Medicine & USMLE


Amino Acids
  1. Serine
  2. Asparagine
  3. Alanine
  4. Valine
  5. Leucine
  6. Isoleucine
  7. Phenylalanine
  8. Tryptophan
  9. Methionine
  10. Proline
  11. Glycine
  12. Threonine
  13. Cysteine
  14. Tyrosine
  15. Glutamine
  16. Aspartate
  17. Glutamate
  18. Histidine
  19. Lysine
  20. Arginine


Proline, which abbreviates to the 3-letter Pro or single letter P, is one of the 20 amino acids that make up proteins in our body. Proline’s R-group is unique in that it bends backwards on itself to connect with the amino group attached to the core or alpha carbon. This forms a 5-sided ring-shaped R-group incorporating the nitrogen of this amino group. Due to the rigid, ring-shape of this R-group, proline causes kinks in any polypeptide chains that include it. Proline is also a non-polar, hydrophobic amino acid, with a neutral charge at physiological pH.

Key Points

  • Proline
    • Abbreviations
      • Pro, P
    • Chemical Structure
    • R-Group: 4-C chain forms 5-membered ring with nitrogen of amino group
      • Hydrogen of nitrogen unaccessible for forming H-bonds in polypeptide chains
      • Kinks polypeptide chain
        • Conformational rigidity of amino group in amide bond connecting amino acids in polypeptide chain causes bends
        • Disrupts secondary structures like alpha-helices or beta-pleated sheets (rarely occurs in those structures)
        • Often found in joining regions between secondary structures
    • Polarity
      • Nonpolar (lipid soluble/hydrophobic)
    • Charge at pH 7
      • Neutral (0)