Chloroplasts electron transfer chain

In plants, the photosynthesis reaction takes place in specialized organelles termed chloroplasts. The chloroplasts are bounded in a two-membrane envelope with an additional third internal membrane called thylakoid membrane. This thylakoid membrane is a highly folded structure, which encloses a distinct compartment called thylakoid lumen. The chlorophyll found in chloroplasts is bound to the protein in the thylakoid membrane. The major photosensitive molecules in plants are the chlorophylls chlorophyll a and chlorophyll b. They are coupled through electron transfer chains to other molecules that act as electron carriers. Structures of chlorophyll a, chlorophyll b, and pheophytin a are shown in Figure 7.9. 

Chapter 6). Other iron-sulfur proteins, so named because they contain iron sulfur clusters of various sizes, include the rubredoxins and ferredoxins. Rubredoxins are found in anaerobic bacteria and contain iron ligated to four cysteine sulfurs. Ferredoxins are found in plant chloroplasts and mammalian tissue and contain spin-coupled [2Fe-2S] clusters. Cytochromes comprise several large classes of electron transfer metalloproteins widespread in nature. At least four cytochromes are involved in the mitrochondrial electron transfer chain, which reduces oxygen to water according to equation 1.29. Further discussion of these proteins can be found in Chapters 6 and 7 of reference 13. 

In addition to NAD and flavoproteins, three other types of electron-carrying molecules function in the respiratory chain a hydrophobic quinone (ubiquinone) and two different types of iron-containing proteins (cytochromes and iron-sulfur proteins). Ubiquinone (also called coenzyme Q, or simply Q) is a lipid-soluble ben-zoquinone with a long isoprenoid side chain (Fig. 19-2). The closely related compounds plastoquinone (of plant chloroplasts) and menaquinone (of bacteria) play roles analogous to that of ubiquinone, carrying electrons in membrane-associated electron-transfer chains. Ubiquinone can accept one electron to become the semi-quinone radical ( QH) or two electrons to form ubiquinol (QH2) (Fig. 19-2) and, like flavoprotein carriers, it can act at the junction between a two-electron donor and a one-electron acceptor. Because ubiquinone is both small and hydrophobic, it is freely diffusible within the lipid bilayer of the inner mitochondrial membrane and can shuttle reducing equivalents between other, less mobile electron carriers in the membrane. And because it carries both electrons and protons, it plays a central role in coupling electron flow to proton movement. 

In the overall reaction catalyzed by the mitochondrial respiratory chain, electrons move from NADH, succinate, or some other primary electron donor through flavoproteins, ubiquinone, iron-sulfur proteins, and cytochromes, and finally to 02. A look at the methods used to determine the sequence in which the carriers act is instructive, as the same general approaches have been used to study other electron-transfer chains, such as those of chloroplasts. 

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 bCyclic electron flow does not involve Photosystem II, so it can be caused by far-red light 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. 

Figure 5-19. Schematic representation of reactions occurring at the photosystems and certain electron transfer components, emphasizing the vectorial or unidirectional flows developed in the thylakoids of a chloroplast. Outwardly directed election movements occur in the two photosystems (PS I and PS II), where the election donors are on the inner side of the membrane and the election acceptors are on the outer side. Light-harvesting complexes (LHC) act as antennae for these photosystems. The plastoquinone pool (PQ) and the Cyt b(f complex occur in the membrane, whereas plastocyanin (PC) occurs on the lumen side and ferredoxin-NADP+ oxidoreductase (FNR), which catalyzes electron flow from ferredoxin (FD) to NADP+, occurs on the stromal side of the thylakoids. Protons (H+) are produced in the lumen by the oxidation of water and also are transported into the lumen accompanying electron (e ) movement along the electron transfer chain.

As with chloroplasts, many questions concerning electron flow and the coupled ATP formation in mitochondria remain unanswered. The first part of the mitochondrial electron transfer chain has a number of two-electron carriers (NAD+, FMN, and ubiquinone) that interact with the cytochromes (one-electron carriers). In this regard, the reduction of O2 apparently involves four electrons coming sequentially from the same Cyt a3. Of... 

It has become clear in the recent years that electron transfer chains of mitochondria, chloroplasts and some bacteria all contain a cytochrome be complex with very similar structural and functional properties (see Refs. 87, 176-180). Although we focus here on the mitochondrial Complex III, much information has, in particular, come from studies on the bacterial chromatophore system [8,87,176,178]. 

