Electron photosynthetic transport chain

In contrast to common usage, the distinction between photosynthetic and respiratory Rieske proteins does not seem to make sense. The mitochondrial Rieske protein is closely related to that of photosynthetic purple bacteria, which represent the endosymbiotic ancestors of mitochondria (for a review, see also (99)). Moreover, during its evolution Rieske s protein appears to have existed prior to photosynthesis (100, 101), and the photosynthetic chain was probably built around a preexisting cytochrome be complex (99). The evolution of Rieske proteins from photosynthetic electron transport chains is therefore intricately intertwined with that of respiration, and a discussion of the photosynthetic representatives necessarily has to include excursions into nonphotosynthetic systems. 

While phototactic action spectra measured in some Phormidium species indicate that chlorophyll a is not involved in the absorption of phototactically active light (see below), the phototactic action spectrum of Anabaena variabilis106) shows slight activity around 440 nm and a distinct peak at around 670 nm, both indicating chlorophyll a. Since blockers of the photosynthetic electron transport, such as DCMU andDBMIB, (see below) do not affect phototactic orientation, the active light seems not to be utilized via the photosynthetic electron transport chain (for further information see below). 

These results were interpreted using the electron pool hypothesis There is an electron pool situated in the linear photosynthetic electron transport chain between photosystems II and I (Fig. 9). A phobic response is triggered by a decrease in the flow rate through the pool. This can be accomplished in two ways ... 

Fig. 10. Redox systems of the photosynthetic electron transport chain incorporated in the thylakoid membrane. Irradiation causes the generation of a proton gradient (after Trebst and Hauska135))...

L. Florin, A. Tsokoglou, T. Happe (2001) A novel type of iron hydrogenase in the green alga Scenedesmus obliquus is linked to the photosynthetic electron transport chain. J. Biol. Chem., 276 6125-6132... 

The key enzyme hydrogenase catalyses the reversible reduction of protons to molecular hydrogen. Inhibitor experiments indicate that the ferredoxin PetF functions as natural electron donor linking the hydrogenase to the photosynthetic electron transport chain [Florin et al., 2001],... 

The first cytochrome to be recognised as a component of the photosynthetic electron transport chain was cytochrome f [142]. The properties of cytochrome f have been reviewed [143,144], and amino-acid sequence information is available for pea, spinach, wheat and tobacco [145]. The axial ligand to the heme-Fe... 

The photosynthetic electron transport chain in plants starts in photosystem II (PS 11 see p. 128). PS 11 consists of numerous protein subunits (brown) that contain bound pigments—i.e., dye molecules that are involved in the absorption and transfer of light energy. 

Triazines are selective herbicides used to control a wide spectrum of grass and broadleaf weeds in cereal, oilseed, and horticultural crops. Triazine herbicides kill weeds by interfering with the electron transport chain in photosystem II (PS II). These herbicides bind to the QB protein in the PS II reaction center and block the flow of electrons through the photosynthetic electron transport chain. 

Triazine (e.g., atrazine, simazine) and substituted urea (e.g., diuron, monuron) herbicides bind to the plastoquinone (PQ)-binding site on the D1 protein in the PS II reaction center of the photosynthetic electron transport chain. This blocks the transfer of electrons from the electron donor, QA, to the mobile electron carrier, QB. The resultant inhibition of electron transport has two major consequences (i) a shortage of reduced nicotinamide adenine dinucleotide phosphate (NADP+), which is required for C02 fixation and (ii) the formation of oxygen radicals (H202, OH, etc.), which cause photooxidation of important molecules in the chloroplast (e.g., chlorophylls, unsaturated lipids, etc.). The latter is the major herbicidal consequence of the inhibition of photosynthetic electron transport. 

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). 

Fig. 6. Sites of inhibitory action of DCMU in photosynthetic electron transport chain. The abbreviations are as follows - Cyt f cytochrome f, Fd ferredoxin, Mn water-splitting complex (manganese-containing), P680 pigment complex of photosystem II, P700 pigment complex of photosystem I, PC plastocyanin, PQ plastquinone, Q quencher, Rd NADP reductase and X direct electron acceptor complex...

Besides water oxidation, endogenous substrate (such as starch) catabolism can al so generate reductants for the algal photosynthetic electron transport chain at the... 

Plastoquinone is one of the most important components of the photosynthetic electron transport chain. It shuttles both electrons and protons across the photosynthetic membrane system of the thylakoid. In photosynthetic electron flow, plastoquinone is reduced at the acceptor side of photosystem II and reoxidized by the cytochrome bg/f-complex. Herbicides that interfere with photosynthesis have been shown to specifically and effectively block plastoquinone reduction. However, the mechanisms of action of these herbicides, i. e., how inhibition of plastoquinone reduction is brought about, has not been established. Recent developments haVe brought a substantial increase to our knowledge in this field and one objective of this article will be to summarize the recent progress. 

Ubiquinones are energy transducers that are obligatory in many respiratory and photosynthetic electron transport chains. The ubiquinone enzymes involved in these reactions usually function in a manner that couples the electron transfer by the ubiquinone to proton translocation across the membrane.The structural makeup of the ubiquinone active site permits varying functional roles that influence the electron and proton chemistry. 

