Photosynthetic photophosphorylation

Owers-Narhi L et al. (1979) Reconstitution of cyanobacterial photophosphory-lation by a latent Ca2+-ATPase, Biochem. Biophys. Res. Comm. 90, 1025-1031. Piccioni RG et al. (1981) A nuclear mutant of Chlamydomonas reinhardtii defective in photosynthetic photophosphorylation, Eur. J. Biochem. 117, 93-102. 

The thylakoid membrane is asymmetrically organized, or sided, like the mitochondrial membrane. It also shares the property of being a barrier to the passive diffusion of H ions. Photosynthetic electron transport thus establishes an electrochemical gradient, or proton-motive force, across the thylakoid membrane with the interior, or lumen, side accumulating H ions relative to the stroma of the chloroplast. Like oxidative phosphorylation, the mechanism of photophosphorylation is chemiosmotic. 

Photosynthetic electron transport, which pumps into the thylakoid lumen, can occur in two modes, both of which lead to the establishment of a transmembrane proton-motive force. Thus, both modes are coupled to ATP synthesis and are considered alternative mechanisms of photophosphorylation even though they are distinguished by differences in their electron transfer pathways. The two modes are cyclic and noncyclic photophosphorylation. 

FIGURE 22.21 The mechanism of photophosphorylation. Photosynthetic electron transport establishes a proton gradient that is tapped by the CFiCFo ATP synthase to drive ATP synthesis. Critical to this mechanism is the fact that the membrane-bound components of light-induced electron transport and ATP synthesis are asymmetrical with respect to the thylakoid membrane so that vectorial discharge and uptake of ensue, generating the proton-motive force. 

Assuming that the concentrations of ATP, ADP, and P in chloroplasts are 3 mM, 0.1 mM, and 10 mM, respectively, what is the AG for ATP synthesis under these conditions Photosynthetic electron transport establishes the proton-motive force driving photophosphorylation. What redox potential difference is necessary to achieve ATP synthesis under the foregoing conditions, assuming an electron pair is transferred per molecule of ATP generated ... 

If noncyclic photosynthetic electron transport leads to the translocation of 3 H /e and cyclic photosynthetic electron transport leads to the translocation of 2 H /A, what is the relative photosynthetic efficiency of ATP synthesis (expressed as the number of photons absorbed per ATP synthesized) for noncyclic versus cyclic photophosphorylation (Assume that the CFiCEq ATP synthase yields 1 ATP/3 H. )... 

Sharkey, T.D. Badger, M.R. (1982). Effects of water stress on photosynthetic electron transport, photophosphorylation and metabolite levels of Xanthium strumarium mesophyll cells. Planta, 156, 199-206. 

Fig. 5.2. The photosynthetic membrane of a green sulfur bacterium. The light-activated bacte-riochlorophyll molecule sends an electron through the electron-transport chain (as in respiration) creating a proton gradient and ATP synthesis. The electron eventually returns to the bacteri-ochlorophyll (cyclic photophosphorylation). If electrons are needed for C02 reduction (via reduction of NADP+), an external electron donor is required (sulfide that is oxidised to elemental sulfur). Note the use of Mg and Fe.

Besides these external processes, formation of ROS may also take place intrac-ellularly. Photooxidative stress, including UVB, stimulates various cellular processes leading to the production of superoxide radicals and hydrogen peroxide, as well as singlet-oxygen and hydroxyl radicals. The sources and production sites of ROS are mainly related to photosynthetic activities such as the pseudocyclic photophosphorylation and the Mehler reaction, which stimulate the accumulation of hydrogen peroxide (Asada 1994 Elstner 1990). 

It can be seen from the normal potentials E° (see p. 18) of the most important redox systems involved in the light reactions why two excitation processes are needed in order to transfer electrons from H2O to NADP"". After excitation in PS II, E° rises from around -IV back to positive values in plastocyanin (PC)—i. e., the energy of the electrons has to be increased again in PS I. If there is no NADP" available, photosynthetic electron transport can still be used for ATP synthesis. During cyclic photophosphorylation, electrons return from ferredoxin (Fd) via the plastoquinone pool to the b/f complex. This type of electron transport does not produce any NADPH, but does lead to the formation of an gradient and thus to ATP synthesis. 

