L-proline is one of the 20 amino acids that are used to synthesize proteins by the human body. This amino acid is encoded in the human genetic code with the codons CCA, CCC, CCG and CCU. L-proline is the only proteinogenic amino acid that is a secondary amine, meaning that its amine nitrogen is bound to two alkyl groups. Virtually all of the proteins in the human body contain L-proline, and the only amino acids that are more abundant are alanine and glutamine. L-proline is especially important in the production of collagen, which is a primary component in skin, cartilage and bone.