Figure 1

A Venn diagram showing different categories of amino acids: abiotic, approximately 80 amino acids which were probably produced by abiotic synthesis before life evolved (see, for example, [53]); biosynthetic, approximately 900 amino acids which are produced by natural biosynthetic pathways [54,55]; and engineered, at least 118 amino acids which have been experimentally engineered and placed into proteins by biomedical research projects [56]. The group of coded amino acids includes the standard amino-acid alphabet of 20 coded amino acids and the coded and biosynthetic amino acids selenocysteine [26] and pyrrolysine [29], as well as at least 30 engineered amino acids which have been cotranslationally incorporated into proteins [28]. One example is shown for each region of the Venn diagram. At least some of the 20 coded amino acids are thought to have originated as biosynthetic modifications of the others. The diagram shows that the 20 coded amino acids of the standard amino-acid alphabet are a small subset of what was chemically and/or biologically possible.