Respiratory electron acceptor

Sulfate reducers can use a wide range of terminal electron acceptors, and sulfate can be replaced by nitrate as a respiratory substrate. Molybdenum-containing enzymes have been discovered in SRB (also see later discussion) and, in particular, D. desulfuricans, grown in the presence of nitrate, generates a complex enzymatic system containing the following molybdenum enzymes (a) aldehyde oxidoreduc-tase (AOR), which reduces adehydes to carboxylic acids (b) formate dehydrogenase (FDH), which oxidizes formate to CO2 and (c) nitrate reductase (the first isolated from a SRB), which completes the enzy-... 

Lorenzen JP, A Kroger, G Unden (1993) Regulation of anaerobic respiratory pathways in Wolinellla succino-genes by the presence of electron acceptors. Arch Microbiol 159 477-483. 

Alcohol dehydrogenases found in certain microorganisms utilize a pyrroloquino-line quinone (PQQ) or flavin cofactor to pass electrons released upon oxidation of alcohols to the heme electron-acceptor protein, cytochrome c. These membrane-associated alcohol dehydrogenases form part of a respiratory chain, and the energy from fuel oxidation therefore contributes to generation of a proton gradient across... 

Baraquet, C., Theraulaz, L., Guiral, M., Lafitte, D., Mejean, V., and Jourlin-Castelli, C. (2006) TorT, a member of a new periplasmic binding protein family, triggers induction of the Tor respiratory system upon trimethylamine N-oxide electron-acceptor binding in Escherichia coli.J. Biol. Chem. 281, 38189-38199. 

Chrysiogenes arsenatis is the only known organism capable of using acetate as the electron donor and arsenate as the terminal electron acceptor for growth. This reduction of arsenate to arsenite is catalyzed by an inducible respiratory arsenate reductase, which has been isolated and characterized by Kraft and Macy (1998). Arsenate reductase (Arr) from C. arsenatis is a... 

The recently isolated Desulfotomaculum strain Ben-RB is able to grow using lactate as a substrate and arsenate as the sole electron acceptor (Macy et al. 2000). It has been proposed that arsenate reductase is associated with the respiratory chain of this organism, because >98% of the arsenate reductase bound to the plasma membrane. 

Anaerobic metabolism occnrs nnder conditions in which the diffusion rate is insufficient to meet the microbial demand, and alternative electron acceptors are needed. The type of anaerobic microbial reaction controls the redox potential (Eh), the denitrification process, reduction of Mu and SO , and the transformation of selenium and arsenate. Keeney (1983) emphasized that denitrification is the most significant anaerobic reaction occurring in the subsurface. Denitrification may be defined as the process in which N-oxides serve as terminal electron acceptors for respiratory electron transport (Firestone 1982), because nitrification and NOj" reduction to produce gaseous N-oxides. hi this case, a reduced electron-donating substrate enhances the formation of more N-oxides through numerous elechocarriers. Anaerobic conditions also lead to the transformation of organic toxic compounds (e.g., DDT) in many cases, these transformations are more rapid than under aerobic conditions. 

In the absence of oxygen—i. e., in anaerobic conditions—the picture changes completely. Since O2 is missing as the electron acceptor for the respiratory chain, NADH+H and QH2 can no longer be reoxidized. Consequently, not only is mitochondrial ATP synthesis halted, but also almost the whole metabolism in the mitochondrial matrix. The main reason for this is the high NADH+H concentration and lack of NAD which inhibit the tricarbox-... 

In the third step, 1, -/3-hydroxyacyl-CoA is dehydrogenated to form /3-ketoacyl-CoA, by the action of /3-hydroxyacyl-CoA dehydrogenase NAD+ is the electron acceptor. This enzyme is absolutely specific for the l stereoisomer of hydroxyacyl-CoA The NADH formed in the reaction donates its electrons to NADH dehydrogenase, an electron carrier of the respiratory chain, and ATP is formed from ADP as the electrons pass to 02. The reaction catalyzed by /3-hydroxyacyl-CoA dehydrogenase is closely analogous to the malate dehydrogenase reaction of the citric acid cycle (p. XXX). 

Oxidative phosphorylation begins with the entiy of electrons into the respiratory chain. Most of these electrons arise from the action of dehydrogenases that collect electrons from catabolic pathways and funnel them into universal electron acceptors—nicotinamide nucleotides (NAD+ or NADP+) or flavin nucleotides (FMN or FAD). 

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 other small blue proteins are only poorly characterized at present It is assumed their function is that of electron transfer. Rusticyanin from Thiobacillus ferrooxidans is thought to be the initial electron acceptor from iron(II) in the respiratory chain at pH 2. Rusticyanin contains 159 residues, with one cysteine, three methionine and five histidine residues. The protein is unusually stable at low pH, in accord with its presence in an acidophilic organism. The midpoint potential of rusticyanin is high (+680 mV), and is second in magnitude only to that of Polyporus laccase. 

The respiratory systems of bacteria are of especial interest1435 and complexity in view of their ability in some cases to use alternative substrates as terminal electron acceptors, depending upon the environmental conditions. This will be illustrated with reference to E. coli, which has been reviewed recently.1436 The advantage of studying respiration in this organism is that, by choice of growth conditions, the pathways of electron transport can be manipulated. In addition, mutants are available which are defective in certain respiratory components. 

E. coli is a facultative anaerobe, and so while it can grow anaerobically in the gut, it can also use dioxygen as the terminal electron acceptor. The three principal respiratory chains of E. coli are linked to fumarate, nitrate and dioxygen. 

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