Electron acceptor, use

King GF, DJ Richardson, JB Jackson, SJ Ferguson (1987) Dimethyl sulfoxide and trimethylamine-A-oxide as bacterial electron acceptors use of nuclear magnetic resonance to assay and characterise the reductase system in Rhodobacter capsulatus. Arch Microbiol 149 47-51. 

Interestingly, non-metallic silver clusters, depending on their sizes, may act either as electron donors or as electron acceptors. Using sulfonatopropyl-viologen, SPV (E° for SPV/SPV- = — 0.41 V/NHE), pulse radiolysis established that small silver clusters (n < 4) acted as electron donors (Le. E° for (Ag /Agn < E° for SPV/SPV - ) while, conversely, large silver clusters (n 2 4) were electron acceptors (i.e. E° for Ag/ /Agn > E° for SPV/SPV- ) [511]. Size-dependent electrochemical potentials of silver aggregates have been elucidated (Fig. 82) [506]. 

Like the homologous Complex III in mitochondria, the cytochrome bci complex of purple bacteria carries electrons from a quinol donor (QH2) to an electron acceptor, using the energy of electron transfer to pump protons across the membrane, producing a proton-motive force. The path of electron flow through this complex is believed to be very similar to that through mitochondrial Complex III, involving a Q cycle (Fig. 19-12) in which protons are consumed on one side of the membrane and released on the other. The ultimate... 

In accordance with the data, the formation of dication 308 from 306 includes the transfer of two electrons, but the intermediate cation radical 307 is too short-lived to allow recording of its ESR spectrum. It was assumed that the exclusive formation of 261 on chemical oxidation of306 is determined by the comparatively low reduction potentials of the electron acceptors used. 

In the dissolved phase, few alternative abiotic oxidants are available in the natural environment. Nitrate, sulfate, and other terminal electron acceptors used by anaerobic microorganisms are thermodynamically capable of oxidizing some organic contaminants, but it appears that these reactions almost always require microbial mediation. 

The pathways for sedimentary microbial metabolism are outlined in Table 2. They are presented in order of decreasing free energy yield for reaction of each oxidant (shown in bold type in the table) with sedimentary organic matter (Froelich et ai, 1979). Pore-water data support the assertion that the electron acceptors are used in this order of decreasing free energy yield. The order of the NO N2 and Mn02 Mn(II) reactions is uncertain, however, and examples exist in the literature for which Mn(IV) appears to be used before NO )" (Froelich et al., 1979 Klinkhammer, 1980) or for which NO appears to be used first (e.g., Shaw et al., 1990 Lohse et al., 1998). Thus, the order of electron acceptor use is O2, NO ... 

Figure 1 Vertical biogeochemical zones in sediments. The top is the sediment-water interface. Processes on the left represent the use of various electron acceptors (respirations) during the degradation of organic matter. Plots on the right represent the chemical profiles most widely used to delineate the vertical extent of each zone. Rotating the figure 90° to the left shows the sequence of electron acceptors used over time (x-axis) if a sample of oxic sediment were enclosed and allowed to become anaerobic over time.

Scheme 8-2. Some organic electron-acceptors used for the formation of CT complexes.

The radical cation can also be obtained by interaction of bicyclobutane with a photoexcited electron acceptor to which it transfers an electron Examples of electron acceptors used are 1-cyanonapthalene and 9,10-dicyanoanthracene. The radical cations obtained in this way undergo various dimerization reactions (the course of which depends on the bridgehead substituents ) and in the presence of nucleophiles such as MeOH or CN " undergo nucleophilic addition reaction (equation 89). 

The environmentally significant and most extensively studied electron acceptors used, along with the reactions that can be mediated by microbes, are illustrated in equations 1-8. 

Attention may also be drawn to dechlorination by anaerobic bacteria of both chlorinated ethenes and chlorophenolic compounds that serve as electron acceptors using electron donors including formate, pyruvate, and acetate. This is more fully discussed in Chapter 6 (Sections 6.4.4 and 6.6). 

T. vaginalis (16), T. foetus (17) and E. histolytica (18), and detected in G. lamblia (19). It is similar in properties and has sequence homology to isofunctional enzymes in anaerobic eubacteria. The enzyme is, however, fundamentally different from the mitochondrial pyruvate dehydrogenase complex, which catalyzes the same overall reaction but involves a different electron acceptor. The electron acceptor used by the enzyme of these anaerobic parasites is known to be ferredoxin, also an iron-sulfur protein (1). Interestingly, however, the trichomonad protein, a [2Fe-2S] ferredoxin with some similarities to mitochondrial proteins, belongs to a different subfamily to the protein in E. histolytica, which is a 2[4Fe-4S] ferredoxin like those in anaerobic eubacteria. The G. lamblia ferredoxin is probably similar to the latter, although it is yet to be fully characterized. 

Desulfomicrobium sp. str. Ben-RB can use sulfate and/or arsenate as terminal electron acceptors, using lactate as the electron donor instead of acetate as does... 

The chemistry by which they obtain energy or recharge the oxidative capacity of the cell (i.e., fermentation or respiration) and the terminal electron acceptors used (e.g., oxygen, ferric ion, sulfate, and organic compounds). 

In correspondance with previous findings [10], CaCU markedly stimulated the oxygen evolution rate. However, the extent of CaCl2 induced stimulation depended severely on the electron acceptor used. This leads to the assumption that CaCl2 induced conformational changes at the acceptor side exceed all possible effects on the donor side or a possible protective role against photoinhibition. 

---