Proline

Proline (symbol Pro or P) is a proteinogenic amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated NH₂⁺ form under biological conditions), an α-carboxylic acid group (which is in the deprotonated −COO⁻ form under biological conditions), and a side chain pyrrolidine, classifying it as a nonpolar (at physiological pH), 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 with a secondary amine, in that the alpha-amino group is attached directly to the main chain, making the α carbon a direct substituent of the side chain