The photoreductive synthetic process that promotes the assimilation of carbon dioxide into carbohydrates, other reduced metabolites, as well as ATP (synthesis of the latter is termed photophosphorylation). Photosynthesis is the primary mechanism for transducing solar energy into biomass, and green plants utilize chlorophyll a to capture a broad spectrum of solar radiant energy reaching the Earth s surface. Photosynthetic bacteria typically produce NADPH, the reductive energy of which is converted to ATP. 

The biological functions of chloroplast ferredoxins are to mediate electron transport in the photosynthetic reaction. These ferredoxins receive electrons from light-excited chlorophyll, and reduce NADP in the presence of ferredoxin-NADPH reductase (23). Another function of chloroplast ferredoxins is the formation oT" ATP in oxygen-evolving noncyclic photophosphorylation (24). With respect to the photoreduction of NADP, it is known that microbial ferredoxins from C. pasteurianum (16) are capable of replacing the spinach ferredoxin, indicating the functional similarities of ferredoxins from completely different sources. The functions of chloroplast ferredoxins in photosynthesis and the properties of these ferredoxin proteins have been reviewed in detail by Orme-Johnson (2), Buchanan and Arnon (3), Bishop (25), and Yocum et al. ( ). 

ATP is the primary high-energy phosphate compound produced by catabolism, in the processes of glycolysis, oxidative phosphorylation, and, in photosynthetic cells, photophosphorylation. Several enzymes then cany phosphoryl groups from ATP to the other nucleotides. Nucleoside diphosphate kinase, found in all cells, catalyzes the reaction... 

In contrast, the reaction centers of green sulfur bacteria resemble PSI of chloroplasts. Their reaction centers also receive electrons from a reduced quinone via a cytochrome be complex.245 However, the reduced form of the reaction center bacteriochlorophyll donates electrons to iron-sulfur proteins as in PSI (Fig. 23-17). The latter can reduce a quinone to provide cyclic photophosphorylation. Cyanobacteria have a photosynthetic apparatus very similar to that of green algae and higher plants. 

The pathways involved in cyclic photophosphorylation in chloroplasts are not yet established. Electrons probably flow from the Fe-S centers Fdx, Fda, or Fdb back to cytochrome b563 or to the PQ pool as is indicated by the dashed line in Fig. 23-18. Cyclic flow around PSII is also possible. The photophosphorylation of inorganic phosphate to pyrophosphate (PP ) occurs in the chromatophores (vesicles derived from fragments of infolded photosynthetic membranes) from Rho-dospirillum rubrum. The PP formed in this way may be used in a variety of energy-requiring reactions in these bacteria.399 An example is formation of NADH by reverse electron transport. 

The photosynthetic process, showing coupling of electron transport and ADP phosphorylation. The dashed line shows electron flow in cyclic photophosphorylation. See text for details. 

Figure E9.1 illustrates the photosynthetic process as it occurs in higher plants. This is called noncyclic photophosphorylation to distinguish it from cyclic photophosphorylation in photosynthetic bacteria. Cyclic photophosphorylation requires only photosystem I and a second series of electron carriers to return electrons to the electron-deficient chlorophyll. The dashed line in Figure E9.1 indicates the flow of electrons in cyclic photophosphorylation. ATP is produced during the cyclic process just as in the noncyclic process, but NADPH is not.

Formation of ATP by photosynthetic systems is often called photophosphorylation. Although photophosphorylation and oxidative phosphorylation are very similar, they do differ in a few details. The respiratory chain pumps protons... 

