Origin & Evolution of Life

Ancient Amino Acid Sets Enable Stable Protein Folds

By Keith Cowing
Status Report
biorxiv.org
November 6, 2025
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Ancient Amino Acid Sets Enable Stable Protein Folds
Experimental characterization and structural analysis of designed ancestral proteins. (A) Schematic representation of the design workflow for ancestral proteins. (B) Summary table reporting general features of the selected designs and their expression outcomes; proteins 5 successfully expressed in the laboratory are indicated in bold. (C) CD spectra of the purified ancestral proteins. (D) Structural comparisons of Ap3, Fd3, and Rn2, respectively. Experimentally determined NMR structures are shown in blue, the corresponding wild-type templates in red, and the designed models from RFdiffusion in grey — biorxiv.org

Early proteins likely arose from a chemically limited set of amino acids available through prebiotic chemistry, raising a central question in molecular evolution: could such primitive compositions yield stable, functional folds?

Using de novo design, we constructed three ancient protein architectures using a reduced, evolution-inspired alphabet of ten amino acids, e.g. lacking all basic and aromatic residues.

The resulting structures adopted their intended topologies and showed exceptional resistance to thermal and chemical denaturation. Computational simulations further revealed that proteins built from this restricted alphabet were as mutation-resilient as those using all twenty canonical residues.

Besides their evolutionary implications, our results provide a foundation for minimalist protein design and generation of simplified, robust systems in minimal cell engineering.

Ancient amino acid sets enable stable protein folds, biorxiv.org (open access)

Astrobiology, evolution, genomics,

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