Proline
C₅H₉NO₂·115.132 g/mol·Amino Acids·CID 145742
Amino AcidStructural
The only amino acid with a cyclic side chain bonded to the backbone nitrogen (an imino acid). Creates rigid kinks in protein chains — abundant in collagen, where every third residue is proline or hydroxyproline. Key to collagen's triple helix structure.