Electron transport chain cyclic

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.

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

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). 

Uncouplers. Uncouplers dissociate electron transport from photophosphorylation. Both noncyclic and cyclic phosphorylation are inhibited, but electron transport reactions are either unaffected or stimulated. Because uncouplers relieve the inhibition of electron transport imposed by energy transfer inhibitors, they are considered to act at a site closer to the electron transport chain than the site of phosphate uptake. In Figure 2, they are shown (site 2) as dissipating some form of conserved energy represented as on the noncyclic and cyclic ATP-gener-ating pathways. Perfluidone is the only herbicide identified to date that functions as a pure uncoupler at pH 8.0 (2). Compounds that uncouple photophosphorylation also uncouple mitochondrial oxidative phosphorylation. 

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. 

How does the cytochrome subunit of the reaction center regain an electron to complete the cycle The reduced quinone (QH2) is reoxidized to Q by complex III of the respiratory electron-transport chain (Section 18.3.3). The electrons from the reduced quinone are transferred through a soluble cytochrome c intermediate, called cytochrome c 2, in the periplasm to the cytochrome subunit of the reaction center. The flow of electrons is thus cyclic. The proton gradient generated in the course of this cycle drives the generation of ATP through the action of ATP synthase. 

The electron transport chain process of photoreactions I and II is noncyclic photophosphorylation. Cyclic photophosphorylation, which may proceed in the case of oxygen deficiency and can be considered as a shortcircuiting of electron transport, presumably does not play a role in the normal photosynthesis energy storing of the cells. 

In cyclic photophosphorylation, the excited chlorophyll of photosystem I passes electrons directly to the electron transport chain that normally links photosystem II to photosystem I. This electron transport chain is coupled to ATP production (see Figure 22.8). 

Electrons, obtained by oxidation of reduced nicotinamides and flavins, flow through the electron transport chain (a series of proteins with associated groups that cyclically become oxidized and reduced) in the inner mitochondrial membrane ultimately to oxygen with regeneration of water, H2O, while pumping protons (acid) across the inner mitochondrial membrane into the intermembrane space between the inner and outer mitochondrial membranes. 

Cyclic photophosphorylation In photosynthesis, Photophosphorylation (light-dependent ATP synthesis) that is linked to a cyclic flow of electrons firom photosystem 11 down an electron transport chain and back to photosystem II it is not coupled to the oxidation of H2O or to the reduction of NADP. Compare non-cyclic photophosphorylation. 

Rieske protain/centar an iron-sulphur protein first isolat from Complex III of the mitochondrial electron transport chain, in which it occurs with cytochromes b and C) [J.S, Rieske el al. Biochem. Biophys Res Commun. IS (1%4) 338-344], but which has now been found in the equivalent cytochrome be complexes in the bacterial plasma membrane and the chloroplast thylakoid membrane. The latter, known as the cytochrome bff complex, partidpates in cyclic and noncyclic electron flow in the light phase of photosynthesis (see Photosynthesis). All Rieske proteins are one-electron redox systems with a standanl redox potential in the + 0.2 to + 0.3V range and have a (2Fe-2S] center, a single membrane-spanning a-helix, and a characteristic electron spin resonance (ESR) spectrum. The chloroplastidic R.p/c, with a M, of - 20,000, is smaller than that of the mitochondnon. It is encoded in the nucleus, synthesized in the cytoplasm and translocated to the chloroplast, where it is inserted into the thylakoid membrane. Within the thylakoid membrane its [2Fe-2S] redox centre (near to its C-terminus) can readily pass electrons to cytochrome /, a c-type cytochrome that projects from the luminal surface cytochrome / then passes electrons to plastocyanin (see) dissolved in the aqueous milieu of the thylakoid lumen. 

Cooley and Vermass, 2001). Thus, electron fluxes in the intersystem electron transport chain may be affected by the supply of electrons from PSII, cytosolic NAD(P)H dehydrogenase, the succinate dehydrogenase-mediated electron transport pathway from respiratory donors, as well as PSI cyclic electron transport. Furthermore, the possibility of electron consumption by either the terminal respiratory Cyt oxidase or by PSI will also affect intersystem electron flux in cyanobacterial thylakoid membranes,... 

A similar conclusion follows from the work of de Kouchkovsky s group who measured the rate of ATP formation driven by a cyclic (PSl) or non-cyclic (PSl -h PS2) electron transport chain as a function of the apparent ApH value [99]. The hypothesis of the direct interaction of PS2 protonic domains with ATPsynthase was also considered by Yaguzhinsky s group [124] who studied the effects of the uncoupler gramicidin D on the photophosphorylation rate in chloroplasts during cyclic or noncyclic electron transport. 

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