Amino acid isomers

Because the α-carbon of an amino acid is attached to four different chemical groups, it is an asymmetric (chiral) atom. Glycine is the exception because its α-carbon has two hydrogen substituents. Amino acids with a chiral α-carbon exist in two different isomeric forms, designated D and L, which are enantiomers, or mirror images (Fig. 1). [Note: Enantiomers are optically active. If an isomer, either D or L, causes the plane of polarized light to rotate clockwise, it is designated the (+) form.] All amino acids found in mammalian proteins are of the L configuration. However, D-amino acids are found in some antibiotics and in bacterial cell walls . [Note: Racemases enzymatically interconvert the D- and L-isomers of free amino acids.]

Figure 1. D and L forms of alanine are mirror images (enantiomers).