Photosynthesis plastocyanin

Blue copper proteins transfer electrons between various biological systems, e.g., between the two photosystems in photosynthesis (plastocyanin). They are characterized by a number of unusual properties, viz., a bright blue color, an unusually high reduction potential, and distinctive... 

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. 

Let us start our examination with the prototypical blue protein plastocyanin, found in the thylacoid membrane of chloroplasts, where it acts as an electron carrier in photosynthesis (see Figure 1). As Figure 30 illustrates, the active site of plastocyanin is formed of a Cu(II) ion (pseudo)tetrahedrally coordinated to two histidine nitrogen atoms and... 

Plastocyanin from parsley, a copper protein of the chloroplast involved in electron transport during photosynthesis, has been reported to have a fluorescence emission maximum at 315 nm on excitation at 275 nm at pH 7 6 (2°8) gjncc the protein does not contain tryptophan, but does have three tyrosines, and since the maximum wavelength shifts back to 304 nm on lowering the pH to below 2, the fluorescence was attributed to the emission of the phenolate anion in a low-polarity environment. From this, one would have to assume that all three tyrosines are ionized. A closer examination of the reported emission spectrum, however, indicates that two emission bands seem to be present. If a difference emission spectrum is estimated (spectrum at neutral pH minus that at pH 2 in Figure 5 of Ref. 207), a tyrosinate-like emission should be obtained. 

This proposition is amply verified by Type I (Tl) copper proteins (37,62). Tl centers are involved in electron transfer with perhaps the most famous example being plastocyanin (Pc), a crucial part of photosynthesis, and the first Tl protein to be... 

Having elucidated, in combination with X-ray structural data, the characteristics of the copper site coordination in blue proteins in extenso, the challenge for EPR spectroscopy (and other techniques) is now to find ways to model the electron transfer (ET) in a realistic fashion. At present EPR is, however, mostly used to ascertain that the coordination of copper in the experimental ET chain models employed is not disturbed prior to ET. Plastocyanin is the electron carrier in photosynthesis. Indications of structural origins of impaired ET in... 

In plants, algae and cyanobacteria the light-induced charge separation of photosynthesis occurs in 2 large membrane proteins, called photosystem (PS) I and II. PS I catalyzes the ET from plastocyanin (or cytochrome c6) on the luminal side to ferrodoxin (or flavodoxin) on the stromal side of the membrane (for review see reference 177). PS I from the cyanobacterium Thermo(Y13)synechococcus (T.) elongatus was crystallized and an X-ray crystallographic structure at 2.5 A resolution has recently been obtained.18,178 Very recently, the structure from plant PS I has also been reported with a resolution of 4.4 A.179... 

Cyanobacteria can synthesize ATP by oxidative phosphorylation or by photophosphorylation, although they have neither mitochondria nor chloroplasts. The enzymatic machinery for both processes is in a highly convoluted plasma membrane (see Fig. 1-6). Two protein components function in both processes (Fig. 19-55). The proton-pumping cytochrome b6f complex carries electrons from plastoquinone to cytochrome c6 in photosynthesis, and also carries electrons from ubiquinone to cytochrome c6 in oxidative phosphorylation—the role played by cytochrome bct in mitochondria. Cytochrome c6, homologous to mitochondrial cytochrome c, carries electrons from Complex III to Complex IV in cyanobacteria it can also carry electrons from the cytochrome b f complex to PSI—a role performed in plants by plastocyanin. We therefore see the functional homology between the cyanobacterial cytochrome b f complex and the mitochondrial cytochrome bc1 complex, and between cyanobacterial cytochrome c6 and plant plastocyanin. 

Photosystems I and II operate in concert. Their interaction is described in the Z scheme (shown in outline in Figure 18). In photosystem II, the primary oxidant is able to remove electrons from water. These electrons are transported to photosystem I via plastoquinone and plastocyanin to replace PSI electrons that have been used in the reduction of iron-sulfur proteins and transferred via NADP to 0O2. Electron flow between PSII and PSI is accompanied by the synthesis of Atp 367 These oxidizing and reducing aspects of photosynthesis can be separated and other substrates incorporated. 

Fig. 26. Z-Scheme of photosynthesis in plants. Chi is chlorophyll, cyt b, f is cytochrome b, / PC is plastocyanine, (Fe-S) is iron sulfer protein. ATP is adenosine triphosphate ADP is adenosine diphosphate Pj is the phosphate ion and NADP is the nicotinamide adenine dinucleotide phosphate ion [203]...

