ABSTRACT
All proteins are made up from the same set of 20 standard amino acids. A typical amino acid has a primary amino group, a carboxyl group, a hydrogen atom and a side-chain attached to a central α-carbon atom. Proline is the exception to the rule in that it has a secondary amino group. All of the 20 standard amino acids, except for glycine, have four different groups arranged tetrahedrally around the Cα atom and thus can exist in either the D or L configuration. These two enantiomers are nonsuperimposable mirror images that can only be distinguished on the basis of their different rotation of plane-polarized light. Only the L isomer is found in proteins. The standard set of 20 amino acids have different side-chains or R groups and display different physicochemical properties. Glycine has a hydrogen atom as its R group.
