Cyclic electron transfer

Proton translocations accompany these cyclic electron transfer events, so ATP synthesis can be achieved. In cyclic photophosphorylation, ATP is the sole product of energy conversion. No NADPFI is generated, and, because PSII is not involved, no oxygen is evolved. The maximal rate of cyclic photophosphorylation is less than 5% of the rate of noncyclic photophosphorylation. Cyclic photophosphorylation depends only on PSI. 

The characteristic derivative-shaped feature at g 1.94 first observed in mitochondrial membranes has long been considered as the sole EPR fingerprint of iron-sulfur centers. The EPR spectrum exhibited by [4Fe-4S] centers generally reflects a ground state with S = I and is characterized by g values and a spectral shape similar to those displayed by [2Fe-2S] centers (Fig. 6c). Proteins containing [4Fe-4S] centers, which are sometimes called HIPIP, essentially act as electron carriers in the photoinduced cyclic electron transfer of purple bacteria (106), although they have also been discovered in nonphotosynthetic bacteria (107). Their EPR spectrum exhibits an axial shape that varies little from one protein to another with g// 2.11-2.14 and gi 2.03-2.04 (106-108), plus extra features indicative of some heterogeneous characteristics (Pig. 6d). 

We discuss this with help of Fig. 2 for a cyclic electron transfer from a molecule in the ground state to the vacuum level, and back to the ionized molecule. After each electron transfer we leave enough time for relaxation of the solvent to reach the new equilibrium state of interaction in which electrostatic forces play a prom-... 

In the Z scheme, photosystem II, the cytochrome b6f complex and photosystem I operate in series to move electrons from H20 to NADP+ and to create an electrochemical potential gradient for protons across the thylakoid membrane. In addition to this linear pathway, chloroplasts in some plant species may use a cyclic electron-transfer scheme that includes photosystem I and the cytochrome b6f... 

Nocek JM, Sishta BP, Cameron JC, Mauk AG, Hoffman BM. Cyclic electron transfer within the [Zn-myoglobin, cytochrome >s] complex. J Am Chem Soc 1997 119 2146-55. 

Crofts, A. R., Meinhardt, S. W., Jones, K. R., and Snozzi, M., 1983, The role of the quinone pool in the cyclic electron transfer chain of Rhodopseudomonas sphaeroides. A modified Q-cycle mechanism. Biochim. Biophys. Acta, 723 2029218. 

As far as we know, the cyclic electron transfer pathways in Chloroflexus are similar to those found in purple bacteria [13,35]. As shown in Fig. 3, there appear to be cyclic pathways involving b- and c-type cytochromes, Fe-S centers, and qui-nones in both kinds of bacteria grown phototrophically. However, Chloroflexus contains only MQ [32], while any given purple bacterium will always contain ubiquinone (UQ) with or without some MQ. On the electron acceptor side of the RC in Chloroflexus there are apparently two MQ molecules in series (MQ and MQb), which are assumed to feed electrons into an MQ pool [30,36]. Electrons presumably enter a Cyt b c complex from the MQ pool and leave the complex via Cyt c = 0.21 V) and Cyt c-554 j = 0.28 V) [35]. The membrane-bound Cyt c-554 ( m,8 +0.26 V, = 43 000) [37] is the direct electron donor to P-865 in the RC [22,30,36]. It contains two hemes with redox potentials of +0.14 and +0.26 V respectively [36] and is absent from aerobically grown cells [38]. 

The membrane-bound ATP synthetase couples phosphorylation to a proton gradient [90] which is generated by the cyclic electron transfer system (Fig. 3). This system includes the RC, a UQ pool [91], a Cyt bic complex [92,93], and a specialized Cyt c (E j = -fO.34 V) for transferring electrons to the oxidized primary donor (P-870 or P-970 ) of the RC. In some bacteria such as Chromatiurn vi-nosum and Rhodopseudomonas viridis this specialized Cyt c is bound to the RC in the membrane [93,94], whereas in other bacteria such as Rb. sphaeroides and Rhodospirillum rubrum this cytochrome is a periplasmic protein (Cyt C2) that binds to the membrane-bound RC [90]. 

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. 

Figure 1 Reaction mechanism of DNA photolyases (A) mechanism of cyclobutane photolyase and (B) mechanism of (6-4) photolyase. Both photolyases harness blue light energy to remove UV-induced damage and contain two noncovalently bound chromophores. They bind UV-damaged DNA in a reaction that is light independent and carry out catalysis in a light-initiated cyclic electron transfer. In (6-4) photolyase, the (6-4) photoproduct is converted to a four-membered oxetane ring thermally (kT) before the photochemical reaction.

Photosynthetic reaction centers plug into the chemiosmotic scheme by using light-excited states to create both an oxidant and a reductant. For the purple bacterial reaction centers, these oxidants and reductants are the redox carriers already described, oxidized cytochrome c and reduced ubiquinone QH2. Thus, in combination with Complex III, light drives a relatively straightforward cyclic electron transfer that generates a transmembrane electric field and proton gradient. 