Golbeck J, ed. Advances in Photosynthesis and Respiration, vol 24. Photosystem 1, The Light-Driven Plastocyanin Ferredoxin Oxidoreductase. 2006. Springer. Dordrecht, The Netherlands. 

Figure 19.22. Pathway of Electron Flow From H2O to NADP in Photosynthesis. This endergonic reaction is made possible by the absorption of light by photosystem II (P680) and photosystem I (P700). Abbreviations Ph, pheophytin Qa and Qg, plastoquinone-binding proteins Pc, plastocyanin Aq and Aj, acceptors of electrons from P700 Fd, ferredoxin Mn, manganese.

The general function of this complex is that of transferring electrons from ubiquinone (or plastoquinone) to a hydrophilic protein acceptor (cytochrome c or plastocyanin). Therefore, in bacterial photosynthesis, it catalyzes the recycling of electrons from the secondary electron acceptor (Qn) to the secondary electron donor (cyt. Cj), completing thereby the cyclic electron transfer system. In chloroplasts and cyanobacteria, an analogous system transfers the electrons from plastoquinone (the secondary acceptor of PSII, A, 3) to plastocyanin (the secondary donor to PSI, 0, 2) and provides in this way an intersystem redox connection between PSII and PSI. The same complex is also involved in the cycling of electrons around PSI. 

Of the other blue copper proteins, only amicyanin shows a similar effect of pH (79), and a TpK of 7.18 has been obtained for the Cu(I) state. As with plastocyanin, no corresponding effect is observed for Cu(II) amicyanin, at least down to pH 4.5. The physiological relevance in the case of both proteins is at present unclear. Because in photosynthesis the pH of the inner thylakoid is less than 5.0, one possibility is that this is related to proton transport. Alternatively, it quite simply may be a control mechanism for electron transport. 

In 1965 Hill elaborated the two-photosystem scheme further as shown in Fig. 15 (B). In this Z-shaped scheme, two groups of chloroplast components with known redox potentials were placed at the bends of the Z Cyt/, plastocyanin and P700, close to -1-0.4 V, and plastoquinone and Cyt b( close to 0 V. Ferredoxin, with a potential of-0.43 V, is close to the midpoint potential ofhydrogen electrode. For oxygen production, the midpoint potential of the unknown component must exceed that of the oxygen electrode. Over the past thirty years, a variety of Z-schemes have been published in the literature to illustrate the electron-transfer processes in green-plant photosynthesis, but their basic features have not deviated from that shown in Fig. 15 (B). For instance, we show a currently accepted, concise Z-scheme in Fig. 15 (C) it includes many more individual components than were originally envisioned, plus a representation of the operation of the so-called Q-cycle in the Cyi-b(,f complex. 

Fig. 12. Plastocyanin and cytochrome tin the pre-docking" state. Figure drawn according to that of Martinez, Huang, Smith and Cramer (1996) Some consequences of the high resolution X-ray structure analysis of cytochrome f. In DR Ort and CF Yocum (eds) Oxygenic Photosynthesis The Light Reactions, p 435. Kluwer Acad PubI, but using ribbon structures. The PC ribbon model is adapted from Fig. 4 on p 608.

DJ Davis (1986) Proposed alignment of ammo acid sequences for cytochromes and f and identification of putative binding sites for cytochrome c and plastocyanin. Proc 7th Intern Photosynthesis Congr 2 473-476... 

Cytochromes/( J from Latin frons for leaf) and were discovered almost half a century ago hy Hill and Scarisbrick and by Hill, respectively. In fact, the redox behavior of these chloroplast c)do-chromes led Robin Hill and Fay Bendall to formulate the so-called Z-scheme for oxygenic photosynthesis. In 1972, Nelson and Neumann" isolated a partially purified complex from a digitonin-fraction-ated PS-I particle obtained from lettuce chloroplasts. The complex was found to contain Cyt/, Cyt and non-heme iron, which led the authors to note its similarity to the Cyt-bci complex (i.e., complex III) of mitochondria. In 1975 Sugahara, Shaw and this author isolated a complex from spinach TSF-I particles and by investigation of its spectroscopic and EPR properties, showed that it also contained Cyt/, Cyt b and nonheme iron, consistent with its being a bjcomplex. The fraction also displayed a distinct EPR signal characteristic of a copper protein, apparently due to plastocyanin co-precipitated during fractionation. 

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