Proline is an organic acid classed as a proteinogenic amino acid , although it does not contain the amino group -NH2 but is rather a secondary amine. The secondary amine nitrogen is in the protonated form under biological conditions, while the carboxyl 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 .