Noncyclic electron transport

Many cytochromes c are soluble but others are bound to membranes or to other proteins. A well-studied tetraheme protein binds to the reaction centers of many purple and green bacteria and transfers electrons to those photosynthetic centers.118 120 Cytochrome c2 plays a similar role in Rhodobacter, forming a complex of known three-dimensional structure.121 Additional cytochromes participate in both cyclic and noncyclic electron transport in photosynthetic bacteria and algae (see Chapter 23).120,122 124 Some bacterial membranes as well as those of mitochondria contain a cytochrome bct complex whose structure is shown in Fig. 18-8.125,126... 

LOCATION AND FUNCTION OF CYTOCHROME b-559 IN THE CHLOROPLAST NONCYCLIC ELECTRON TRANSPORT CHAIN... 

C2 plays a similar role in Rhodobacter, forming a complex of known three-dimensional structure. Additional cytochromes participate in both cyclic and noncyclic electron transport in photosynthetic bacteria and algae (see Chapter Some bacterial mem-... 

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. 

The mode of action studies suggest that nitro-diphenyl ethers act on the photosynthetic systems of plants. Two pathways are involved, one requiring light, the other not. Thus they can inhibit noncyclic electron transport and coupled photophosphorylation in chloroplasts, and in mitochondria they inhibit electron transport. (Matsunaka, 1969a, 1969b and Moreland et al.. 1970). 

The rate of a noncyclic electron transport was determined by the reduction of NADP from a reduced dichlorphe-nolindophenol in the presence of ferredoxin and montiron. 

Fig. 5.16. The dependence of a steady state transmembrane pH difference, ApH, in bean chloroplasts on the pH value of chloroplast suspension (after [78]). ApH was determined from the partition of spin label TEMPOamine during noncyclic electron transport driven by photosystem 1 and photosystem 2 (H2O - methylviologen). Triangles are without added ADP light or dark circles are in the presence of 2 or 4 mM MgADP.

There are several lines of argument to support the notion of the nonuniform distribution of transmembrane pH gradients. One of them arises from the ApH measurements, while another one is from the comparison between the time-courses of proton accumulation inside the thylakoids during cyclic and noncyclic electron transport. The kinetic study of protons and TA uptake enables us to discriminate between the events associated with loading the protonic pools of grana- and stroma-exposed thylakoids. Comparing the uptake of protons and spin label TA by chloroplasts suspended in the media with different osmomolarity (and thus having different internal volumes), we... 

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. 

Fig. 5.28. The intrathylakoid pH values, pHj , versus external pH in the suspension of bean chloroplasts, noncyclic electron transport (H2O -> methylviologen). The dark symbols are the data obtained with the kinetic method (after [76, 77]), and the light symbols are the data obtained with the use of the TEMPOamine partition technique (after [78]).

In the course of photosynthesis water must be cleaved. All of the processes that are related to the cleavage of water-photolyse, are called the primary processes of photosynthesis. To these belong not only photolysis but also the noncyclic electron transport, which is tightly coupled to it, and cyclic electron transport. 

Thus, electrons flow from one end of the chain to the other, from the electron donor, water, to the terminal electron acceptor, NADP. In this case one speaks of a noncyclic electron transport In addition to oxygen, which is derived from the photolysis of water, its products are NADPH+ H+and ATP. 

Of the products of noncyclic electron transport, O2, ATP, and NADPH + H+, O2 will not interest us further. On the other hand, ATP and NADPH are the substances that link the primary processes with the secondary processes. They do so in that they arise in the primary processes and can then be utilized in the secondary processes for the fixation and reduction of CO2. 

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. 

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. )... 

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. ( ). 

The proton gradient drives ATP synthesis via an ATP synthase located in the thylakoid membrane (photophosphorylation). Since the electron transport involves a linear array of electron carriers, the system is called noncyclic photophosphorylation. 

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. 

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. 

Inhibitory Uncouplers. Inhibitory uncouplers inhibit the reactions affected by both electron transport inhibitors and uncouplers. Hence, they inhibit basal, methylamine-uncoupled, and coupled electron transport with ferricyanide as electron acceptor and water as the electron donor, much like electron transport inhibitors. Coupled noncyclic photophosphorylation is inhibited and the phosphorylation reaction is slightly more sensitive than the reduction of ferricyanide. Cyclic photophosphorylation is also inhibited. NADP reduction, when photosystem II is circumvented with ascorbate + DPIP, is not inhibited however, the associated phosphorylation is inhibited. Inhibitory uncouplers act at both sites 1 and 2 (Figure 2). 

As mentioned in Chapter 35, the Cyt b(Jcomplex is involved not only in noncyclic, or linear, electron transport but also in cyclic transfer around PS I. In the latter case, the electrons received from photosystem I by Fd, instead of going to reduce NADP, are transferred to the plastoquinone pool via b f. During this cyclic process, protons are translocated across the thylakoid membrane, contributing to the transmembrane proton gradient. This cyclic electron-transfer pathway, which is independent of PS II, functionally resembles that of the bacterial photosynthetic system. The existence of a cyclic electron-transfer pathway also helps to account for the observation that chloroplasts often require more than 8 photons for the evolution of one O2 molecule. The physiological function of the cyclic pathway, just as it is for the Q-cycle, is to increase the amount of ATP produced relative to the amount of NADPH formed, and thus provide a mechanism for the cell to adjust the relative amounts of the two substances according to its needs. 

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. 

The NADPHj formed is very rich in energy and is able to realise the first steps of carbon dioxide assimilation. The electron transport mechanism of light reactions I and II is a noncyclic electron transfer process. Its by-product is oxygen, the formation mechanism of which is unknown. 

The proton gradient is created by the operation of the electron transport chain that links the two photosystems in noncyclic photophosphorylation. 

The photochemical activity of PS2 was determined according to the reduction of dichlorphenolindophenol (DCPIPl from HoO in white light, 70 W/m (sattirating) and 4 W/m (limiting). The rates of noncyclic and cyclic electron transport in PSI were determined with whole chloroplasts by the reduction of MDP from DCPIPH in the presence of monu-ron and by the photophosphorylation in aerobic conditions in the presence of phenazine methosulphate (PMS) in white light (70 W/m ). The methods used are described elsewhere (6). The Chi a/Chl b ratio was determined following the described procediire (9). 

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