu²⁺Pc + e^-

P700 become P700⁺. Photosystem I, now actived, can produces NADPH. This NADPH will be used in the dark reaction of photosynthesis.

References

1.Coller, C.A., Guss, J.M., Sugimura, Y., and Yoshizaki, F. (1989) Crystal Structure of Plastocyanin from a Green Alga, Enteromorpha prolifera, J. Mol. Biol. (1990) 211, 617-632.[[1]]

2.Colman, P.M., Freeman, H.C., Guss, J.M., Murata, M., Norris, V.A., Ramshaw, J.A.M. and Venkatappa M.P.(1978)Nature (Lond.) 272, 319-324. [[2]]

3.Simpson J. R., Moritz, L.R., Nice, E.C., Grego, B. and Yoshizak, F. (1986)Complete amino acid sequence of plastocyanin from a green alga, Enteromorpha prolifera, Eur.J. Biochem. 157, 497-506. [[3]]

4.Cookson, D. J., Hayes, M. T. and Wright, P. E. (1980) Nature (Lond.) 283, 682-683. [[4]]

5.Handford, P. M., Hill, H. A. O., Lee, R. W.-K., Henderson, R.A. and Sykes, A. G. (1980) J. Inorg. Biochem. 13, 83-88.[[5]]

6.Chothia and Lesk, 1982; Guss and Freeman, 1983.

Proteopedia Page Contributors and Editors

Mathilde BICHELBERGER and Morgane RAPHALEN