Any of a class of molecules that are combined to form proteins. The sequence of amino acids in a protein and hence protein function are determined by the genetic code. Amino acids contain a basic amino (NH2) group, an acidic carboxyl (COOH) group and a side chain (R - of a number of different kinds) attached to an alpha carbon atom. The genetic code common to all Terragen organisms specifies 20 different amino acids on a regular basis, and two more by nonstandard codings; a number of additional amino acids are also employed by Terragen organisms by modifying the original 22 once a protein is already synthesized. Neogens, and some naturally evolved xenobionts with biochemistry similar to that of Terragens, may employ alternative or additional amino acids beyond those of traditional Old Earth biochemistry, but typically there is significant overlap.