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Brown SM, Mayer-Bacon C, Freeland S. Xeno Amino Acids: A Look into Biochemistry as We Do Not Know It. Life (Basel) 2023; 13:2281. [PMID: 38137883 PMCID: PMC10744825 DOI: 10.3390/life13122281] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/30/2023] [Revised: 11/18/2023] [Accepted: 11/20/2023] [Indexed: 12/24/2023] Open
Abstract
Would another origin of life resemble Earth's biochemical use of amino acids? Here, we review current knowledge at three levels: (1) Could other classes of chemical structure serve as building blocks for biopolymer structure and catalysis? Amino acids now seem both readily available to, and a plausible chemical attractor for, life as we do not know it. Amino acids thus remain important and tractable targets for astrobiological research. (2) If amino acids are used, would we expect the same L-alpha-structural subclass used by life? Despite numerous ideas, it is not clear why life favors L-enantiomers. It seems clearer, however, why life on Earth uses the shortest possible (alpha-) amino acid backbone, and why each carries only one side chain. However, assertions that other backbones are physicochemically impossible have relaxed into arguments that they are disadvantageous. (3) Would we expect a similar set of side chains to those within the genetic code? Many plausible alternatives exist. Furthermore, evidence exists for both evolutionary advantage and physicochemical constraint as explanatory factors for those encoded by life. Overall, as focus shifts from amino acids as a chemical class to specific side chains used by post-LUCA biology, the probable role of physicochemical constraint diminishes relative to that of biological evolution. Exciting opportunities now present themselves for laboratory work and computing to explore how changing the amino acid alphabet alters the universe of protein folds. Near-term milestones include: (a) expanding evidence about amino acids as attractors within chemical evolution; (b) extending characterization of other backbones relative to biological proteins; and (c) merging computing and laboratory explorations of structures and functions unlocked by xeno peptides.
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Makarov M, Sanchez Rocha AC, Krystufek R, Cherepashuk I, Dzmitruk V, Charnavets T, Faustino AM, Lebl M, Fujishima K, Fried SD, Hlouchova K. Early Selection of the Amino Acid Alphabet Was Adaptively Shaped by Biophysical Constraints of Foldability. J Am Chem Soc 2023; 145:5320-5329. [PMID: 36826345 PMCID: PMC10017022 DOI: 10.1021/jacs.2c12987] [Citation(s) in RCA: 3] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/25/2023]
Abstract
Whereas modern proteins rely on a quasi-universal repertoire of 20 canonical amino acids (AAs), numerous lines of evidence suggest that ancient proteins relied on a limited alphabet of 10 "early" AAs and that the 10 "late" AAs were products of biosynthetic pathways. However, many nonproteinogenic AAs were also prebiotically available, which begs two fundamental questions: Why do we have the current modern amino acid alphabet and would proteins be able to fold into globular structures as well if different amino acids comprised the genetic code? Here, we experimentally evaluate the solubility and secondary structure propensities of several prebiotically relevant amino acids in the context of synthetic combinatorial 25-mer peptide libraries. The most prebiotically abundant linear aliphatic and basic residues were incorporated along with or in place of other early amino acids to explore these alternative sequence spaces. The results show that foldability was likely a critical factor in the selection of the canonical alphabet. Unbranched aliphatic amino acids were purged from the proteinogenic alphabet despite their high prebiotic abundance because they generate polypeptides that are oversolubilized and have low packing efficiency. Surprisingly, we find that the inclusion of a short-chain basic amino acid also decreases polypeptides' secondary structure potential, for which we suggest a biophysical model. Our results support the view that, despite lacking basic residues, the early canonical alphabet was remarkably adaptive at supporting protein folding and explain why basic residues were only incorporated at a later stage of protein evolution.
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Affiliation(s)
- Mikhail Makarov
- Department of Cell Biology, Faculty of Science, Charles University, BIOCEV, Prague 12843, Czech Republic
| | - Alma C Sanchez Rocha
- Department of Cell Biology, Faculty of Science, Charles University, BIOCEV, Prague 12843, Czech Republic
| | - Robin Krystufek
- Department of Physical Chemistry, Faculty of Science, Charles University, Prague 12843, Czech Republic.,Institute of Organic Chemistry and Biochemistry, Czech Academy of Sciences, Prague 16610, Czech Republic
| | - Ivan Cherepashuk
- Department of Cell Biology, Faculty of Science, Charles University, BIOCEV, Prague 12843, Czech Republic
| | - Volha Dzmitruk
- Institute of Biotechnology of the Czech Academy of Sciences, BIOCEV, Vestec 25250, Czech Republic
| | - Tatsiana Charnavets
- Institute of Biotechnology of the Czech Academy of Sciences, BIOCEV, Vestec 25250, Czech Republic
| | - Anneliese M Faustino
- Department of Chemistry, Johns Hopkins University, Baltimore, Maryland 21218, United States
| | - Michal Lebl
- Institute of Organic Chemistry and Biochemistry, Czech Academy of Sciences, Prague 16610, Czech Republic
| | - Kosuke Fujishima
- Earth-Life Science Institute, Tokyo Institute of Technology, Tokyo 1528550, Japan.,Graduate School of Media and Governance, Keio University, Fujisawa 2520882, Japan
| | - Stephen D Fried
- Department of Chemistry, Johns Hopkins University, Baltimore, Maryland 21218, United States.,T. C. Jenkins Department of Biophysics, Johns Hopkins University, Baltimore, Maryland 21218, United States
| | - Klara Hlouchova
- Department of Cell Biology, Faculty of Science, Charles University, BIOCEV, Prague 12843, Czech Republic.,Institute of Organic Chemistry and Biochemistry, Czech Academy of Sciences, Prague 16610, Czech Republic
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