prolines

Prolines refer to the amino acid proline, one of the 20 standard amino acids used in protein synthesis. Proline is unique among amino acids because its side chain forms a pyrrolidine ring that connects to the amino group, creating a secondary amine. This cyclic structure makes proline relatively nonpolar and markedly rigid, reducing the conformational flexibility of polypeptide chains and influencing protein folding and structure.

In proteins, proline residues affect secondary structure in characteristic ways. The rigidity of proline tends to disrupt alpha helices and can promote turns and kinks, contributing to the shaping of protein tertiary structures. Proline-rich regions also participate in specific protein–protein interactions, sometimes adopting the polyproline II helix conformation that binds certain interaction domains.

Proline is especially abundant in collagen, the main structural protein in connective tissue. A large portion of collagen proline is post-translationally converted to hydroxyproline, a modification carried out by prolyl hydroxylase enzymes. Hydroxyproline enhances hydrogen-bonding networks that stabilize the collagen triple helix. Vitamin C (ascorbic acid) deficiency impairs this hydroxylation, weakening collagen and leading to connective-tissue symptoms observed in scurvy.

Biosynthetically, proline is produced from glutamate through a pathway involving gamma-glutamyl phosphate, glutamate-5-semialdehyde, pyrroline-5-carboxylate, and reduction to proline. In catabolism, proline is oxidized by proline dehydrogenase to pyrroline-5-carboxylate and ultimately to glutamate, linking proline metabolism with energy production in cells.

Overall, proline's distinctive structure and its role in structural proteins like collagen make it a notable amino acid in biochemistry and molecular biology.