There are twenty (20) amino acids in nature. Generally, each amino acid is structurally made up of a central carbon atom called alpha carbon attached to a hydrogen, carboxylic acid group (-COOH) and an amine group (-NH2). However, one particular amino acid called PROLINE posseses an exception to this.
Proline, which is the only cyclic amino acid, is also the only amino acid that forms a secondary amine group i.e. loss of hydrogen atoms in its amine group when in a protein structure. This means that when in a protein, PROLINE does not have an AMINE GROUP.
