Chloroplasts electron carriers

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. 

Let us start our examination with the prototypical blue protein plastocyanin, found in the thylacoid membrane of chloroplasts, where it acts as an electron carrier in photosynthesis (see Figure 1). As Figure 30 illustrates, the active site of plastocyanin is formed of a Cu(II) ion (pseudo)tetrahedrally coordinated to two histidine nitrogen atoms and... 

Artificial cell-free systems have been investigated, to test models of photosynthetic production of H2. Benemann et al. (1973) demonstrated that it was possible to produce H2 and O2 by combining chloroplasts from green plants and bacterial hydrogenase, with ferredoxin as the intermediate electron carrier ... 

Selected entries from Methods in Enzymology [vol, page(s)] Electron-transport chain [components, 69, 205, 206 sites of inhibition, 69, 676, 677] chloroplast [autoxidizable carriers, 69, 416, 417 DBMIB, 69, 422, 423 dichlorophenolindophenol and related carriers, 69, 418 ferricyanide, 69, 417, 418 isolated, 69,... 

The 2Fe2S (S, acid-labile sulfur) ferredoxins have a redox active binuclear center, with each of the two iron atoms attached to the protein by two cysteinyl sulfur ligands and connected by two inorganic acid-labile sulfur ligands. At cty-ogenic temperatures these clusters are EPR detectable, with characteristic features in the vicinity of g = 1.94. Spinach ferredoxin has principal g values of 2.03, 1.96, and 1.88 and a broad absorbance spectrum with a weak maximum around 420 nm, giving these proteins a reddish brown color which bleaches on reduction. Ferredoxins are low potential electron carriers chloroplast ferredoxins function in photosynthetic electron transfer, but related proteins such as adrenal ferredoxin are involved in steroidogenic electron transfer in mitochondria in tissues which produce steroid hormones. 

Ubiquinone (also called coenzyme Q) and plasto-quinone (Fig. 10-22d, e) are isoprenoids that function as lipophilic electron carriers in the oxidation-reduction reactions that drive ATP synthesis in mitochondria and chloroplasts, respectively. Both ubiquinone and plasto-quinone can accept either one or two electrons and either one or two protons (see Fig. 19-54). 

Ubiquinones and plastoquinones, also isoprenoid derivatives, function as electron carriers in mitochondria and chloroplasts, respectively. 

We examine the function of flavoproteins as electron carriers in Chapter 19, when we consider their roles in oxidative phosphorylation (in mitochondria) and photophosphorylation (in chloroplasts), and we describe the photolyase reactions in Chapter 25. 

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. 

The extent to which an electron carrier is oxidized or reduced during photosynthetic electron transfer can sometimes be observed directly with a spectrophotometer. When chloroplasts are illuminated with 700 nm light, cytochrome/, plastocyanin, and plastoquinone are oxidized. When chloroplasts are illuminated with 680 nm light, however, these electron carriers are reduced. Explain. 

In addition to its role as an intermediate in cholesterol biosynthesis, isopentenyl pyrophosphate is the activated precursor of a huge array of biomolecules with diverse biological roles (Fig. 21-48). They include vitamins A, E, and K plant pigments such as carotene and the phytol chain of chlorophyll natural rubber many essential oils (such as the fragrant principles of lemon oil, eucalyptus, and musk) insect juvenile hormone, which controls metamorphosis dolichols, which serve as lipid-soluble carriers in complex polysaccharide synthesis and ubiquinone and plastoquinone, electron carriers in mitochondria and chloroplasts. Collectively, these molecules are called isoprenoids. More than... 

A crucially important finding is that submitochon-drial particles or vesicles from broken chloroplasts will synthesize ATP from ADP and P , when an artificial pH gradient is imposed.172186 Isolated purified FjF0 ATPase from a thermophilic Bacillus has been coreconstituted into liposomes with the light-driven proton pump bacteiiorhodopsin (Chapter 23). Illumination induced ATP synthesis.187 These observations support Mitchell s proposal that the ATP synthase is both spatially separate from the electron carriers in the membrane and utilizes the protonmotive force to make ATP. Thus, the passage of protons from the outside of the mitochondria back in through the ATP synthase induces the formation of ATP. What is the stoichiometry of this process ... 

The soluble electron carriers released from the reaction centers into the cytoplasm of bacteria or into the stroma of chloroplasts are reduced single-electron carriers. Bacterial ferredoxin with two Fe4S4 clusters is formed by bacteria if enough iron is present. In its absence flavodoxin (Chapter 15), which may carry either one or two electrons, is used. In chloroplasts the carrier is the soluble chloroplast ferredoxin (Fig. 16-16,C), which contains one Fe2S2 center. Reduced ferredoxin transfers electrons to NADP+ (Eq. 15-28) via ferredoxin NADP oxidoreductase, a flavoprotein of known three-dimensional structure.367 369... 

