Carbon atoms are capable of forming four covalent bonds with other atoms. A carbon atom bound to four other atoms has a tetrahedral structure, with the carbon in the center and the groups in the vertices.
If the four radical groups bound to a carbon are all different, they can be arranged in one of two conformations. Despite being identical in their composition, these two conformations are not equal: one is the mirror image of the other. These types of molecules are called enantiomers, and the central carbon is called the chiral carbon.
The alpha carbon of all amino acids, with the exception of glycine, is a chiral carbon, because it is bound to four distinct groups: amino, carboxyl, hydrogen and the R group. So amino acids can exist in two different forms: the enantiomers D and L. This classification is based on the structure of a molecule called glyceraldehyde. So, amino acids with a structure similar to the L-glyceraldehide are called L amino acids, and D amino acids are similar to the D-glyceraldehide.
Enantiomers have a different 3D structure, and because of that, their functions are different in living organisms. Typically, L-amino acids are the ones with biological relevance, and cells are only capable of synthesizing and using this conformation to create proteins.
