{"id":3439,"date":"2019-05-03T14:53:33","date_gmt":"2019-05-03T05:53:33","guid":{"rendered":"http:\/\/163.180.4.222\/lab\/?p=3439"},"modified":"2019-05-03T14:53:33","modified_gmt":"2019-05-03T05:53:33","slug":"a-possible-non-biological-reaction-framework-for-metabolic-processes-on-early-earth","status":"publish","type":"post","link":"https:\/\/biochemistry.khu.ac.kr\/lab\/?p=3439","title":{"rendered":"A possible non-biological reaction framework for metabolic processes on early Earth"},"content":{"rendered":"

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Early life forms established a network of reactions for converting carbon dioxide into organic compounds. A non-biological system of reactions that could have formed the network\u2019s core on ancient Earth has been reported.<\/h5>\n

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All biological molecules used by living organisms are themselves synthesized by living organisms. The development of routes for making organic matter was therefore an essential early step in the emergence of life on Earth. A complex network of reactions must have arisen to make organic molecules from carbon dio xide, or possibly from other inorganic sources of carbon such as carbon monoxide or cyanides, but the process involved remains largely unknown. In\u00a0a paper in\u00a0Nature<\/i><\/a>, Muchowska\u00a0et al<\/i>.1<\/a><\/sup>\u00a0demonstrate experimentally that a suitable complex reaction network can develop from just two simple organic constituents, namely, glyoxylate (HCOCO2<\/sub>\u2212<\/sup>) and pyruvate (CH3<\/sub>COCO2<\/sub>\u2212<\/sup>), in the presence of ferrous iron (Fe2+<\/sup>).<\/p>\n

The identified network produces nine of the eleven main components of the tricarboxylic acid (TCA) cycle \u2014 the series of reactions by which present-day organisms metabolize organic matter to convert it into energy (Fig. 1), producing the nucleotide ATP as an energy carrier and CO2<\/sub>\u00a0as a by-product. The TCA cycle can also work in reverse, in which case it is known as the reductive tricarboxylic acid (rTCA) cycle. The rTCA cycle could have been an early route by which CO2<\/sub>\u00a0was converted (fixed) into the organic molecules that are used as the basic components of living organisms. Muchowska and colleagues\u2019 work suggests that the rTCA cycle, as well as other processes that are associated with the metabolism of carbon, could have emerged from a network of abiotic reactions that, at least partly, matched the pattern of the biological reaction network that is now catalysed by enzymes.<\/p>\n

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Figure 1 | The tricarboxylic acid (TCA) cycle.<\/b>\u00a0The TCA cycle is one of the core metabolic pathways in many present-day organisms. Muchowska\u00a0et al.<\/i>1<\/a><\/sup>\u00a0report that nine (red) of the eleven intermediates in the TCA cycle are also formed in a complex network of reactions that is established when glyoxylate and pyruvate are combined in water with ferrous iron (Fe2+<\/sup>). The authors propose that their network might have formed a non-biological framework for metabolic pathways when life emerged on early Earth. (Adapted from ref. 1.)<\/span><\/p>\n<\/figcaption><\/figure>\n

The authors also show that, in the presence of hydroxylamine (NH2<\/sub>OH) and metallic iron, their chemical network can be extended to include the formation of four kinds of amino acid, the building blocks of proteins. Both hydroxylamine and metallic iron could have been available on early Earth: hydroxylamine would probably have formed as a result of the rich, abiotic nitrogen chemistry that is known to have occurred early in the planet\u2019s existence2<\/a><\/sup>, whereas metallic iron is abundant in certain meteorites that peppered our planet.<\/p>\n

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Muchowska\u00a0et al.<\/i>\u00a0suggest that their pathway could have developed further to facilitate the subsequent emergence of functional polymers, including peptides and nucleic acids. This would require that abiotic processes that fix CO2<\/sub>\u00a0fed the system with glyoxylate and pyruvate. The authors identify evidence from the scientific literature that supports the existence of such processes, but it is unclear whether these processes could have produced sufficient concentrations of glyoxylate and pyruvate to sustain emergent living organisms. This does not invalidate the authors\u2019 reaction network as a potential key player in the origins of life, however.<\/p>\n

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