Proline is a kind of protein α- Amino acid is one of the 20 DNA codes, and its corresponding codons are CCU, CCC, CCA and CCG. Proline is not an essential amino acid, which can be synthesized by human body. The most special feature of amino acids formed in 20 proteins is that amino nitrogen is bound to not one but two alkyl groups, so it has secondary ammonia, and L-form usually has s stereochemistry.