Photosynthesis occurs in the plant cell organelle called the chloroplast. During the process of photosynthesis, electrons are transferred from H20 to NADP+ via an electron carrier system. The energy released by electron transport is converted into the form of a proton gradient and coupled to ADP phosphorylation. In this experiment a method is introduced to demonstrate the formation of the proton gradient across the chloroplast membranes. 

The photosynthetic process in green plants occurs in subcellular organelles called chloroplasts. These organelles resemble mitochondria they have two outer membranes and a folded inner membrane called the thy-lakoid. The apparatus for photosynthesis, including the chlorophyll reaction centers and electron carriers, is in the thylakoid membrane. The chemical reactions of the Calvin cycle take place in the stroma, the region around the thylakoid membrane. 

The plastocyanins are found in plant chloroplasts and other photosynthetic organisms, and act as membrane-bound electron carriers between photosystems II and I in the photosynthetic pathway of higher plants, green algae and some blue-green algae. 

Triazine (e.g., atrazine, simazine) and substituted urea (e.g., diuron, monuron) herbicides bind to the plastoquinone (PQ)-binding site on the D1 protein in the PS II reaction center of the photosynthetic electron transport chain. This blocks the transfer of electrons from the electron donor, QA, to the mobile electron carrier, QB. The resultant inhibition of electron transport has two major consequences (i) a shortage of reduced nicotinamide adenine dinucleotide phosphate (NADP+), which is required for C02 fixation and (ii) the formation of oxygen radicals (H202, OH, etc.), which cause photooxidation of important molecules in the chloroplast (e.g., chlorophylls, unsaturated lipids, etc.). The latter is the major herbicidal consequence of the inhibition of photosynthetic electron transport. 

Figure 4. Scheme for proton transfer by plastoquinone as a mobile carrier in membrane lipid. Electrons are transferred one by one to a bound plastoquinone A (PQA) which in turn reduces external plastoquinone. When reduced, the anionic plastoquinone takes up protons to become a hydroquinone which is oxidized by the cytochrome bb f complex on the inside of the membrane to release protons. A second quinone, vitamin K, (KQ) is also involved in chloroplast electron transport, but its role in proton movement is not known. 

Electron transport through oxidases in the plasma membrane contributes to, or controls, part of the proton release from the cell. The details of oxidase function and the mechanism of control remain to be elucidated. The NADPH oxidase of neutrophils is a special case in which proton transport is coupled to the cytochrome >557 electron carrier. This type of proton transport has its precedents in the well-characterized proton pumping through electron carriers in mitochondrial and chloroplast membranes and prokaryotic plasma membranes. 

Most of the components involved in electron transport in mitochondria are contained in four supramolecular protein complexes that traverse the inner mitochondrial membrane. Complex I, which contains FMN and various iron-sulfur clusters as active sites, transfers electrons from NADH to ubiquinone (Fig. 6-8). Complex II, which contains FAD, various iron-sulfur clusters, and a Cyt >, transfers electrons from succinate also to a ubiquinone. Ubiquinone functions as a pool of two-electron carriers, analogous to the function of plastoquinone A in the lamellar membranes of chloroplasts, which accepts electrons from Complexes I and II and delivers them to the... 

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... 

The photosynthetic apparatus of green plants and cyanobacteria oxidizes water and transfers electrons to NADP, with a net gain in electrochemical potential of 1.13 eV (at pH 7), utilizing the energy of two light quanta per electron. The complete system is contained in the chloroplasts, and is localized within the thylakoid membranes, with the exception of the electron carrier ferredoxin, which is in solution in the stroma, and serves to transfer electrons from the reducing end of photosystem I (PS I) to a membrane-bound flavoprotein which then reduces NADP, and of the copper protein plastocyanin (PC, the electron donor to PS I), which is in solution in the internal phase of thylakoids. 

If the model proposed by Andersson and Anderson [109] of total separation of PS I and PS II in the granal chloroplasts were to be accepted, electron transport from the PS II acceptors to P-700 would require a mobile electron carrier(s) which should diffuse laterally in the membrane fast enough to account for the observed electron transport rate. Plastoquinone [112] and plastocyanin are the candidates of choice for this role. The former has been shown to be present at approximately the same activity in the partitions and in the stroma-exposed membranes [43], while PC is known to be located in the intrathylakoid space [113],... 

The areas in the electron transport pathway where energy conservation is observed are termed coupling sites. One site, between plastoquinone and Cyt/, was originally identified in thylakoid preparations by the cross-over phenomenon [14] when ADP, for example, is added to illuminated chloroplasts when electron flow is severely limited by the phosphorylation reaction, all electron carriers which precede the coupling site will be oxidized, while all carriers which follow the coupling site will be reduced. 

Fig. 4.4. Optical and ESR spectra of electron carriers in PSII-RC of higher plants and algae. (A) Primary electron donor (D ,) chlorophyll a. Light-dark differential spectrum of P-682 in chloroplasts obtained by plotting the extent of the fast-decaying component of the flash-induced optical changes including both P-682 and P-700 oxidation (from Ref. 291).

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