Several observations made in whole membranes or in the isolated complexes are in line with these concepts the shifts induced by antimycin A [110,137] and myxothiazol on the absorption spectra of cytochromes b and the alterations of the ESR spectrum of the FeS protein by UHDBT or DBMIB [131]. Moreover, the oxidant-induced reduction of cytochromes b, the key observation for accepting these electron transfer schemes, has been demonstrated in all h/c, complexes isolated so far from mitochondria [134], chloroplasts [111], cyanobacteria [112] and photosynthetic bacteria [110]. In the chloroplast b /f complex this reaction has been demonstrated also in the absence of any exogenously added quinol, indicating that possibly a structurally bound quinone (quinone is always present in the isolated complexes with a stoicheiometry of about 0.5-0.7 mol/mol of cyt. c, [110,111]) is sufficient to drive the reduction of cytochromes [138]. Since a detailed treatment of the genera] mechanism, as well as of the more specific problems of the mitochondrial respiratory chain, are reported in Chapter 3 of this volume, the following discussion will deal only with the specific features of the electron transfer chains in photosynthetic membranes. 

Once metals have been transported to their target tissue, they need to be distributed within the subcellular compartments where they are required, and need to be safely stored when they are in excess. Nearly 90% of Fe in plants is located in the chloroplasts, where it is required in the electron transfer chain, and in the synthesis of chlorophylls, haem, and Fe—S clusters. Fe, Cu, and Zn are also required in chloroplasts as cofactors for superoxide dismutases to protect against damage by reactive oxygen species during chloroplast development, and Cu is also required in other enzymes including the essential Cu protein plastocyanin. Pathways of intracellular metal transport in plant cells are illustrated in Fig. 8.10. Transport into the chloroplast is best characterised for Cu,... 

The extent of coupling between photophosphorylation and electron transport in chloroplasts is usually expressed by the ratio of ATP formed per pair of electrons transferred, written as ATP/Cj or P/Cj. This parameter expresses the amount of ATP formed divided by the number of pairs of electrons transferred through the electron-transfer chain. The P/ej ratio for phosphorylation coupled to the transfer of electrons from water to photosystem I can be computed by taking the HVe ratio of 2 (4 protons per electron-pair transferred) and the HVATP ratio of 3 (three protons required to flow through CFo F to produce one ATP), to obtain the P/c2 value of 1.33. 

Figure 28.12 showed cytochromes to be vital members of the mitochondrial electron-transfer chain they are also essential components in plant chloroplasts for photosynthesis. Cytochromes are haem proteins, and the ability of the iron centre to undergo reversible Fe(III) Fe(II) changes allows them to act as one-electron transfer centres. Many different cytochromes are known, with the reduction potential for the Fe /Fe " " couple being tuned by the surrounding protein environment. Cytochromes belong to various families, e.g. cytochromes a, cytochromes b and cytochromes c, which are denoted according to the substituents on the... 

When investigating the electron transport processes in mitochondria and chloroplasts and bacteria, it is simplest to assume that the interaction of the chain components follows the mass action law.11-14 That would mean that the free movement of individual elements of the chain in the membrane is possible and also transfer of the charge by accidental collisions. However, transfer of electrons during both respiration and photosynthesis passes along the electron transfer chain organized into definite structural complexes. Consequently a molecule possessing an electron can donate it to a... 

The true nature of oxidative phosphorylation, i.e. the synthesis of ATP linked to the transfer of electrons along a chain of electron (or hydrogen) carriers, was emphasized by the discovery of photosynthetic phosphorylation in chloroplasts by Arnon in 1954.< > Here neither oxidizable substrates nor oxygen play a role. Electrons derived from the photolytic cleavage of water are fed into an electron-transfer chain and withdrawn at the other end of the chain by the oxidizing equivalents formed by the photolytic reaction. 

Figure 2-16 Beta cylinders. (A) Stereoscopic a-carbon plot of plastocyanin, a copper-containing electron-transferring protein of chloroplasts. The copper atom at the top is also shown coordinated by the nitrogen atoms of two histidine side chains. The side chains of the aromatic residues phenylalanines 19,29, 35,41, 70, and 82 and tyrosines 80 and 83 are also shown. Most of these form an internal cluster.

---