The [2Fe 2S], [3Fe S], and [4Fe S] clusters that are found in simple Fe S proteins are also constituents of respiratory and photosynthetic electron transport chains. Multicluster Fe S enzymes such as hydrogenase, formate dehydrogenase, NADH dehydrogenase, and succinate dehydrogenase feed electrons into respiratory chains, while others such as nitrate reductase, fhmarate reductase, DMSO reductase, and HDR catalyze the terminal step in anaerobic electron transport chains that utihze nitrate, fumarate, DMSO, and the CoB S S CoM heterodisulfide as the respiratory oxidant. All comprise membrane anchor polypeptide(s) and soluble subunits on the membrane surface that mediate electron transfer to or from Mo cofactor (Moco), NiFe, Fe-S cluster or flavin active sites. Multiple Fe-S clusters define electron transport pathways between the active site and the electron donor or... 

Spectroscopic and crystallographic studies have identified four Fe S clusters in the membrane-bound photosynthetic electron transport chain of plant and cyanobacterial chloro-plasts. One is the Rieske-type [2Fe-2S] + + center in the cyt b(,f complex, which catalyzes electron transfer from plasto-quinol to plastocyanin with concomitant proton translocation, and is functionally analogous to the cyt bc complex, with cyt / in place of cyt The remainder are low-potential [4Fe 4S] + + centers in Photosystem I which constitute the terminal part of the electron transfer chain that is initiated by the primary donor chlorophyll. One is a very low-potential [4Fe S] + + center, Fx (Em =-705 mV), that bridges two similar subunits (PsaA and PsaB) and is coordinated by two cysteines from each subunit in a C-Xg-C arrangement. This cluster transfers electrons to the 2Fe-Fd acceptor via an electron transfer chain composed of Fa, a [4Fe S] + + cluster with Em = -530 mV, and Fb, a [4Fe S] + + clusters with Em = -580 mV. Fa and Fb are in a low-molecular weight subunit (PsaC, 9 kDa) that shows strong sequence and structural homology with bacterial 8Fe-Fds. The center-to-center distance between Fx and Fa and between Fa and Fb are 14.9 A and 12.3 A, respectively, well... 

Mehler activity is generally considered a process which can only consume photosyntheticaUy derived O2, and it cannot cause net consumption of O2 because PSI activity rehes on photosyntheticaUy derived electrons (Kana, 1993). Yet, the shared-arrangement of photosynthetic and respiratory electron transport chains in cyanobacteria aUows electrons from respiratory derived NAD(P)H to feed into the plastoquinone pool of the photosynthetic electron transport chain and reduce PSI (Schmetterer, 1994). Through the translocation of reductant (i.e. glucose 6-phosphate) from ceUs with functional PSII, Mehler activity can result in a net consumption of O2 in ceUs (or heterocysts) which have no PSII activity and in which nitrogen is fixed (Fig. 35.3). 

Michel, K. P., and Pistorius, E. K. (2004). Adaptation of the photosynthetic electron transport chain in cyanobacteria to iron deficiency The function of idia and isia. Physiol. Plant. 120, 36—50. 

Nonetheless, photosynthesis did not evolve immediately at the origin of life. The failure to discover photosynthesis in the domain of Archaea implies that photosynthesis evolved exclusively in the domain of Bacteria. Eukaryotes appropriated through endosymbiosis the basic photosynthetic units that were the products of bacterial evolution. All domains of life do have electron-transport chains in common, however. As we have seen, components such as the ubiquinone-cytochrome c oxidoreductase and cytochrome hf family are present in both respiratory and photosynthetic electron-transport chains. These components were the foundations on which light-energy-capturing systems evolved. 

Gorman DS and Levine RP. (1965). Cytochrome f and plastocyanin their sequence in the photosynthetic electron transport chain of Chlamydomonas reinhardtii. Proc. Natl. Acad Sci. USA 54,1665-1669. 

Bacterial copper proteins are found only in the plasma membrane (Gram-positive bacteria) or in the plasma membrane and the periplasm (Gram-negative bacteria), not in the bacterial cytoplasm. However, cyanobacteria do have copper proteins in their cytoplasm. These important photosynthetic bacteria require copper for plastocyanin, which plays a critical role in the photosynthetic electron transport chain. Both plastocyanin and cytochrome c oxidase are found in the thylakoid compartments within the cytoplasm. In Synechocystis, the Cu(I) Pie-ATPase CtaA imports Cu(I). A second ATPase, PacS, imports Cu(I) into the thylakoid, and the Atxl-like copper chaperone ScAtxl is believed to deliver Cu(I) from CtaA to PacS (Fig. 8.6). 

Fig. 15. The Z-scheme for two photosystems (A) The original Z-scheme of Hill and Bendall (1960) (B) an expanded Z-scheme of Hill (1965) (C) a contemporary Z-scheme. (A) modified from Hill and Bendall (1960) Function of two cytochrome components in chloroplasts A working hypothesis. Nature 186 137 (B) from Hill (1965) The biochemist s green mansions the photosynthetic electron-transport chain in plants. In PN Campbell and GD Greville (eds) Essays in Biochemistry, 1 143. Acad Press.

Fig. 8. Simplified free energy diagram of the photosynthetic electron transport chain (for details see Ref.27, 69X For the sake of simplicity only the reduced forms of the corresponding redox carriers are given. The electronic excitations are symbolized by thick open arrows, thermal redox reactions in the dark are indicated by thin arrows. Abbreviations Cyt fre(j = reduced cytochrome f, NADPH = reduced nicotinamide adenine dinucleotidphosphate, PCjgj = reduced plastocyanine, PQH2 = plastohydroquinone, Xf and X 320- are the reduced forms of the primary electron acceptor of C[ and Qj, respectively [see Eq. (14a, b)l, Y = watersplitting enzyme system...

On the basis of the corresponding midpoint potentials there is given in Fig. 8 a free energy diagram of the main steps of the photosynthetic electron transport chain. According to the relation AG = — n AE (n = number of electrons transferred, 5 = Faraday... 

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