Photosynthetic electron transport bacterial

Spectroscopic and crystallographic studies have identified four Fe S clusters in the membrane-bound photosynthetic electron transport chain of plant and cyanobacterial chloro-plasts. One is the Rieske-type [2Fe-2S] + + center in the cyt b(,f complex, which catalyzes electron transfer from plasto-quinol to plastocyanin with concomitant proton translocation, and is functionally analogous to the cyt bc complex, with cyt / in place of cyt The remainder are low-potential [4Fe 4S] + + centers in Photosystem I which constitute the terminal part of the electron transfer chain that is initiated by the primary donor chlorophyll. One is a very low-potential [4Fe S] + + center, Fx (Em =-705 mV), that bridges two similar subunits (PsaA and PsaB) and is coordinated by two cysteines from each subunit in a C-Xg-C arrangement. This cluster transfers electrons to the 2Fe-Fd acceptor via an electron transfer chain composed of Fa, a [4Fe S] + + cluster with Em = -530 mV, and Fb, a [4Fe S] + + clusters with Em = -580 mV. Fa and Fb are in a low-molecular weight subunit (PsaC, 9 kDa) that shows strong sequence and structural homology with bacterial 8Fe-Fds. The center-to-center distance between Fx and Fa and between Fa and Fb are 14.9 A and 12.3 A, respectively, well... 

Nonetheless, photosynthesis did not evolve immediately at the origin of life. The failure to discover photosynthesis in the domain of Archaea implies that photosynthesis evolved exclusively in the domain of Bacteria. Eukaryotes appropriated through endosymbiosis the basic photosynthetic units that were the products of bacterial evolution. All domains of life do have electron-transport chains in common, however. As we have seen, components such as the ubiquinone-cytochrome c oxidoreductase and cytochrome hf family are present in both respiratory and photosynthetic electron-transport chains. These components were the foundations on which light-energy-capturing systems evolved. 

A band with a molecular weight of 25 000 of the bacterial oxidoreductase has been identified with the high-potential Fe-S protein, by means of cross reaction with a monospecific antibody against the analogous electron carrier from Neurospora crassa mitochondria. The existence of this type of Fe-S center in photosynthetic bacteria was first discovered by ESR spectroscopy [125] and its involvement in photosynthetic electron transport was demonstrated. The midpoint potential in Rps. sphaeroides is 0.285 V, and is pH dependent above pH 8, with a decrease of 60 mV per pH unit [125]. 

Bacterial copper proteins are found only in the plasma membrane (Gram-positive bacteria) or in the plasma membrane and the periplasm (Gram-negative bacteria), not in the bacterial cytoplasm. However, cyanobacteria do have copper proteins in their cytoplasm. These important photosynthetic bacteria require copper for plastocyanin, which plays a critical role in the photosynthetic electron transport chain. Both plastocyanin and cytochrome c oxidase are found in the thylakoid compartments within the cytoplasm. In Synechocystis, the Cu(I) Pie-ATPase CtaA imports Cu(I). A second ATPase, PacS, imports Cu(I) into the thylakoid, and the Atxl-like copper chaperone ScAtxl is believed to deliver Cu(I) from CtaA to PacS (Fig. 8.6). 

Chromatophores 1. plastids of higher plants Chloroplasts (see), Chromoplasts (see) and Leuco-plasts (see). 2. The photosynthetic organelle of Photosynthetic bacteria (see). Bacterial C. are intraplasmatic membranes originating from the cell membrane. They may exist as closed vesicles or as flattened stacks, whose membranes contain the photosynthetic pigments, and the components of photosynthetic electron transport and photophosphorylation. 

The development of herbicides that inhibit photosynthetic electron transport has proved to be outstandingly successful. Furthermore, because of the efficiency of these compounds and their use as tools to study photosynthesis, our knowledge of photosystem II in particular has been greatly enhanced by their use. Recent developments involving X-ray structural analysis of photosynthetic bacterial reaction centers as well as the ability to engineer herbicide resistance into crop plants have been outstanding scientific achievements. 

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. 

Nishimura M I to T and Chance B (1962) Studies on bacterial phosphox ylation. Ill A sensitive and rapid method of determination of photophosphorylation, Biochim. Biophys. Acta. 59, 177-182 Renger G Erixon K Do ring G and Wolff Ch (1976) Studies on the nature of the inhibitory effect of trypsin on the photosynthetic electron transport of system II in spinach chloropleists, Biochim. Biophys. Acta. 440, 278-286. 