In a study designed to determine the mode of action of atrazine in higher plants, Shimabukuro and Swanson (1969) concluded that atrazine inhibits the Hill reaction and its noncyclic phosphorylation, while being ineffective against cyclic photophosphorylation. Atrazine readily penetrated the chloroplast of resistant as well as susceptible plants. In tolerant plants such as sorghum, the metabolism of atrazine was postulated to occur outside the chloroplasts to form water-soluble and insoluble residues that reduced the concentration of photosynthetic inhibitors in the chloroplasts. 

Photosynthesis can be affected in many ways. Metals can influence biosynthesis of biomembranes and photosynthetic pigments, especially chlorophyll. They may inactivate enzymes by oxidising SH-groups necessary for catalytic activity or by substitution for other divalent cations in metalloenzymes. They finally can also interact with the photosynthetic electron transport and with the related photophosphorylation. 

In-vitro approach Data are available in abundance concerning metal effects on isolated chloroplasts (for a review, see Clijsters and Van Assche, 1985). All the metals studied were found to be potential inhibitors of photosystem 2 (PS 2) photosystem 1 (PS 1) was reported to be less sensitive. From the in-vitro experiments, at least two potential metal-sensitive sites can be derived in the photosynthetic electron transport chain the water-splitting enzyme at the oxidising side of PS 2, and the NADPH-oxido-reductase (an enzyme with functional SH-groups) at the reducing side of PS 1 (Clijsters and Van Assche, 1985). Moreover, in vitro, non cyclic photophosphorylation was very sensitive to lead (Hampp et al., 1973 b) and mercury (Honeycutt and Korgmann, 1972). Both cyclic and non-cyclic photophosphorylation were proven to be inhibited by excess of copper (Uribe and Stark, 1982) and cadmium (Lucero et al, 1976). 

In contrast to the previous results, Weigel (1985 a, b) reported that in mesophyll protoplasts of Valerianella locusta and in intact chloroplasts of Spinacea oleraceae cadmium affects photosynthesis by inhibition of several reaction steps of the Calvin cycle and not by interaction with the electron transport or photophosphorylation (cf. section on photosynthetic C02 fixation). 

Although photophosphorylation was reported to be very sensitive to metals in vitro, the reduction of photosynthetic ATP production was shown to be related to an inhibition of the electron flow rate in Euglena gracilis treated with toxic concentrations of zinc, cadmium and mercury (De Filippis et al., 1981 b), in cadmium-treated Lycoper-sicon esculentum (Bazinsky et al., 1980) and in zinc-treated Phaseolus vulgaris (Van Assche and Clijsters, 1986a). 

Amon, D.I. and Chain, R.K. 1977. Ferredoxin-catalyzed photophosphorylations concurrence, stoichiometry, regulation and quantum efficiency. In Photosynthetic Organelles (Ed. S.Miyachi, S.Katoh, Y.Fujita and K. Shibata). (Japanese Society of Plant Physiologists, Japan), p. 433. 

Answer Plants have two photosystems. Photosystem I absorbs light maximally at 700 nm and catalyzes cyclic photophosphorylation and NADP+ reduction (see Fig. 19-56). Photosystem II absorbs light maximally at 680 nm, splits H20 to 02 and H+, and donates electrons and H+ to PSI. Therefore, light of 680 nm is better in promoting 02 production, but maximum photosynthetic rates are observed only when plants are illuminated with light of both wavelengths. 

The role of ferredoxin in reactions of photosynthetic bacteria is summarized in Fig. 11. Reactions which should now be possible to show, but so far have not been observed in cell-free systems, are the fcrredoxin-dependent photoproduction of hydrogen gas and photoreduction of pyridine nucleotide. Hood (56) reported a two-fold stimulation by ferredoxin in the photoreduction of DPN by chlorophyll-containing particles from Chromatium. Hood s results were inconsistent and Hinkson (54) found that the ferredoxin requirement is not specific and may be satisfied by serum albumin. In addition, there is still no evidence for a role of ferredoxin in photophosphorylation by photosynthetic bacteria, similar to that in chloroplasts. 

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