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


Cysteine, which is abbreviated with the letters Cys or just C, is one of the 20 amino acids that make up proteins in our body. Cysteine’s R group is made up of a 1-carbon structure with a sulfhydryl group attached to the end. This makes it a polar, hydrophilic, and uncharged amino acid. Cysteine’s ability to form disulfide bonds with other cysteine residues makes it very important in determining the tertiary and quaternary structure of proteins. Finally, cysteine is the only amino acid with a naturally occurring R configuration. 

Key Points

  • Cysteine
    • Abbreviations
      • Cys, C
    • Chemical Structure
    • R-group: -CH2-SH
      • Thiol/Sulfhydryl group (-SH)
        • Forms covalent disulfide bonds (-S-S-) with other cysteine residues
          • Cysteine is only amino acid that covalently bonds side chains
        • Can act as a nucleophile in enzyme-catalyzed reactions, especially after deprotonation
    • Polarity
      • Polar (water soluble/hydrophilic)
    • Charge at pH 7
      • Neutral (0)
    • Other Information 
      • Only chiral AA with a natural R configuration
        • All other chiral amino acids are in S configuration