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WIREs Syst Biol Med
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Physiological, metabolic and biotechnological features of extremely thermophilic microorganisms

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The current upper thermal limit for life as we know it is approximately 120°C. Microorganisms that grow optimally at temperatures of 75°C and above are usually referred to as ‘extreme thermophiles’ and include both bacteria and archaea. For over a century, there has been great scientific curiosity in the basic tenets that support life in thermal biotopes on earth and potentially on other solar bodies. Extreme thermophiles can be aerobes, anaerobes, autotrophs, heterotrophs, or chemolithotrophs, and are found in diverse environments including shallow marine fissures, deep sea hydrothermal vents, terrestrial hot springs—basically, anywhere there is hot water. Initial efforts to study extreme thermophiles faced challenges with their isolation from difficult to access locales, problems with their cultivation in laboratories, and lack of molecular tools. Fortunately, because of their relatively small genomes, many extreme thermophiles were among the first organisms to be sequenced, thereby opening up the application of systems biology‐based methods to probe their unique physiological, metabolic and biotechnological features. The bacterial genera Caldicellulosiruptor, Thermotoga and Thermus, and the archaea belonging to the orders Thermococcales and Sulfolobales, are among the most studied extreme thermophiles to date. The recent emergence of genetic tools for many of these organisms provides the opportunity to move beyond basic discovery and manipulation to biotechnologically relevant applications of metabolic engineering. WIREs Syst Biol Med 2017, 9:e1377. doi: 10.1002/wsbm.1377 This article is categorized under: Biological Mechanisms > Metabolism
3‐Hydroxypropionate/4‐hydroxybutyrate (3‐HP/4‐HB) cycle from Metallosphaera sedula. The cycle consists of two major portions: carbon incorporation (via bicarbonate) occurs in the first half (blue) of the cycle and is followed by subsequent reduction and reformation of two acetyl‐CoA molecules in the second half (red). Enzymes listed and their references: acetyl‐coA carboxylase (ACC), acetoacetyl‐CoA β‐ketothiolase (ACCT), acryloyl‐CoA reductase (ACR), crotonyl‐CoA hydratase (CCH), 4‐hydroxybutyrate‐CoA dehydratase (HBCD), 4‐hydroxybutyrate‐CoA synthase (HBCS), 3‐hydroxypropionate‐CoA dehydratase (HPCD), 3‐hydroxypropionate‐CoA synthase (HPCS), methylmalonyl‐CoA epimerase (MCE), methylmalonyl‐CoA mutase (MCM), malonyl‐CoA/succinyl‐CoA reductase (MCR), malonate semialdehyde reductase (MSR), and succinate semialdehyde reductase (SSR).
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Co‐culture of Thermotoga maritima yellow/green rods) and Methanocaldococcus jannaschii (red cocci)—permission pending.
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Conserved metabolic pathways in all Caldicellulosiruptor species. This includes sugar uptake, glycolytic, and fermentative pathways. The figure includes only the major steps, or start and end products. P is the abbreviation for phosphate, NADH for reduced nicotinamide adenine dinucleotide, ATP for adenosine triphosphate, Fdred for reduced ferrodoxin, and GDP for guanosine diphosphate.
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Comparison of traditional Embden‐Meyerhof‐Parnas pathway with the modified pathway in the archaeon P. furiosus. Included are three fermentative pathways which utilize the reduced ferredoxin produced via glycolysis. Enzyme abbreviations: hydrogenase (hyd), ferredoxin:NADP oxidoreductase (FNOR), glutamate deaminase (GD), alanine aminotransferase (AT), and NADP:sulfur oxidoreductase (NSOR).
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Chemolithotrophic pathways in the Sulfolobales. The first half of the figure (blue) shows the hypothetical pathways for sulfur utilization in the Sulfolobales, including both oxidizing and reducing pathways, beginning with elemental sulfur. Sulfur reducing complexes: hydrogenase (Hyd), sulfur reductase (SR). Sulfur oxidizing enzymes: sulfur oxygenase reductase (SOR), thiosulfate:quinone oxidoreductase (TQO), Sulfite:acceptor oxidoreductase (SAOR), adenylylsulfate reductase (APSR), adenylylsulfate:phosphate adenyltransferase (APAT). The second panel shows a hypothetical pathway for the oxidation of ferrous iron using several fox stimulon proteins as well as some iron‐responsive respiratory proteins. Ferrous‐oxidation (Fox), rusticyanin (Rus), cystathionine‐β‐synthase containing protein subunits A and B (CbsAB), sulfur oxidation (Sox), NADH dehydrogenase (NAD).
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