Detailed Explanation
Amino acid (Pro, P) unique among the 20 standard amino acids for having a cyclic side chain that connects to both the Cα carbon and the backbone nitrogen, forming a five-membered pyrrolidine ring. This ring constrains the backbone φ dihedral angle to approximately −60° and prevents nitrogen from donating a hydrogen bond, making proline a helix breaker that introduces rigid kinks in peptide chains.
Proline is abundant in collagen (~10% of residues) as part of the characteristic Gly-Pro-Hyp repeat, where its rigidity contributes to the triple-helix structure. In turns, proline frequently occupies the i+1 position of type II β-turns. Proline isomerization (cis/trans switching around the prolyl peptide bond) is often the rate-limiting step in protein folding.
Key Facts
- Amino acid (Pro, P) unique among the 20 standard amino acids for having a cyclic side chain that connects to both the Cα carbon and the backbone nitrogen, forming a five-membered pyrrolidine ring.
- This ring constrains the backbone φ dihedral angle to approximately −60° and prevents nitrogen from donating a hydrogen bond, making proline a helix breaker that introduces rigid kinks in peptide chains.
- Proline is abundant in collagen (~10% of residues) as part of the characteristic Gly-Pro-Hyp repeat, where its rigidity contributes to the triple-helix structure.
- In turns, proline frequently occupies the i+1 position of type II β-turns.
- Proline isomerization (cis/trans switching around the prolyl peptide bond) is often the rate-limiting step in protein folding.
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