Proline
Proline — A unique amino acid (Pro, P) with a cyclic side chain bonded to both the nitrogen and alpha-carbon, introducing rigid kinks in peptide chains.
What Is Proline?
Proline (Pro, P) is the only cyclic amino acid, with its side chain bonded to both the alpha-carbon and the backbone nitrogen, forming a five-membered pyrrolidine ring (MW: 115.13 Da). This unique structure imposes severe conformational constraints on the peptide backbone, making proline a key determinant of peptide shape and function.
Structural Effects
- Helix breaker: Proline cannot donate a backbone N-H hydrogen bond and introduces a kink in alpha-helices
- Cis-trans isomerism: The Xaa-Pro peptide bond can adopt cis (10-30%) or trans conformations, creating slow folding steps
- Polyproline helix: Consecutive prolines form PPII helices, an extended left-handed structure common in collagen and CPP sequences
- Turn induction: Pro at positions i+1 of beta-turns stabilizes reverse turn conformations
Proline-Rich Peptides
Bradykinin (Pro-Pro-Gly core), collagen peptides (Gly-Pro-Hyp repeats), KPV (central proline), and many antimicrobial peptides rely on proline for their bioactive conformations.
Frequently Asked Questions
What is Proline?
A unique amino acid (Pro, P) with a cyclic side chain bonded to both the nitrogen and alpha-carbon, introducing rigid kinks in peptide chains.
Why is Proline important in peptide research?
Proline is a fundamental concept in amino acid as it relates to peptide science. It directly influences experimental design, compound characterization, and the reliability of research outcomes across biochemistry and molecular biology disciplines.