Cyclic electron flow

Joliot P, Joliot A (2006) Cyclic electron flow in C3 plants. Biochim Biophys Acta 1757 362-368... 

Using an alternative path of light-induced electron flow, plants can vary the ratio of NADPH to ATP formed in the light this path is called cyclic electron flow to differentiate it from the normally unidirectional or noncyclic electron flow from H20 to NADP+, as discussed thus far. Cyclic electron flow (Fig. 19-49) involves only PSI. Electrons passing from P700 to ferredoxin do not continue to NADP+, but move back through the cytochrome bef complex to plastocyanin. The path of... 

Figure 23-32 Simplified diagram of cyclic electron flow in purple bacteria. Two protons from the cytoplasm bind to QB2 in the reaction center to form QH2 (ubiquinol), which diffuses into the ubiquinone pool. From there it is dehydrogenated by the cytochrome kq complex with expulsion of two protons into the periplasm. A third and possibly a fourth proton may be pumped (green arrows) across the membrane, e.g., via the Q cycle (Fig. 18-9). The protons are returned to the cytoplasm through ATP synthase with formation of ATP. Some electrons may flow to the reaction centers from such reduced substrates as S2 and some electrons may be removed to generate NADPH using reverse electron transport.345...

Cyclic electron flow is represented as a shunt in Figure 2. 

For cyclic electron flow, an electron from the reduced form of ferredoxin moves back to the electron transfer chain between Photosystems I and II via the Cyt blight absorbed only by Photosystem I — a fact that is often exploited in experimental studies. In particular, when far-red light absorbed by Photosystem I is used, cyclic electron flow can occur but noncyclic does not, so no NADPH is formed and no O2 is evolved (cyclic electron flow can lead to the formation of ATP, as is indicated in Chapter 6, Section 6.3D). When light absorbed by Photosystem II is added to cells exposed to far-red illumination, both CO2 fixation and O2 evolution can proceed, and photosynthetic enhancement is achieved. Treatment of chloroplasts or plant cells with the 02-evolution inhibitor DCMU [3-(3,4-dichlorophenyl)-l, 1-dimethyl urea], which displaces QB from its binding site for electron transfer, also leads to only cyclic electron flow DCMU therefore has many applications in the laboratory and is also an effective herbicide because it markedly inhibits photosynthesis. Cyclic electron flow may be more common in stromal lamellae because they have predominantly Photosystem I activity. 

We now recapitulate the accomplishments of the various processes described previously. O2 is evolved inside a thylakoid and readily diffuses out. The protons from the C>2-evolving step plus those transported by the Cyt bef complex are released in the thylakoid lumen, where the membranes prevent their ready escape. In cyclic electron flow (Fig. 5-18), electrons from P qo move to ferredoxin and thence to the Cyt complex, which also causes... 

A. The 710-nm light absorbed by Photosystem I leads to cyclic electron flow and accompanying ATP formation. In the idealized example given, 550-nm light by itself leads to excitation of Photosystem II only, and no photophosphorylation occurs. 

In many organisms, a cyclic process takes place, in which the reduced electron acceptor transfers its electron through a series of carriers back to the oxidized donor. Energy conservation is achieved by coupling proton translocation across a membrane to the electron flow. This type of cyclic electron flow occurs in eukaryotes under some conditions and in many anoxygenic photosynthetic bacteria. No NADPFl is produced, only ATP. This process occurs when cells may require additional ATP, or when there is no NADP+ to reduce to NADPFl. In other organisms, noncyclic electron flow takes... 

Most photosynthetic eubacteria appear to contain cyclic electron transfer pathways driven by the RCs. Electrons from the secondary acceptor of the RC are transferrred first to a quinone pool and then to the secondary donor (Cyt c) via a Cyt bic complex which stores some of the electron redox energy as potential energy in the form of a transmembrane proton gradient. Evidence for cyclic electron flow in the gram-positive line has not yet been found, but it would be surprising not to find it. 

