L -proline

Proline (symbol Pro or P) is an organic acid classed as a proteinogenic amino acid (used in the biosynthesis of proteins), although it does not contain the amino group -NH

2 but is rather a secondary amine. The secondary amine nitrogen is in the protonated NH2+ form under biological conditions, while the carboxy group is in the deprotonated −COO− form. The "side chain" from the α carbon connects to the nitrogen forming a pyrrolidine loop, classifying it as a aliphatic amino acid. It is non-essential in humans, meaning the body can synthesize it from the non-essential amino acid L-glutamate. It is encoded by all the codons starting with CC (CCU, CCC, CCA, and CCG).

Proline is the only proteinogenic amino acid which is a secondary amine, in that the nitrogen atom is attached to the α-carbon by a chain of three other carbon atoms.