The general function of this complex is that of transferring electrons from ubiquinone (or plastoquinone) to a hydrophilic protein acceptor (cytochrome c or plastocyanin). Therefore, in bacterial photosynthesis, it catalyzes the recycling of electrons from the secondary electron acceptor (Qn) to the secondary electron donor (cyt. Cj), completing thereby the cyclic electron transfer system. In chloroplasts and cyanobacteria, an analogous system transfers the electrons from plastoquinone (the secondary acceptor of PSII, A, 3) to plastocyanin (the secondary donor to PSI, 0, 2) and provides in this way an intersystem redox connection between PSII and PSI. The same complex is also involved in the cycling of electrons around PSI. 

Deactivation of S2 and S3 states could proceed either via a redox component which replaces water as the electron donor or through a back reaction within PSII. A class of reagents, including CCCP and substituted thiophenes (the so-called ADRY compounds) has been demonstrated to accelerate up to 50-fold the decay of S2 and S3 states. In the presence of these compounds deactivation probably occurs through a cyclic electron transfer u ound PSII which involves an endogenous reductant [199]. S2 and S3 states are on the contrary stabilized by ammonia, an inhibitor of O2... 

Less clear is the pathway of cyclic electron transfer around PSI. This pathway involves cyt. 6 and cyt. /, and an energy conserving site (demonstrated by the formation of ATP, proton pumping and a slow rising electronchromic signal. 

In bacterial chromatophores the RC and the b/c, complex are arranged to form a cyclic electron transfer system possibly mediated by the diffusion of ubiquinone and cyt. Cj these carriers are, however, also coupled to other multienzyme complexes forming the respiratory chain and perform the aerobic metabolism of these facultative photosynthetic organisms [254]. The electrogenic steps of the photosynthetic cycle take place both within the RC and the 6/cj complexes and can be monitored by the electrochromic spectral shift of endogenous carotenoids and on the basis of their response to specific inhibitors and kinetics. When induced by a short laser flash the carotenoid signal displays three distinct kinetic phases (r,/2 10 h/i 5 jas... 

Note that this cyclic electron-transfer process produces no net oxidation or reduction. However, in the process, protons acquired from the cytoplasm are translocated across the plasma membrane to establish a transmembrane electrochemical potential gradient. The dissipation of such a proton gradient then provides the necessary energy to drive ATP synthesis. A similar simplified cyclic electron-transport diagram has been shown earlier in Chapter 3 as Fig. 12 (C) on p. 81, in coimection with a discussion of a LHl-RC-Cyt6c, supercomplex of Rb. sphaeroides. More detailed discussion of the cytochromeic] and bff complexes and ATP synthesis will be presented in Chapters 35 and 36, respectively. 

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. 

An appropriate electron carrier such as PMS (5-N-methyl-phenazonium methylsulfate) can mediate a cyclic electron transfer around photosystem 1. PMS mediates the cyclic electron flow by serving as an electron acceptor on the reducing side of PS 1 with the reduced PMS serving as a donor of electrons directly to photooxidized P700, as illustrated in Fig. 9 (A). Thus a simple system containing just photosystem I embedded in a closed membrane system that also contains the ATP synthase plus the electron carrier PMS should in principle carry out photophosphorylation. Indeed, Hauska, Samoray, Orlich and Nelson prepared such a simple, minimum system using purified individual membrane complexes of chloroplasts, namely, photosystem I and ATP synthase, plus a soybean phospholipid (asolectin) as the membrane matrix to demonstrate its expected effectiveness. 

Alternatively, formation of the 1,3,5-triazines may occur due to an energy gain during cycliza-tion involving a cyclic electron transfer.146... 

These reactions are cyclic electron transfers, with the same reaction scheme as in Figure 17 except that no escape (no separation of the radical ions by diffusion) is possible D corresponds to the special pair and A to a pheophytin. In all cases, CIDNP is of the S-Tq type but there are three competing mechanisms of CIDNP generation, the relative amounts of which strongly depend on the chemical and magnetic parameters of the system ... 

Amino Acids, Peptides, and Proteins. The determination of the accessibility of amino acid residues is the standard application of CIDNP to proteins and larger peptides, the key idea being that only amino acids exposed to the surface can react with a photoexcited dye. The photoreactions must be reversible to avoid unwanted structural changes of the biopolymer that are induced by the experiment itself. This can be realized with cyclic electron-transfer (cf. Section V.A.2, Chart VI) or hydrogen-transfer reactions. Because of the photochemistry of amino acids, the only... 

A wide variety of different cytochrome-linked electron-transfer systems is encountered in bacteria respiratory chains with oxygen, nitrate or sulphate as electron acceptors, fumarate reductase systems and light-driven cyclic electron-transfer systems (Fig. 3). All these systems are composed of several electron-transfer carriers, the nature of which varies considerably in different organisms. Electron carriers which are most common in bacterial electron-transfer systems are flavoproteins (dehydrogenases), quinones, non-heme iron centres, cytochromes and terminal oxidases and reductases. One common feature of all electron-transfer systems is that they are tightly incorporated in the cytoplasmic membrane. Another important general property of these systems is that electron transfer results in the translocation of protons from the cytoplasm into the external medium. Electron transfer therefore... 

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