An electron moves from to Qg in about 200 p,s [28-31,51]. Excitation of the reaction center by a second photon sends another electron from P to Q, and then on to Qg with similar kinetics. The fully reduced Qg now probably picks up two protons from the solvent, dissociates from the reaction center as the quinol (QgH2), and is replaced by a fresh molecule of ubiquinone. Electrons from OgH2 return to P" via a Cyt bc complex and a high-potential, c-type cytochrome. This cyclic electron flow drives proton translocation across the chromatophore membrane, and is coupled to the formation of ATP. 

In all oxygen-evolving organisms, the PS I reaction centres finally reduce a water-soluble ferredoxin. This small protein of around 10 kDa has a (2Fe-2S) cluster and a rather low midpoint reduction potential of -400 mV. Ferredoxin binds to the PS I centre after reduction it participates both in linear electron flow to NADP, via ferredoxin-NADP reductase, and in cyclic electron flow around the PS I centre. Two membrane-bound iron-sulfur centres, designated Centre A (or F, ) and Centre B (or Fg), appear to be the terminal acceptors in the reaction centre. Their mode of functioning is not clearly established and their structure is not well known, mainly because they cannot be extracted without their complete denaturation. F and Fq can be photoreduced at low temperature in cells or in purified PS I centres. Characteristic EPR spectra are thus obtained with g values of 1.86, 1.94, 2.05 for F, and 1.89, 1.92, 2.05 for F -. 

Additional polypeptides ascribed to the Cyt b-f complex are a bound form of ferredoxin-NADP reductase (FNR) [99] and one or more smaller polypeptides [100]. An association of the complex with ferredoxin-NADP reductase may be expected in view of the reported role of FNR in cyclic electron flow from PS I to the Cyt complex [101]. FNR remains associated with the complex during the early stages of the purification of the complex but there is no evidence that it is an intrinsic component of the complex necessary for plastoquinol-plastocyanin oxido-reductase. The presence of small polypeptides in the complex requires further investigation. Polypeptides of about 5 kDa have been reported to be associated with the spinach complex [100]. 

Cyclic Electron Flow Through Photosystem I Leads to the Production of ATP Instead of NADPH... 

There is one bacterial system where such reversed electron transfer is of great importance. In Rps. sphaeroides the generated by cyclic electron flow through the reaction centre and cytochrome system is used to induce reversed electron flow from the level of the ubiquinone pool to the NADH/NAD" pool, in a manner analogous to that described for mitochondria. The role of this is to supply low potential electrons for the biosynthetic functions of the cell [38]. 

Chloroplast ferredoxin is a small water soluble protein M W 000) containing an Fe-S center [245]. Its midpoint potential ( — 0.42 V [246]) is suitable for acting as an electron acceptor from the PSI Fe-S secondary acceptors (Centers A and B) and as a donor for a variety of functions on the thylakoid membrane surface and in the stroma. Due to its hydrophylicity and its abundance in the stromal space, ferredoxin is generally considered as a diffusable reductant not only for photosynthetic non-cyclic and cyclic electron flow, but also for such processes as nitrite and sulphite reduction, fatty acid desaturation, N2 assimilation and regulation of the Calvin cycle enzyme through the thioredoxin system [245]. Its possible role in cyclic electron flow around PSI has already been discussed. The mobility of ferredoxin along the membrane plane could be an essential feature of this electron transfer process the actual electron acceptor for this function and the pathway of electron to plastoquinone is, however, still undefined. 

In C. littorale, the inhibition of photosynthetic oxygen evolution and carbon uptake, and the growth of air-grown cells subjected to H-CO2 conditions can be explained by the activity change in PS II. The increase in PS I activity found in the adaptation period suggests that ATP produced by cyclic electron flow around PS I should be used to cope with H-COj stress and for the recovery of PS II activity. 

B) Cyclic electron flow, illustrated in Fig. 3 (B), is facilitated by the introduction of both the oxidized and reduced forms of a single electron carrier, in this case, TMPD. While the reduced form of TMPD can donate electrons to P700, the oxidized form of TMPD (Wurster s Blue, abbreviated as TMPDT can serve as an artificial electron acceptor in photosystem I. Since TMPD has an absorbance band in the region of 575 nm, observation of its absorbance changes is quite convenient in this system. 

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