Prolin

Proline, or L-proline, is an α-amino acid that is incorporated into proteins during translation. It is one of the 20 standard amino acids and is typically found in the L- configuration in biological systems. Proline is considered non-essential for humans because it can be synthesized endogenously, although dietary intake can contribute to overall amino acid availability under certain conditions.

Its side chain forms a pyrrolidine ring that bonds back to the amino group, making proline a secondary amine. This cyclic structure renders proline relatively nonpolar and rigid compared with other amino acids, and it influences the conformational properties of polypeptides, notably by constraining backbone rotation.

Proline is highly abundant in collagen, the main structural protein in connective tissues. Proline and its hydroxylated derivative hydroxyproline contribute to the stability of the collagen triple helix. The imino ring in proline also tends to disrupt regular alpha helices and promotes turns and polyproline-rich regions, contributing to distinct structural motifs in proteins.

Biosynthesis and metabolism in humans begin with the conversion of glutamate to Δ1-pyrroline-5-carboxylate, followed by reduction to proline. Proline can be catabolized back to glutamate, linking to energy production and the urea cycle. The hydroxylation of proline residues to hydroxyproline during collagen formation requires prolyl hydroxylases and vitamin C.

Proline occurs in most dietary proteins and is found in a wide range of foods. As a non-essential amino acid, it is synthesized by the body and does not require dietary intake in healthy individuals, though diet contributes to the overall pool of amino acids available for protein synthesis.