This view shows enzymes only for those organisms listed below, in the list of taxa known to possess the pathway. If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.
|Superclasses:||Generation of Precursor Metabolites and Energy → Respiration → Anaerobic Respiration|
Expected Taxonomic Range:
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
(Per)chlorate-reducing bacteria use the highly oxidized chlorate ion as the sole electron acceptor for oxidation of organic matter. The reduction of chlorate occurs in a two-step reaction: chlorate reduction to chlorite, followed by the decomposition of the latter to chloride ions and oxygen. The second step is one of the rare occasions where molecular oxygen is formed in a metabolic reaction [other examples include photosynthesis, the detoxification of reactive oxygen species (see for example superoxide radicals degradation), and potentially the reaction catalyzed by "nitric oxide lyase" (see nitrite-dependent anaerobic methane oxidation)].
The two enzymes that catalyze chlorate reduction, chlorate reductase and chlorite O2-lyase, are periplasmic, and have been purified from several organisms [Thorell03, deGeus09]. Electron transport from the bacterial inner membrane to the soluble periplasmic chlorate reductase is mediated by soluble c cytochromes [Bohlin09], while additional c cytochromes present in the periplasm appear to serve as electron donors for the terminal oxidase [Backlund09].
The oxygen that is produced in the process is believed to be used by either terminal oxidases or by monooxygenases that attack hydrocarbons, enabling the organisms to use oxygen-requiring enzymes under anaerobic conditions [Chakraborty04].
Perchlorate and chlorate reduction has also been observed in the archaea. However, in those organisms chlorite is not enzymatically split into chloride and oxygen. Instead, it is eliminated by an interplay of abiotic and biotic redox reactions involving sulfur compounds [Liebensteiner13].
Bohlin09: Bohlin J, Backlund AS, Gustavsson N, Wahlberg S, Nilsson T (2009). "Characterization of a cytochrome c gene located at the gene cluster for chlorate respiration in Ideonella dechloratans." Microbiol Res. PMID: 20015627
deGeus09: de Geus DC, Thomassen EA, Hagedoorn PL, Pannu NS, van Duijn E, Abrahams JP (2009). "Crystal structure of chlorite dismutase, a detoxifying enzyme producing molecular oxygen." J Mol Biol 387(1);192-206. PMID: 19361444
Liebensteiner13: Liebensteiner MG, Pinkse MW, Schaap PJ, Stams AJ, Lomans BP (2013). "Archaeal (per)chlorate reduction at high temperature: an interplay of biotic and abiotic reactions." Science 340(6128);85-7. PMID: 23559251
Mehboob09: Mehboob F, Wolterink AF, Vermeulen AJ, Jiang B, Hagedoorn PL, Stams AJ, Kengen SW (2009). "Purification and characterization of a chlorite dismutase from Pseudomonas chloritidismutans." FEMS Microbiol Lett 293(1);115-21. PMID: 19228194
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