Electron carriers in photosynthesis

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... 

Having elucidated, in combination with X-ray structural data, the characteristics of the copper site coordination in blue proteins in extenso, the challenge for EPR spectroscopy (and other techniques) is now to find ways to model the electron transfer (ET) in a realistic fashion. At present EPR is, however, mostly used to ascertain that the coordination of copper in the experimental ET chain models employed is not disturbed prior to ET. Plastocyanin is the electron carrier in photosynthesis. Indications of structural origins of impaired ET in... 

Tagawa, K. and D. I. Arnon Ferredoxins as electron carriers in photosynthesis and in the biological production and consumption of hydrogen gas. Nature 195, 537-543 (1962). 

Copper is present in chloroplasts in plastocyanin which is essential as an electron carrier in photosynthesis (48). Copper is also essential for the functioning of phenolases, which are plastid enzymes. 

Simple quinones or benzoquinones are found commonly in nature. Some quinonoid compounds such as the electron carrier ubiquinone (13), sometimes known as coenzyme Q, play important primary roles in plants. Structurally related plasto-quinones are important as electron carriers in photosynthesis. 

Cytochromes. Heme-containing proteins that function as electron carriers in oxidative phosphorylation and photosynthesis. 

The cytochromes are the electron carrier heme proteins occurring in the mitochondrial respiratory chain.449 There are five cytochromes linking coenzymes Q (ubiquinone) and 02 in this electron transport chain (Scheme 7). Cytochromes are also involved in energy transfer in photosynthesis. The iron atom in cytochromes cycles between the Fe11 and Fe111 states, i.e. they are one-electron carriers, in contrast to CoQ and the NADH flavins they act upon which are two-electron carriers. Thus, one molecule of reduced CoQ transfer its two high potential electrons to two molecules of cytochrome b, the next member of the electron transport chain. 

Arlt, T., Schmidt, S., Kaiser, W., Lauterwasser, C., Meyer, M., Scheer, H., and Zinth, W., 1993, The accessory bacteriochlorophyll A real electron carrier in primary photosynthesis. Proc. Natl. Acad. Sci. USA, 90 11757911761. 

Ciurli S, Musiani F. High potential iron-sulfur proteins and their role as soluble electron carriers in bacterial photosynthesis tale of a discovery. Photosynth. Res. 2005 85 115-131. 

We have seen the Z-scheme for the two photosystems in green-plant photosynthesis and the electron carriers in these photosystems. We have also described how the photosystems of green plants and photosynthetic bacteria all appear to function with basically the same sort ofmechanisms of energy transfer, primary charge separation, electron transfer, charge stabilization, etc., yet the molecular constituents of the two reaction centers in green plants, in particular, are quite different from each other. Photosystem I contains iron-sulfur proteins as electron acceptors and may thus be called the iron-sulfur (FeS) type reaction center, while photosystem 11 contains pheophytin as the primary electron acceptor and quinones as the secondary acceptors and may thus be called the pheophytin-quinone (0 Q) type. These two types of reaction centers have also been called RCI and RCII types, respectively. 

Even before the fluorescence-quencher hypothesis was proposed, however, it had been suggested that plastoquinone may be an important electron carrier in green-plant photosynthesis. Bishops reported in 1959 the significant finding that extraction of PQ from chloroplasts results in the loss ofthe ability to evolve oxygen but that upon reconstitution with plastoquinone, oxygen-evolution activity is restored. 

The iron-sulfur proteins play important roles as electron carriers in virtually all living organisms, and participate in plant photosynthesis, nitrogen fixation, steroid metabolism, and oxidative phosphorylation, as well as many other processes (Chapter 7). The optical spectra of all iron-sulfur proteins are very broad and almost featureless, due to numerous overlapping charge-transfer transitions that impart red-brown-black colors to these proteins. On the other hand, the EPR spectra of iron-sulfur clusters are quite distinctive, and they are of great value in the study of the redox chemistry of these proteins. 

