Protein design at the interface of the pre-biotic and biotic worlds

A single aromatic core mutation converts a designed "primitive" protein from halophile to mesophile folding

The halophile environment has a number of compelling aspects with regard to the origin of structured polypeptides (i.e., proteogenesis) and, instead of a curious niche that living systems adapted into, the halophile environment is emerging as a candidate "cradle" for proteogenesis. In this viewpoint, a subsequent halophile-to-mesophile transition was a key step in early evolution. Several lines of evidence indicate that aromatic amino acids were a late addition to the codon table and not part of the original "prebiotic" set comprising the earliest polypeptides. We test the hypothesis that the availability of aromatic amino acids could facilitate a halophile-to-mesophile transition by hydrophobic core-packing enhancement. The effects of aromatic amino acid substitutions were evaluated in the core of a "primitive" designed protein enriched for the 10 prebiotic amino acids (A,D,E,G,I,L,P,S,T,V)-having an exclusively prebiotic core and requiring halophilic conditions for folding. The results indicate that a single aromatic amino acid substitution is capable of eliminating the requirement of halophile conditions for folding of a "primitive" polypeptide. Thus, the availability of aromatic amino acids could have facilitated a critical halophile-to-mesophile protein folding adaptation-identifying a selective advantage for the incorporation of aromatic amino acids into the codon table.

Structure of a simplified β-hairpin and its ATP complex

The capacity of three designed duodecamer peptides with the low diversity sequence: H1ϕ2I3K4I5D6G7K8ϕ9I10K11H12 where ϕ is His, Phe or Trp, to adopt a β-hairpin conformation was studied using NMR spectroscopy. Whereas KIAβH, the variant with His at positions two and nine, is disordered, KIAβF, the peptide with Phe at these positions, adopts a small population of β-hairpin. A high population of β-hairpin structure was detected for KIAβW, the variant with Trp. Utilizing NMR data, the structure of KIAβW was solved and it reveals a β-hairpin stabilized by hydrophobic interactions between Ile residues on one face and Trp-Trp and cation-π interactions on the opposite face. Upon adding ATP, these peptides show chemical shift changes indicative of ATP binding. The binding of ATP to KIAβW shows a KD ≈ 20 μM at pH 5, 5 °C and has a 1:1 stoichiometry. The KIAβW-ATP complex was determined using NMR spectroscopy and reveals the adenine ring sandwiched between the two Trp indole rings and that ATP binding induces important conformational changes in His1, Trp2, Lys4, Trp9 and Lys11 in the β-hairpin. The implications of these results for the hypothetic presence of β-hairpins and amyloids alongside RNAs on the prebiotic Earth are discussed.

Reconstructing a flavodoxin oxidoreductase with early amino acids

Primitive proteins are proposed to have utilized organic cofactors more frequently than transition metals in redox reactions. Thus, an experimental validation on whether a protein constituted solely by early amino acids and an organic cofactor can perform electron transfer activity is an urgent challenge. In this paper, by substituting "late amino acids (C, F, M, T, W, and Y)" with "early amino acids (A, L, and V)" in a flavodoxin, we constructed a flavodoxin mutant and evaluated its characteristic properties. The major results showed that: (1) The flavodoxin mutant has structural characteristics similar to wild-type protein; (2) Although the semiquinone and hydroquinone flavodoxin mutants possess lower stability than the corresponding form of wild-type flavodoxin, the redox potential of double electron reduction Em,7 (fld) reached -360 mV, indicating that the flavodoxin mutant constituted solely by early amino acids can exert effective electron transfer activity.

Simplified protein design biased for prebiotic amino acids yields a foldable, halophilic protein

A compendium of different types of abiotic chemical syntheses identifies a consensus set of 10 "prebiotic" α-amino acids. Before the emergence of biosynthetic pathways, this set is the most plausible resource for protein formation (i.e., proteogenesis) within the overall process of abiogenesis. An essential unsolved question regarding this prebiotic set is whether it defines a "foldable set"--that is, does it contain sufficient chemical information to permit cooperatively folding polypeptides? If so, what (if any) characteristic properties might such polypeptides exhibit? To investigate these questions, two "primitive" versions of an extant protein fold (the β-trefoil) were produced by top-down symmetric deconstruction, resulting in a reduced alphabet size of 12 or 13 amino acids and a percentage of prebiotic amino acids approaching 80%. These proteins show a substantial acidification of pI and require high salt concentrations for cooperative folding. The results suggest that the prebiotic amino acids do comprise a foldable set within the halophile environment.