Thylakoid membrane pathways

Traditionally, the electron and proton transport pathways of photosynthetic membranes (33) have been represented as a "Z" rotated 90° to the left with noncycHc electron flow from left to right and PSII on the left-most and PSI on the right-most vertical in that orientation (25,34). Other orientations and more complex graphical representations have been used to depict electron transport (29) or the sequence and redox midpoint potentials of the electron carriers. As elucidation of photosynthetic membrane architecture and electron pathways has progressed, PSI has come to be placed on the left as the "Z" convention is being abandoned. Figure 1 describes the orientation in the thylakoid membrane of the components of PSI and PSII with noncycHc electron flow from right to left. 

Most of the arguments described in the sections on bacterial signal peptides and membrane proteins seem to be valid for the eukaryotic systems, as well as the translocation phenomena across the ER membrane (Sakaguchi, 1997). They seem to be also true for the translocation system across the mitochondrial inner membrane protein into the intermembrane space and the system across the thylakoid membrane in chloroplasts. Although the TAT-dependent pathway has not been found in the ER, it exists on the thylakoid membrane (and possibly on the inner membrane of mitochondria). 

Chloroplasts are a typical type of plastid that performs various metabolic reactions as well as photosynthesis. Their envelope consists of two membranes the outer envelope membrane and the inner membrane (Fig. 7). The space between these two membranes is called the intermembrane space, and the space enclosed by the inner envelope membrane is called the stroma. In addition, chloroplasts have another membrane system within the stroma the thylakoid membrane forms the lumen. Therefore, there are six different localization sites and, of course, multiple pathways to each site. Naturally, their sorting mechanisms are very complicated. 

All thylakoidal proteins seem to be first translocated into the stroma through the previously mentioned general import pathway all of them have a cleavable N-terminal transit peptide. However, there are at least four different pathways into the thylakoid membrane (Robinson et al, 1998 Schnell, 1998). Most of them are reminiscent of the pathways of bacteria, described in Section II,B,1. It is not surprising because chloroplasts are most likely evolved from a prokaryotic endosymbioint, but there are certain differences. 

The transport of proteins into chloroplasts also occurs by more than one mechanism. An SRP-dependent pathway may be needed only for insertion of proteins into membranes.594 Other proteins, among which are the 23-kDa and 16-kDa photosystem II proteins (Chapter 23), enter by a pathway related to the Tat pathway of bacteria. In thylakoids this pathway is directly dependent upon the large pH difference (A pH) across die thylakoid membrane. In contrast to the bacterial Sec pathway, the A pH pathway seems to be able to transport completely folded proteins. 

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... 

An elegant example of this is the monitoring of herbicide residues via the photosynthetic electron transport (PET) pathway by utilising cyanobacteria or thylakoid membranes (5). For many herbicides the mode of action is as inhibitors of PET, often acting between the 2 photosystems as indicated in figure 3, and the result is a decrease in the photocurrent. 

The respiratory and photosynthetic electron-transfer pathways are proton pumps operating with the same polarity as does the A TP synthase when hydrolyzing A TP Since it is difficult to detect protons circulating in the steady-state, Mitchell and Moyle [19] studied the transient extrusion of protons when a small amount of oxygen is injected into an anaerobic incubation of mitochondria in the presence of substrate. Prior to this, Neumann and Jagendorf [20] had observed a light-dependent proton uptake into chloroplast thylakoid membranes. 

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. 

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. 

Cytochrome b /also serves as an intermediate in a non-linear, or so-called cyclic, electron-transport pathway around PS I, as formulated in Fig. 1 (B). A third function of Cyt b /is translocation of protons across the thylakoid membrane during electron transfer from plastoquinol to plastocyanin [Fig. 1 (C)]. The unique effects resulting from electron transport and proton translocation in the cytochrome b(f complex are the production of an electrochemical potential and a pH gradient across the thylakoid membrane to provide energy in a form needed for ATP synthesis (see the following chapter). 

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. 

A Q cycle similar to that observed in the electron transport pathway that links PSI and PSII is believed to be responsible for pumping 2 protons across the thylakoid membrane for each electron transported. The proton flow drives ATP synthesis. No NADPH is produced. 

Dalbey, R. E., and A. Kuhn. 2000. Evolutionarily related insertion pathways of bacterial, mitochondrial, and thylakoid membrane proteins. Ann. Rev. Cell Devel. Biol. 16 51-87. 

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