Because four different carbon groups are atteched to the central carbon of an amino acid, it is called an asymmetric carbon, identified as the -carbon. Two mirror-isomers, or enantiomorphs, are possible, as with L-alanine and D-alanine, compared on the right. The two molecules have been set to rotate in opposing directions to emphasise their mirror-image relationship.

The L- originally meant "levo-" or left, and D- meant "dextro-" or right, which refers to the rotation of polarized light by a solution of the molecules. This nomenclature has only a formal meaning today, since a molecule synthesized from an L-molecule also is called an L-form, even though the new molecule might rotate polarised light in the opposite direction. Only L-amino acids are found in living organisms, with the exception of a few cell-wall components in some bacteria. This seemingly arbitrary asymmetry of molecules in living organisms puzzled Pasteur and early biochemists, and we will return to this question later.

The drawing on the right is a convenient memory device to fix the structure of L-amino acids in your mind. Imagine that an amino acid is transformed into a hump-backed bridge, and that you are crossing it form the O to the N atom (remember it as "ONward"). Then at the crest of the bridge, the side chain of an L-amino acid will be on your left ("levo") and that of a D-amino acid will be on your right ("dextro").