Plastocyanins

Electron Transport Between Photosystem I and Photosystem II Inhibitors. The interaction between PSI and PSII reaction centers (Fig. 1) depends on the thermodynamically favored transfer of electrons from low redox potential carriers to carriers of higher redox potential. This process serves to communicate reducing equivalents between the two photosystem complexes. Photosynthetic and respiratory membranes of both eukaryotes and prokaryotes contain stmctures that serve to oxidize low potential quinols while reducing high potential metaHoproteins (40). In plant thylakoid membranes, this complex is usually referred to as the cytochrome b /f complex, or plastoquinolplastocyanin oxidoreductase, which oxidizes plastoquinol reduced in PSII and reduces plastocyanin oxidized in PSI (25,41). Some diphenyl ethers, eg, 2,4-dinitrophenyl 2 -iodo-3 -methyl-4 -nitro-6 -isopropylphenyl ether [69311-70-2] (DNP-INT), and the quinone analogues,... 

Structure and electron transfer reactivity of the blue copper protein, plastocyanin. A. G. Sykes, Chem. Soc. Rev., 1985,14, 283 (117). 

Ribbon structures of two redox proteins, cytochrome c (a) and plastocyanin (b). The blowups show the active sites where transition metal atoms are located. 

Blue copper proteins. A typical blue copper redox protein contains a single copper atom in a distorted tetrahedral environment. Copper performs the redox function of the protein by cycling between Cu and Cu. Usually the metal binds to two N atoms and two S atoms through a methionine, a cysteine, and two histidines. An example is plastocyanin, shown in Figure 20-29Z>. As their name implies, these molecules have a beautiful deep blue color that is attributed to photon-induced charge transfer from the sulfur atom of cysteine to the copper cation center. 

Fragata, M., Ohnishi, S., Asada, K., Ito, T. and Takahashi, M. (1984) Lateral diffusion of plastocyanin in multilamellar mixed-hpid bilayers studied by fluorescence recovery after photobleaching. Biochemistry, 23, 4044—4051. 

In the blue, Type I copper proteins plastocyanin and azurin, the active-site structure comprises the trigonal array [CuN2S] of two histidine ligands and one cysteine ligand about the copper,... 

Copper proteins present interesting problems of structure for the copper(I) oxidation state. They are difficult to probe in detail, and what we do know of them suggests they are rarely regular or predictable.58 In plastocyanin the copper(I) coordination sphere is made up of three strongly... 

Bai, Y W., Chung, J., Dyson, H. J., and Wright, P. E. (2001). Structural and dynamic characterization of an unfolded state of poplar apo-plastocyanin formed under nondenaturing conditions. Protein Sci. 10, 1056-1066. 

Fig. 5.8. When photosystem II is activated by absorbing photons, electrons are passed along an electron-acceptor chain and are eventually donated to photosystem I and finally to NAPD+. Photosystem II is responsible for the photolytic dissociation of water and the production of atmospheric oxygen. This pathway is sometimes referred to as the Z scheme because of its zigzag route, as depicted here, but the two arms are in fact remote in space. (Note Plastocyanin (Cu) is an alternative late replacement for an Fe cytochrome complex).

F.A. Armstrong, A.M. Bond, H.A.O. Hill, B.N. Oliver, and I.S.M. Psalti, Electrochemistry of cytochrome c, plastocyanin, and ferredoxin at edge- and basal-plane graphite electrodes interpreted via a model based on electron transfer at electroactive sites of microscopic dimensions in size. J. Am. Chem. Soc. 111,91859189 (1989). 

Integration of PS II and PS I via cyt b6f in chloroplasts (Z scheme of photosynthesis). Pheo a represents pheophytin pQA and pQB represent phytoquinone A and B, respectively A, and At represent electron acceptor chlorophylls, respectively pc represents plastocyanin and Fd represents ferridoxin. 

The electrons subsequently pass to plastocyanin (PC), which is a copper-containing protein. The Cu-containing redox center of this 10.5 kD monomer cycles between Cu(I) and Cu(II) oxidation states. The structure of PC shows that... 

The reaction-center proteins for Photosystems I and II are labeled I and II, respectively. Key Z, the watersplitting enzyme which contains Mn P680 and Qu the primary donor and acceptor species in the reaction-center protein of Photosystem II Qi and Qt, probably plastoquinone molecules PQ, 6-8 plastoquinone molecules that mediate electron and proton transfer across the membrane from outside to inside Fe-S (an iron-sulfur protein), cytochrome f, and PC (plastocyanin), electron carrier proteins between Photosystems II and I P700 and Au the primary donor and acceptor species of the Photosystem I reaction-center protein At, Fe-S a and FeSB, membrane-bound secondary acceptors which are probably Fe-S centers Fd, soluble ferredoxin Fe-S protein and fp, is the flavoprotein that functions as the enzyme that carries out the reduction of NADP+ to NADPH. 

Results obtained for the blue Cu protein plastocyanin are considered here in detail as illustrative of different approaches yielding relevant information. The reduction of plastocyanin PCu(II) with cytochrome c(II) is also considered as an example of a protein-protein reaction. 

Metalloproteins fall into three main structure categories depending on whether the active site consists of a single coordinated metal atom, a metal-porphyrin unit, or metal atoms in a cluster arrangement. In the context of electron-transfer metalloproteins, the blue Cu proteins, cytochromes, and ferre-doxins respectively are examples of these different structure types. Attention will be confined here mainly to a discussion of the reactivity of the blue Cu protein plastocyanin. Reactions of cytochrome c are also considered, with brief mention of the [2Fe-2S] ferredoxin, and high potential Fe/S protein [HIPIP]. 

It is timely to review the reactivity of plastocyanin in the light of recent aqueous solution studies, and the elegant structural work of Freeman and colleagues on both the PCu(I) and PCu(II) forms (1 2) Plastocyanin now ranks alongside cytochrome c (3) as the electron-transfer metalloprotein for which there is most structural information. 

From 13 completed amino-acid sequences and 54 partial sequences (>40 residues) of plastocyanins from higher plants it appears that sixty residues are invariant and 7 are conservatively substituted 02,7). With three algal plastocyanins included there are 39 invariant or conservatively substituted groups. It is believed that the same structural features apply to the whole family, and that highly conserved residues are an indication of functional sites on the protein surface. The upper hydrophobic and right-hand-side surfaces are believed to be particularly relevant in this context, the latter including four consecutive... 

Figure 1. Structure of plastocyanin (2) showing the positions of a-carbon atoms of amino acid residues. The active site and positions of the conserved (plant) negative patch (42-45) and Tyr 83 are indicated (%).

Rate Constants and Reactivity. Electron-transfer reactions of plastocyanin (and other metalloproteins) are so efficient that only a narrow range of redox partners (having small driving force) can be employed. Rates are invariably in the stopped-flow range, Table I. Unless otherwise stated parsley plastocyanin... 

---