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n human dietary guides it is pointed out that 10 amino acids are essential and must be supplied in the diet. Since all 20 coded amino acids are required by every organism, we may assume that the remaining 10 are synthesized by human metabolism from other ingested molecules. Keeping the above in mind, let us consider the synthesis of amino acids in reductive chemoautotrophs, believed to be the earliest organisms. In these bacterial species, all molecules are synthesized from carbon dioxide, ammonia, phosphoric acid, water, hydrogen, and hydrogen sulfide. Every single macromolecule and intermediate is made from these simple starting materials. In addition, every synthetic pathway starts from a molecule in the reductive citric acid cycle, usually acetate, pyruvate, oxaloacetate, a-keto gluarate, and succinate. It is therefore possible to index each amino acid by the number of reaction steps between it and its starting molecule in the reductive tricarboxylic acid cycle. These index numbers and mole percentages (taken from the Swiss Prot Data Base) averaged over a collection of proteins are then:
The nonessential amino acids are alanine, asparagine, aspartic acid, cysteine, glutamic acid, glutatmine, glycine, proline, and serine. Tyrosine is usually listed as a nonessential amino acid, but since it is made in humans by a one step catabolic reaction from phenylalanine, it seems more appropriate to consider it as an essen-