Abstract
The present study proposes to search our solar system (Mars, Enceladus, Europa) for patterns of organic molecules that are universally associated with biological functions and structures. The functions are primarily catalytic because life could only have originated within volume/space-constrained compartments containing chemical reactions catalyzed by certain polymers. The proposed molecular structures are specific groups in the side chains of amino acids with the highest catalytic propensities related to life on Earth, that is, those that most frequently participate as key catalytic groups in the active sites of enzymes such as imidazole, thiol, guanidinium, amide, and carboxyl. Alternatively, these or other catalytic groups can be searched for on non-amino-acid organic molecules, which can be tested for certain hydrolytic catalytic activities. The first scenario assumes that life may have originated in a similar manner as the terrestrial set of α-amino acids, while the second scenario does not set such a requirement. From the catalytic propensity perspective proposed in the first scenario, life must have invented amino acids with high catalytic propensity (His, Cys, Arg) in order to overcome, and be complemented by, the low catalytic propensity of the initially available abiogenic amino acids. The abiogenic and the metabolically invented amino acids with the lowest catalytic propensity can also serve as markers of extraterrestrial life when searching for patterns on the basis of the following functional propensities related to protein secondary/quaternary structure: (1) amino acids that are able to form α-helical intramembrane peptide domains, which can serve as primitive transporters in protocell membrane bilayers and catalysts of simple biochemical reactions; (2) amino acids that tend to accumulate in extremophile proteins of Earth and possibly extraterrestrial life. The catalytic/structural functional propensity approach offers a new perspective in the search for extraterrestrial life and could help unify previous amino acid–based approaches.
Highlights
The present study proposes to search our solar system (Mars, Enceladus, Europa) for patterns of organic molecules that are universally associated with biological functions and structures
The proposed molecular structures are specific groups in the side chains of amino acids with the highest catalytic propensities related to life on Earth, that is, those that most frequently participate as key catalytic groups in the active sites of enzymes such as imidazole, thiol, guanidinium, amide, and carboxyl
The abiogenic and the metabolically invented amino acids with the lowest catalytic propensity can serve as markers of extraterrestrial life when searching for patterns on the basis of the following functional propensities related to protein secondary/quaternary structure: (1) amino acids that are able to form a-helical intramembrane peptide domains, which can serve as primitive transporters in protocell membrane bilayers and catalysts of simple biochemical reactions; (2) amino acids that tend to accumulate in extremophile proteins of Earth and possibly extraterrestrial life
Summary
Why should we search for amino acids as biomarkers in life-detection missions to other planetary objects? The simplest answers are that they are unique constituents of terrestrial life and some are found in meteorites. Finding cyanide compounds (CH3CN, HC3N and HCN) in protoplanetary disks such as the young star MWC 480 is of special interest because of its cometlike composition This supports the idea that comets once seeded early Earth with the water and organics needed for life to originate, as well as the role of C-N bonds for abiotic amino acid synthesis (Oberg et al, 2015). GEORGIOU proposed for the detection and identification of life in distant worlds by linear polarization signals from biomolecules that can capture photons of particular wavelengths and can be distinguishable from mineral components of sands and rocks (Berdyugina et al, 2016) It is questionable whether the polarization spectra calibrated from Earth plant pigments (such as chlorophyll, carotenoids, and others) would be produced by the structurally unknown pigments in the surface, oceans, and clouds of exoplanets that are inhabited by extraterrestrial life. Since acetylene has been detected in Titan’s upper atmosphere, and may originate within Enceladus by thermal processes, it was proposed that stable carbon isotope fractionation (12C/13C) due to biotic acetylene fermentation could be used for life detection in hydrocarbon-rich volatiles of icy planet(oid)s (Miller et al, 2015)
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