Fig. 5.2. The photosynthetic membrane of a green sulfur bacterium. The light-activated bacte-riochlorophyll molecule sends an electron through the electron-transport chain (as in respiration) creating a proton gradient and ATP synthesis. The electron eventually returns to the bacteri-ochlorophyll (cyclic photophosphorylation). If electrons are needed for C02 reduction (via reduction of NADP+), an external electron donor is required (sulfide that is oxidised to elemental sulfur). Note the use of Mg and Fe.

Many cytochromes c are soluble but others are bound to membranes or to other proteins. A well-studied tetraheme protein binds to the reaction centers of many purple and green bacteria and transfers electrons to those photosynthetic centers.118 120 Cytochrome c2 plays a similar role in Rhodobacter, forming a complex of known three-dimensional structure.121 Additional cytochromes participate in both cyclic and noncyclic electron transport in photosynthetic bacteria and algae (see Chapter 23).120,122 124 Some bacterial membranes as well as those of mitochondria contain a cytochrome bct complex whose structure is shown in Fig. 18-8.125,126... 

Fujita, Y. and Murakami, A. 1987. Regulation of electron transport composition in cyano-bacterial photosynthetic system stoichiometry among PSI and PSII complexes and their light harvesting antenna and Cyt bt, f complex. Plant Cell Physiol. 28, 1547-1553. 

This chapter covers general characteristics of photosynthetic bacteria, with special emphasis on RCs, light-harvesting, electron transport and bacterial phylo-geny. 

C2 plays a similar role in Rhodobacter, forming a complex of known three-dimensional structure. Additional cytochromes participate in both cyclic and noncyclic electron transport in photosynthetic bacteria and algae (see Chapter Some bacterial mem-... 

We now summarize in Fig. 11 the reaction-center structure and the known electron-transport reactions in purple bacteria. A simplified representation of the reaction-center and the light-harvesting complexes contained in the bacterial membrane is shown in Fig. 11 (A), followed by a column model and a cofactor model in Fig. 11 (B). The cofactor model is used to illustrate the various electron-transport steps with the associated rate constants in Fig. 11 (C), where the cofactors in the starting state (oxidized or reduced) are shown in solid black. When a cofactor first becomes reduced or oxidized, it is shown as an open symbol. We will also use this cofactor model and reaction sequence as a framework for introducing the remaining chapters throughout the section on photosynthetic bacteria. 

Based on the nature of the cytochromes, there are two kinds of photosynthetic bacterial reaction centers. The first kind, represented by that of Rhodobacter sphaeroides, has no tightly bound cytochromes. For these reaction centers, as shown schematically in Fig. 2, left, the soluble cytochrome C2 serves as the secondary electron donor to the reaction center the RC also accepts electrons from the cytochrome bc complex by way ofCytc2- The rate of electron transfer from cytochrome to the reaction center is sensitive to the ionic strength of the medium. Functionally, cytochrome C2 is positioned in a cyclic electron-transport loop. In Rb. sphaeroides, Rs. rubrum and Rp. capsulata cells, the two molecules of cytochromes C2 per RC are located in the periplasmic space between the cell wall and the cell membrane. When chromatophores are isolated from the cell the otherwise soluble cytochrome C2 become trapped and held by electrostatic forces to the membrane surface at the interface with the inner aqueous phase. These cytochromes electrostatically bound to the membrane can donate electrons to the photooxidized P870 in tens of microseconds at ambient temperatures, but are unable to transfer electrons to P870 at low temperatures. 

The presence of the two new chlorophyll molecules ( A and A ) is significant in that it points to the similarity between the photosystem-1 and the purple-bacterial reaction centers with regard to the electron-transport pathway and the kinds of pigment molecules involved, as well as their locations. While the involvement of an intermediary chlorophyll in electron transport in photosynthetic bacteria has gradually become clear (see Chapter 7), a similar involvement of an intermediary chlorophyll in photosystem 1 can only be surmised at present. With regard to the various cofactors involved, it is not known yet which ofthe two branches, primed or unprimed in Fig. 3, constitutes the photoactive electron-transport pathway. In any event, a unidirectional electron flow along a P700->(A[Chl] )->Ao->-A ->-FeS-X- FeS-(A/B) pathway is clearly indicated. 

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. 

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