These organic acids exist naturally in a zwitterion state where the carboxylic acid moiety is ionized and the basic amino group is protonated. The entire class of amino acids has a common backbone of an organic carboxylic acid group and an amino group attached to a saturated carbon atom. The simplest member of this group is glycine, where the saturated carbon atom is unsubstituted, rendering it optically inactive. The rest of the 20 most common amino acids are optically active existing as both D and L stereoisomers. There are also acidic and basic side chains as well as thiol chains that can be oxidized to dithiol linkages between two similar amino acids.
Amino acids are the principal building blocks of proteins and enzymes.
They are incorporated into proteins by transfer RNA according to the genetic code while messenger RNA is being decoded by ribosomes. During and after the final assembly of a protein, the amino acid content dictates the spatial and biochemical properties of the protein or enzyme. The amino acid backbone determines the primary sequence of a protein, but the nature of the side chains determines the protein's properties. Amino acid side chains can be polar, non-polar, or practically neutral. Polar side chains tend to be present on the surface of a protein where they can interact with the aqueous environment found in cells.