If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.
Locations of Mapped Genes:
|Superclasses:||Generation of Precursor Metabolites and Energy → Electron Transfer|
|Generation of Precursor Metabolites and Energy → Respiration → Aerobic Respiration|
Pathway Summary from MetaCyc:
Like fermentation, respiration is a process by which electrons are passed from an electron donor to a terminal electron acceptor. However, in respiration the electrons do not pass directly from the donor to the acceptor. Instead, they pass a number of membrane-bound electron carriers that function as a transport chain, passing the electrons from one to another in steps that follow the electrochemical gradients between the electron donor and the acceptor.
Each oxidized member of the electron transfer chain (which can be a flavoprotein, an electron-transfer quinone, a cytochrome, or other type of electron carrier) can be reduced by the reduced form of the preceding member, and the electrons flow through the chain all the way to the terminal acceptor, which could be oxygen in the case of aerobic respiration, or another type of molecule in anaerobic respiration.
Known terminal acceptors include organic compounds (fumarate, dimethyl sulfoxide, or trimethylamine N-oxide), or inorganic compounds (nitrate, nitrite, nitrous oxide, chlorate, perchlorate, oxidized manganese ions, ferric iron, gold, selenate, arsenate, sulfate and elemental sulfur).
During the process of electron transfer, a proton gradient is formed across the membrane due to three potential processes:
1. The use of some of the energy associated with the electron transfer for active pumping of protons out of the cell.
2. Exporting protons out of the cell during electron-to-hydrogen transfers.
3. Scalar reactions that consume protons inside the cell, or produce them outside the cell, without actually moving a proton from one compartment to another.
Upon passage of protons back into the cytoplasm, they drive multisubunit ATP synthase enzymes that generate ATP.
About This Pathway
In the respiratory chain formed by NADH dehydrogenase I (NDH-1) and cytochrome bo oxidase the transfer of electrons from NADH to oxygen is coupled to the generation of a proton-motive force across the cytoplasmic membrane [Matsushita87].
By analogy to the related enzyme from mitochondria, NDH-I is thought to function as a proton pump translocating 4H+ per NADH oxidised (2e-) [H+/e- = 2] however a lower ratio of 3H+/2e- has also been proposed [Bogachev96, Wikstrom12]. Cytochrome bo oxidase contributes to the PMF (H+/e-=2) through its action as a proton pump (H+/e-=1) [Puustinen89] and through a redox loop mechanism (H+/e-=1) ([Puustinen91] and see review by [Unden97]).
During glucose limited aerobic growth E. coli directs electron flux through both NDH-I and NADH dehydrognease II (NDH-II) [Calhoun93a]. Electron transport from NADH to oxygen is not affected in a strain lacking NDH-I but is significantly reduced in a strain lacking NDH-II which suggests that NDH-II is used preferentially in aerobic respiration [Tran97]
Pathway Evidence Glyph:
Bogachev96: Bogachev AV, Murtazina RA, Skulachev VP (1996). "H+/e- stoichiometry for NADH dehydrogenase I and dimethyl sulfoxide reductase in anaerobically grown Escherichia coli cells." J Bacteriol 178(21);6233-7. PMID: 8892824
Calhoun93a: Calhoun MW, Oden KL, Gennis RB, de Mattos MJ, Neijssel OM (1993). "Energetic efficiency of Escherichia coli: effects of mutations in components of the aerobic respiratory chain." J Bacteriol 175(10);3020-5. PMID: 8491720
Cotter90: Cotter PA, Chepuri V, Gennis RB, Gunsalus RP (1990). "Cytochrome o (cyoABCDE) and d (cydAB) oxidase gene expression in Escherichia coli is regulated by oxygen, pH, and the fnr gene product." J Bacteriol 172(11);6333-8. PMID: 2172211
Kita84: Kita K, Konishi K, Anraku Y (1984). "Terminal oxidases of Escherichia coli aerobic respiratory chain. I. Purification and properties of cytochrome b562-o complex from cells in the early exponential phase of aerobic growth." J Biol Chem 1984;259(5);3368-74. PMID: 6365921
Tran97: Tran QH, Bongaerts J, Vlad D, Unden G (1997). "Requirement for the proton-pumping NADH dehydrogenase I of Escherichia coli in respiration of NADH to fumarate and its bioenergetic implications." Eur J Biochem 244(1);155-60. PMID: 9063459
Tseng96: Tseng CP, Albrecht J, Gunsalus RP (1996). "Effect of microaerophilic cell growth conditions on expression of the aerobic (cyoABCDE and cydAB) and anaerobic (narGHJI, frdABCD, and dmsABC) respiratory pathway genes in Escherichia coli." J Bacteriol 178(4);1094-8. PMID: 8576043
Unden97: Unden G, Bongaerts J (1997). "Alternative respiratory pathways of Escherichia coli: energetics and transcriptional regulation in response to electron acceptors." Biochim Biophys Acta 1320(3);217-34. PMID: 9230919
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