Concept 5 Review

L- and D-Amino Acids

Amino acids can occur in L- and D-forms, but only L-forms are used by cells.

Every amino acid (except glycine) can occur in two isomeric forms, because of the possibility of forming two different enantiomers (stereoisomers) around the central carbon atom. By convention, these are called L- and D- forms, analogous to left-handed and right-handed configurations.

L- and D-alanine: b&s and formulas

Only L-amino acids are manufactured in cells and incorporated into proteins. Some D-amino acids are found in the cell walls of bacteria, but not in bacterial proteins.

Glycine, the simplest amino acid, has no enantiomers because it has two hydrogen atoms attached to the central carbon atom. Only when all four attachments are different can enantiomers occur.

To review enantiomers (stereoisomers), see the section on enantiomers in the Building Biomolecules Lab Simulations activity.