L-Proline is one of the twenty proteinogenic units which are used in living organisms as the building blocks of proteins. The other nineteen units are all primary amino acids, but due to the (3-carbon) cyclic sidechain binding back to the nitrogen of the backbone, proline lacks a primary amine group (−NH2). The nitrogen in proline is properly referred to as a secondary amine. Proline is sometimes called an imino acid, but this is not correct, as imines contain a carbon-nitrogen double bond. The side chain binding to the nitrogen prevents rotation around the phi torsion angle, giving proline unique structural properties.
Proline has an aliphatic side chain with a distinctive cyclic structure. The secondary amino (imino) group of proline residues is held in a rigid conformation that reduces the structural flexibility of polypeptide regions containing proline
Proline is a non-polar amino acid. In proteins it does not have a hydrogen on the amide group and can therefore not act as a hydrogen bond donor. Proline can act as a structural disruptor for (α) helices, and as a turning point in β sheets. Multiple prolines and/or hydroxyprolines in a row can create a proline helix; this is the predominant structure in collagen. Sequences of proline and 2-aminoisobutyric acid (Aib) form a helical turn structure.
Proline is biosynthetically derived from the amino acid L-glutamate and its direct precursor is the real imino acid (S)-Δ1-pyrroline-5-carboxylate (P5C).
Proline and its derivatives are often used as asymmetric catalysts in organic reactions. The CBS reduction or proline catalysed aldol condensation are prominent examples.
Proline has a sweet flavor with a distinct aftertaste. Proline also causes slight irritation to the tongue not unlike Sichuan Pepper.