Evolution of Protein Amino Acids

Abstract

In his article “Researchers create first autonomous synthetic life form” (31 Jan., p. 640), Robert F. Service reports that Peter Schultz and his team at Scripps Research Institute have metabolically engineered Escherichia coli bacteria to manufacture and incorporate into its growing proteins a 21st amino acid, p-aminophenylalanine. They are now experimenting to see if the new bacterial variety will outcompete and fare better than the 20-amino acid variety. If so, Schultz says that “it would suggest that although biology has made do with 20 amino acids for billions of years, evolution could make use of plenty more.” These are important and exciting results, but it is worth noting that about a billion years ago, evolution had already performed similar experiments with breakthrough success. Two new protein amino acids evolved: hydroxyproline and hydroxylysine. Probably originating from within fungal-like protists (1), this development permitted the subsequent evolution of the structural glycoproteins collagen and extensin. Functional “make-do” substitutes for these proteins using some combination of the 20 “standard” amino acids had not been achieved. Thus, the evolution of these two amino acids and their insertion into fibrous proteins was a major biochemical breakthrough for complex life. It opened the way for the origin, evolution, and diversification of the Metazoa and Metaphyta. These “rare” amino acids require molecular oxygen and oxygenase enzymes for their synthesis. The proteins also use oxygen and oxidases for the intermolecular cross-links that are important for fibril strengthening and chemical resistance. Because of this mandatory, energy-expensive, and metabolically competitive need for molecular oxygen, it is likely that the delayed appearance of complex multicellular life until the latter part of the Precambrian could be due, in part, to the lower amounts of atmospheric free oxygen available earlier on (1–3).