Amino Acid Overview
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• With rare exception, most organisms use only 20 amino acids (AAs) for protein synthesis.
• Proteins are synthesized as linear sequences of AA "residues," or polypeptide chains, formed by the enzymatic joining of AAs in sequences precisely dictated by the genetic code (i.e., translation).
• The special relationship between the genetic code and these 20 amino acids define the "standard set" of amino acids, or the "standard" amino acids. 
• Dynamic molecular images of the physiologically-relevant chemical isoforms of the standard AAs can all be viewed in the windows below by clicking on the "AA buttons" on the left panel, listed according to the conventional 3-letter abbreviations, or on the right panel, listed according to the conventional 1-letter abbreviations.The dotted"surface" briefly surrounding each AA is an imprint of the van der Waals radius of each atom in the structure.  (The color formats and image manipulation tools are described below.)
• Chemically, the standard AAs are similar with a central carbon, the tetrahedral a-carbon (Ca), covelently bonded to an amino group (a-NH3+), a carboxyl group (a-COO-), a hydrogen atom, and a variable sidechain group referred to at the "R-group."
• 1  AA, i.e., proline, is an "imino" acid with the Ca-H bond replaced by a Ca-N bond from the R-group thereby forming a cyclic sidechain.
• 19 of the standard AAs are classified  as "L"-amino acid by virture of their stereochemistry in relation to the a-carbon; the 1 exception is glycine which lacks an asymmetric carbon center.
• A variety of additional "nonstandard" AAs are biosynthetically manufactured or acquired in the diets of most organisms.
• Polypeptide residues that appear to derive from nonstandard AAs frequently appear as part of the covalent structure of polypeptides or proteins.  Such "unusual" residues generally arise from post-translational modifications of mature proteins (rather than from de novo synthesis of the polypeptide chain with a modified genetic code).

The structural relationships between standard and nonstandard AAs are best understood by first exploring the structures of the standard AAs.

Standard Amino Acids

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©	Duane W. Sears September 28, 1998