Enantiomers are molecules that share the same chemical structure and chemical bonds but differ in the three-dimensional placement of atoms so that they are non-superimposable mirror images. Figure shows an amino acid alanine example, where the two structures are nonsuperimposable. In nature, only the L-forms of amino acids make proteins. Some D forms of amino acids are seen in the cell walls of bacteria, but never in their proteins. Similarly, the D-form of glucose is the main product of photosynthesis and we rarely see the molecule's L-form in nature.

Molecular models of D-and L-alanine are shown. The two molecules, which contain the same number of carbon, hydrogen, nitrogen atoms, are mirror images of one another.
D-alanine and L-alanine are examples of enantiomers or mirror images. Only the L-forms of amino acids are used to make proteins.