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


Serine, which abbreviates to the 3-letters Ser or one-letter S, is one of the 20 amino acids that make up proteins in our body. Serine’s R-group is a hydroxymethyl group; that is, an alcohol attached to a single bridging carbon. Because of its hydroxyl group, serine can be phosphorylated; in fact, it is the amino acid most likely to be phosphorylated in our body. Serine is also a polar, hydrophilic amino acid, with a neutral charge at physiological pH.

Key Points

  • Serine
    • Abbreviations
      • Ser, S
    • Chemical Structure
    • R-Group: -CH2-OH
      • 1 bridging methylene β carbon (-CH2-)
        • Contrast vs. Threonine (which has a methyl substituent in place of one of the hydrogens on the β carbon)
      • Hydroxy group
        • Phosphorylatable by serine kinases
        • Serine is the amino acid most likely to be phosphorylated
    • Polarity
      • Polar (water soluble/hydrophilic)
    • Charge at pH 7
      • Neutral (0)