Equation (25) describes a process of electron transfer from donor molecules to CO2 (for both oxygenic and anoxygenic photosynthesis). Unlike the anaerobic processes described in Section 3, in which the electron transfer is frequently extracellular (between cells), the electron transfer in photosynthesis is intracellular. As such, it can occur completely via biological electron carriers, and without need for free electron carriers such as H2. However, the presence of hydrogenase enzymes in representatives from both classes of phototrophs [2,24,25] makes H2 metabolism possible in both accessory and primary roles. 

Ubiquinones (UQ), often called coenzyme Qio, are electron carriers in oxidative phosphorylation and photosynthesis, respectively. Ubiquinones consist of quinoid nucleus (derived from the shikimate pathway), 4-hydroxybenzoate (derived from chorismate or tyrosine), and terpenoid moiety. Zeatin, a phytohormone, is a member of the cytokinin family involved in various processes of growth and development in plants. Most cytokinins are adenine-type, where the hydrogen of amino group at Ce position of adenine is replaced with an isoprenoid. 

Arnon and his group have definitively established that ferredoxins (iron-sulfur proteins noted for their strongly electronegative redox potentials) are the primary electron acceptors in photosynthesis, and that they are essential electron carriers for the light-induced generation of reducing power and ATP formed in the processes of cyclic and non-c clic photophosphorylation. Reducing power—either reduced ferredoxin or reduced nicotinamide adenine dinucleotides, NAD(P)H—and ATP constitute the assimilatory power required for the further assimilation in the dark of carbon dioxide, nitrate and sulfate. ... 

Fig. 6.15 In plant photosynthesis, light-induced electron transfer processes lead to the oxidation of water to O2 and the reduction of NADP+ to NADPH, with concomitant production of ATP. The energy stored in ATP and NADPH is used to reduce CO2 to carbohydrate in a separate set of reactions. The scheme summarizes the general patterns of electron flow and does not show aU the intermediate electron carriers in photosystems 1 and 11, the cytochrome b f complex, andferredoxin NADP oxidoreductase.

The cytochromes plays a major role as electron carriers in the respiratory chain, as well as taking part in photosynthetic reactions in green plants, algae, and anaerobic photosynthetic bacteria A comprehensive survey of their occurrence, properties, structure, and function has been presented by Lemberg and Barrett (1973). ESR continues to play an important role in the identification of mitochondrial cytochromes (Kilpatrick and Erecinska, 1977), and delineating events in bacterial and plant photosynthesis (Prince et a/., 1978). The role played by the cytochromes of higher plants and algae in photosynthetic electron transport has been reviewed recently (Knaf 1978). The present work deals with some recent developments in the properties of those cytochromes where ESR information has accumulated. 

RCII may subsequently have been transformed into RCI by formation of the Fx cluster and eventually the capturing of a soluble 2[4Fe-4S] protein as an RC-associated subunit. These additions would have allowed electrons to leave the space of the membrane and serve for reductive processes in the dark reactions of photosynthesis. Our present knowledge concerning distribution of HiPIPs among species indicate that this electron carrier would have been invented only lately within the branch of the proteobacteria. Tbe evolutionary driving... 

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. 

The cytochromes are another group of haem proteins found in all aerobic forms of life. Cytochromes are electron carriers involving a Fe(ii)/Fe(m) redox system. They are a crucial part of the electron transfer reactions in mitochondria, in aspects of the nitrogen cycle, and in enzymic processes associated with photosynthesis. 

Most mechanisms which control biological functions, such as cell respiration and photosynthesis (already discussed in Chapter 5, Section 3.1), are based on redox processes. In particular, as shown again in Figure 1, it is evident that, based on their physiological redox potentials, in photosynthesis a chain of electron carriers (e.g. iron-sulfur proteins, cytochromes and blue copper proteins) provides a means of electron transport which is triggered by the absorption of light. 

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. 

---