Electron transport chain, bacterial

Recent work has shown that bacteria, in common with chloroplasts and mitochondria, are able, through the membrane-bound electron transport chain aerobically, or the membrane-bound adenosine triphosphate (ATP) anerobically, to maintain a gradient of electrical potential and pH such that the interior of the bacterial cell is negahve and alkaline. This potential gradient and the electrical equivalent of the pH difference (1 pH unit = 58 mV at 37°C) give a potential difference across the membrane of 100-180 mV, with the inside negative. The membrane is impermeable to protons, whose extmsion creates the potential described. 

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.

In contrast to the flavin oxidases, flavin dehydrogenases pass electrons to carriers within electron transport chains and the flavin does not react with 02. Examples include a bacterial trimethylamine dehydrogenase (Fig. 15-9) which contains an iron-sulfur duster that serves as the immediate electron acceptor167 169 and yeast flavocytochrome b2, a lactate dehydrogenase that passes electrons to a built-in heme group which can then pass the electrons to an external acceptor, another heme in cytochrome c.170-173 Like glycolate oxidase, these enzymes bind their flavin coenzyme at the ends of 8-stranded a(i barrels similar... 

The lipid-soluble ubiquinone (Q) is present in both bacterial and mitochondrial membranes in relatively large amounts compared to other electron carriers (Table 18-2). It seems to be located at a point of convergence of the NADH, succinate, glycerol phosphate, and choline branches of the electron transport chain. Ubiquinone plays a role somewhat like that of NADH, which carries electrons between dehydrogenases in the cytoplasm and from soluble dehydrogenases in the aqueous mitochondrial matrix to flavoproteins embedded in the membrane. Ubiquinone transfers electrons plus protons between proteins within the... 

Cytochrome Ch is similar in most respects to other typical bacterial cytochromes c and C2- However its X-ray structure (Read et al., 1999) shows a number of unusual features it bears a closer gross resemblance to mitochondrial cytochrome c than to the bacterial cytochrome C2 and the left hand side of the haem cleft is unique. In particular it is highly hydrophobic, the usual water is absent, and the iconservediTyr67 is replaced by tryptophan. A number of features of the structure demonstrate that the usual hydrogen bonding network involving water in the haem channel is not essential, and that other mechanisms for modulation of redox potentials may exist in this cytochrome. It should be emphasised that the unique character of this cytochrome does not appear to be related in any way to its special involvement in oxidising cytochrome Ci in the methylotroph electron transport chain. 

A clearer picture has emerged for Complex II and crystal sfructures are available for the E. coli succinate quinone oxidoreductase (SQR) and closely related bacterial quinol frnnarate oxidoreductases (QFR) that catalyze the reverse reaction in order to use frnnarate as a terminal electron donor in anaerobic respiration. The sfructure of the SQR monomer showing the location of the redox cofactors is shown in Figure 7. The [2Fe-2S] + + (Tim = 4-10 mV), [4Fe-ASf+ + ( = -175 mV) and [3Fe S]+ ° Em = +65 mV) clusters in the SdhB subunit provide a linear electron transport chain for transferring electrons from the... 

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. 

The biochemical pathway of both assimilatory and dissimilatory sulfate reduction is illustrated in Figure 1. The details of the dissimilatory reduction pathway are useful for understanding the origin of bacterial stable isotopic fractionations. The overall pathways require the transfer of eight electrons, and proceed through a number of intermediate steps. The reduction of sulfate requires activation by ATP (adenosine triphosphate) to form adenosine phosphosulfate (APS). The enzyme ATP sulfurylase catalyzes this reaction. In dissimilatory reduction, the sulfate moiety of APS is reduced to sulfite (SO3 ) by the enzyme APS reductase, whereas in assimilatory reduction APS is further phosphorylated to phospho-adenosine phosphosulfate (PAPS) before reduction to the oxidation state of sulfite and sulfide. Although the reduction reactions occur in the cell s cytoplasm (i.e., the sulfate enters the cell), the electron transport chain for dissimilatory sulfate reduction occurs in proteins that are peiiplasmic (within the bacterial cell wall). The enzyme hydrogenase... 

Two amicyanins from the methylotropic bacteria Pseudomonas AMI and T. versutus have been a recent focus of attention (42, 43). Their function is to mediate electron transfer between bacterial cytochrome c and methylamine dehydrogenase in a relatively short electron transport chain. 

Lactate in CSF normally parallels blood levels, but not in children. With biochemical alterations in the CNS, however, CSF lactate values change independently of blood values. Increased CSF concentrations are noted in cerebrovascular accidents, intracranial hemorrhage, bacterial meningitis, epilepsy, inborn errors of the electron transport chain, and other CNS disorders. In aseptic (viral) menmgitis, lactate concentrations in CSF are not usually increased hence, CSF lactate has been used to help discriminate between viral and bacterial meningitis,but the clinical utility has been questioned. In a few children with inherited metabolic diseases, CSF lactate concentrations may be increased despite a plasma lactate in the reference interval. 

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). 

This approach has been adopted to investigate the function of COX from the proteobacterium Rhodohacter sphaeroides [110], the last enzyme in the respiratory electron transport chain of bacteria, located in the bacterial inner membrane. It receives one electron from each of four ferrocytochrome c molecules, located on the periplasmic side of the membrane, and transfers them to one oxygen molecule, converting it into two water molecules. In the process, it binds four protons from the cytoplasm to make water, and in addition translocates four protons from the cytoplasm to the periplasm, to establish a proton electrochemical potential difference across the membrane. In this ptBLM, the orientation of the protein with respect to the membrane normal depends on the location of the histidine stretch... 

Iron-sulfur clusters were probably the first systematic structures of bacterial electron-transport chains. They are found in all classes of organisms from bacteria to man. The development of porphyrins, the basic material for the synthe-... 

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