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Exoplanet Exploration

Precursors To The Citric Acid Cycle And Metabolism: A Mechanistic Investigation Of Metabolic Intermediates Detected In Carbonaceous Meteorites

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Precursors To The Citric Acid Cycle And Metabolism: A Mechanistic Investigation Of Metabolic Intermediates Detected In Carbonaceous Meteorites
The citric acid cycle is central to cellular metabolism. It is responsible for processing chemical energy and using intermediary metabolites as synthetic precursors for bio-molecular building blocks. It is thought that cycles related to that of the citric acid cycle may have spawned a primitive metabolism. A fundamental goal in the study of the origin of life is to understand how metabolism emerged in the prebiotic environment. This proposal is rooted in the search to explain the prebiotic origin of metabolic intermediates in a citric acid cycle-type predecessor. The proposal follows on a recent report on the detection of many (non-enzymatic) citric acid cycle intermediates in carbonaceous meteorites and, simultaneously, the laboratory demonstration that pyruvate (a citric acid cycle precursor) leads to the formation of several of these key compounds without enzymes. However, the chemical mechanisms of how such abiotic reactions proceed are largely unknown. The proposed work involves detailed laboratory investigations and will (1) attempt to elucidate the chemical mechanisms of pyruvate chemistry that lead to oxaloacetate, malate, fumarate, and other citric acid cycle compounds and (2) perform quantitative analyses of the corresponding meteoritic compounds. Meteoritic analyses will also include searches for (and quantitative analysis of) intermediates in the formation of the above compounds. If the molecular and abundance profiles of this suite of meteoritic compounds are consistent with laboratory pyruvate chemistry, the results would lend evidence to the suggestion that early in the history of the Solar System pyruvate was responsible for the production of several (currently known) metabolic intermediates. By following the time-course of isotope-labeled laboratory reactions the steady state abundances of the compounds can also be assessed, e.g., could relatively fragile compounds such as oxaloacetate have persistent long enough on the ancient Earth for incorporation into proto-cells? This proposal will involve both synthetic and analytical experimentation to investigate the scenario that pyruvate may have been the precursor of key metabolic intermediates on the early Earth as well as extraterrestrial settings. The major analytical techniques will be gas chromatography-mass spectrometry (GC-MS), liquid chromatography-MS and nuclear magnetic resonance (NMR). GC-isotope ratio mass spectrometry (GC-IRMS) will be used for compound-specific isotope (13C) analysis in year three of the proposal. Relevance of Proposed Work: Included in NASA's strategic plan is the objective of understanding how life begins and evolves. Carbonaceous meteorites provide the only record yet available for the laboratory study of a wide range of organic compounds that were present on (delivered to) the early planets. Organic compounds in ancient extraterrestrial bodies are indicators of chemical processes that occurred before the dawn of life and possibly lead to life itself. By studying carbonaceous meteorites and the fate of their organic compounds, we start to understand the apparently general process of chemical evolution of molecules from interstellar space to their incorporation into planetesimals, i.e., other solar systems. This increases our knowledge of the processes in the distant past (and today) that lead to life throughout the universe. More »

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