This is an interesting article by Casey Luskin at Evolution and News:
A recent paper in Nature‘s journal Scientific Reports, “Extraordinarily Adaptive Properties of the Genetically Encoded Amino Acids,” has found that the twenty amino acids used by life are finely tuned to explore “chemistry space” and allow for maximal chemical reactions. Considering that this is a technical paper, they give an uncommonly lucid and concise explanation of what they did:
[W]e drew 108 random sets of 20 amino acids from our library of 1913 structures and compared their coverage of three chemical properties: size, charge, and hydrophobicity, to the standard amino acid alphabet. We measured how often the random sets demonstrated better coverage of chemistry space in one or more, two or more, or all three properties. In doing so, we found that better sets were extremely rare. In fact, when examining all three properties simultaneously, we detected only six sets with better coverage out of the 108 possibilities tested.
That’s quite striking: out of 100 million different sets of twenty amino acids that they measured, only six are better able to explore “chemistry space” than the twenty amino acids that life uses. That suggests that life’s set of amino acids is finely tuned to one part in 16 million.
Of course they only looked at three factors — size, charge, and hydrophobicity. When we consider other properties of amino acids, perhaps our set will turn out to be the best: