Electron transport cyclic

The participation of the phycobiliproteins in the absorption ofphotokinetically active light has been demonstrated above. Peaks of around 565 and 615 nm in the action spectra indicate the involvement of C-phycoerythrin andC-phycocanin. These pigments transfer energy to the reaction center of PS II and suggest the participation of the non-cyclic electron transport and coupled phosphorylation. 

The very pronounced EPR signal of P7 0, which is easily detected in all photosynthetic material, can be used to monitor light-induced ET. This has been employed recently to study cyclic electron-transport around PS I in chloroplasts... 

A Cyclic Electron-Transport Chain Moves Protons Outward across the Membrane That Drive the Formation of ATP... 

In summary, when electron transport is operating in noncyclic mode, via PSI and PSII, the products are NADPH and ATP. In cyclic electron transport, on the other hand, the sole product is ATP. 

Ghanotakis, D.F., Yocum, C.F. and Babcock, G.T. 1986. ESR spectroscopy demonstrates that cytochrome b-559 remains low potential in Ca2+-reactivated, salt-washed PS II particles. Photosynthesis Research, 9,125-134. Hervas, M., Ortega, J.M., de la Rosa, M.A., de la Rosa, F.F. and Losada, M. 1985. Location and function of cytochrome b-559 in the chloroplast non-cyclic electron transport chain. Physiol. Veg., 23,593-604. 

Reduced Fd is known to be the electron donor for a number of different acceptors, both artificial, such as mammalian Cyt c [58], and of physiological importance, such as nitrite reductase [59]. It is also known to be a catalyst of cyclic electron transport around PS I [59], a process coupled to the synthesis of ATP (cyclic photophosphorylation), in which electrons are transferred back to a component (Cyt b(, ) of the chain between the two photochemical reactions. 

Most experimental results indicate that the ATP/2 e ratio observed with isolated chloroplasts (or washed thylakoids) ranges between 1 and 1.3 (see review by Ort and Melandri [5]). Only a few reports of higher ratios have appeared in the literature [86,87] related to class 1 chloroplasts in spite of the correction applied [87] it is difficult to rule out completely the possibility that some cyclic electron transport occurring together with NADP reduction might have contributed to ATP synthesis under the conditions of the experiments. 

The number of protons translocated into the thylakoids per electron transported (H /e ratio) is still controversial (see Refs. 5 and 6 for reviews). The reason for the controversy lies in the still unclear mechanism of the PQ-Cyt b cycle and its role in cyclic and/or non-cyclic electron transport (see Section 2), and in the differences in the methods used. However, most authors find a ratio H /e of = 2 in isolated thylakoids, while a few reports of higher values have appeared [5,6]. 

Early fractionation studies with thylakoids that had been fragmented by detergents [54] or mechanical means [55] followed by collection of the thylakoid submembrane fractions by centrifugation provided the first evidence for lateral heterogeneity. Submembrane fractions derived from granal stacks were enriched in PS II but they also contained PS 1, whereas the stroma thylakoids had mainly PS I. Sane et al. [55] proposed that appressed membranes were the site of non-cyclic electron transport, while the non-appressed membranes carried out cyclic photo-... 

Note that this cyclic electron-transfer process produces no net oxidation or reduction. However, in the process, protons acquired from the cytoplasm are translocated across the plasma membrane to establish a transmembrane electrochemical potential gradient. The dissipation of such a proton gradient then provides the necessary energy to drive ATP synthesis. A similar simplified cyclic electron-transport diagram has been shown earlier in Chapter 3 as Fig. 12 (C) on p. 81, in coimection with a discussion of a LHl-RC-Cyt6c, supercomplex of Rb. sphaeroides. More detailed discussion of the cytochromeic] and bff complexes and ATP synthesis will be presented in Chapters 35 and 36, respectively. 

Based on the nature of the cytochromes, there are two kinds of photosynthetic bacterial reaction centers. The first kind, represented by that of Rhodobacter sphaeroides, has no tightly bound cytochromes. For these reaction centers, as shown schematically in Fig. 2, left, the soluble cytochrome C2 serves as the secondary electron donor to the reaction center the RC also accepts electrons from the cytochrome bc complex by way ofCytc2- The rate of electron transfer from cytochrome to the reaction center is sensitive to the ionic strength of the medium. Functionally, cytochrome C2 is positioned in a cyclic electron-transport loop. In Rb. sphaeroides, Rs. rubrum and Rp. capsulata cells, the two molecules of cytochromes C2 per RC are located in the periplasmic space between the cell wall and the cell membrane. When chromatophores are isolated from the cell the otherwise soluble cytochrome C2 become trapped and held by electrostatic forces to the membrane surface at the interface with the inner aqueous phase. These cytochromes electrostatically bound to the membrane can donate electrons to the photooxidized P870 in tens of microseconds at ambient temperatures, but are unable to transfer electrons to P870 at low temperatures. 

Fig. 7. (A) Reaction scheme involving a cyclic electron transport around photosystem I mediated by the TMPDVTMPD couple. (B) Semi-logarithmic plots of decay of absorbance changes at 700 and 575 nm in spinach D144 particles following a flash at different TMPD concentrations [TMPD at 67 pM o for 0.2 pM, for 0.6 pM, and for 1.2 pM TMPD, respectively] Figure source (B) Hiyama and Ke (1972) Difference spectra and extinction coefficient of P700. Biochim Biophys Acta 267 162.

Cytochrome b /also serves as an intermediate in a non-linear, or so-called cyclic, electron-transport pathway around PS I, as formulated in Fig. 1 (B). A third function of Cyt b /is translocation of protons across the thylakoid membrane during electron transfer from plastoquinol to plastocyanin [Fig. 1 (C)]. The unique effects resulting from electron transport and proton translocation in the cytochrome b(f complex are the production of an electrochemical potential and a pH gradient across the thylakoid membrane to provide energy in a form needed for ATP synthesis (see the following chapter). 

As described, the absorption of a photon by P700 leads to the release of an energized electron. This electron is then passed through a series of electron carriers, the first of which is a chlorophyll a molecule (A0). As the electron is donated sequentially to phylloquinone (Q) and to several iron-sulfur proteins (the last of which is ferre-doxin), it is moved from the lumenal surface of the thylakoid membrane to its stromal surface. Ferredoxin, a mobile, water-soluble protein, then donates each electron to a flavoprotein called ferredoxin-NADP oxidoreductase (FNR). The flavoprotein uses a total of 2 electrons and a stromal proton to reduce NADP+ to NADPH. The transfer of electrons from ferredoxin to NADI is referred to as the noncyclic electron transport pathway. In some species (e.g., algae), electrons can return to PSI by way of a cyclic electron transport pathway (Figure 13.13). In this process, which typically occurs when a chloroplast has a high NADPH/NADP1 ratio, no NADPH is produced. Instead, electrons are used to pump additional protons across the thylakoid membrane. Consequently, additional molecules of ATP are synthesized. 

Non-c/clic electron transport Cyclic electron transport Q cycle... 

Fig. 12. (a) The relationship between the quantum efficiency of Photosystem I electron transport (Opj,) and the quantum efficiencies for Photosystem II electron transport d>psu) and the quantum efficiencies for open Photosystera II reaction centers These data were obtained from leaves of JuanuUoa aurantiaca ( , O) and Begonia luzonensis ( , ) under conditions of increasing irradiance and a leaf of JuanuUoa aurantiaca kept at constant irradiance, but subjected to decreasing CO2 concentrations (350-35 ppm) in a non-photorespiratory atmosphere (A, A), (b) The irradiance response of the rate constants for non-cyclic electron transport. 

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