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[alpha]-Amino acids can undergo peptide formation by activation with carbon monoxide (CO) under hot aqueous conditions in the presence of freshly coprecipitated colloidal (Fe,Ni)S. We now show that CO-driven peptide formation proceeds concomitantly with CO-driven, N-terminal peptide degradation by racemizing N-terminal hydantoin and urea derivatives to [alpha]-amino acids. This establishes a peptide cycle with closely related anabolic and catabolic segments. The hydantoin derivative is a purin-related heterocycle. The (Fe,Ni)S-dependent urea hydrolysis could have been the evolutionary precursor of the nickelenzyme urease. The results support the theory of a chemoautotrophic origin of life with a CO-driven, (Fe,Ni)S-dependent primordial metabolism.
Extant metabolisms depend on cyclic protein turnover with anabolic condensation and catabolic degradation segments. In a primordial organism, the proteins would have been preceded by peptides, but a cyclic turnover from amino acids to peptides and back to amino acids should already have been operative. Otherwise, functionless peptides would have been futile sinks for valuable amino acids. Previously, we demonstrated the anabolic segment from amino acids to peptides, driven by CO in the presence of coprecipitated (Fe, Ni)S and Na^sub 2^S (1). We have now investigated the course of the catabolic segment from peptides back to amino acids.
Experiments for testing CO-driven peptide formation in the presence of (Fe,Ni)S and Na^sub 2^S are complicated by a pH problem. CO is continuously hydrated to HCOOH and oxidized to H^sub 2^CO^sub 3^, causing progressive acidification. Adding Mg(OH)^sub 2^ stabilizes the pH and renders the peptide-forming reaction more reproducible for study in a pH-controlled manner (2). Preliminary experiments in the presence of Mg(OH)^sub 2^ (pH 9.8) showed that with NiS alone dipeptide of L-phenylalanine (Phe) is formed and that with a 50% replacement of NiS by FeS the dipeptide yield is increased, whereas with FeS alone or with a mixture of Ni(OH)^sub 2^ and Fe(OH)^sub 2^ no dipeptide is formed. Under geochemical conditions, FeS is the companion of NiS.
Reaction of Phe with CO in the presence of (Fe,Ni)S (2) produced the dipeptide Phe-Phe. Byproducts with molecular weights exceeding that of Phe-Phe by 26 or 44 were detected by high-performance liquid chromatography mass spectrometry (HPLC-MS) (fig. S1). Similarly, tyrosine formed its dipeptide and byproducts with molecular weights exceeding that of the dipeptide by 26 or 44. Byproducts obtained from...