Inferring the protein architecture chronology is one of central topics in origin of life study and has been given much attention. Based on an amino acid evolutionary model that late amino acids were bio-synthesized prior to early counterparts, we addressed the issue by examining the structures of amino acid synthases. Despite the limited structural information on amino acid synthases, our deduction revealed that α/β was the oldest protein class, which is in good agreement with the prior fold-usage-based conclusion.

Key words: Protein; Fold; Evolution; and Amino Acid Synthase.