Phosphorylation Resistance in Amino Acids

Phosphorylation is a common post-translational modification where a phosphate group (PO₄^3-) is added to an amino acid residue within a protein. This process typically requires the amino acid to have a nucleophilic group capable of accepting the phosphate, most commonly a hydroxyl group ($-OH$).

Amino Acids Targeted by Phosphorylation

The primary amino acids in proteins that undergo phosphorylation are:

• Serine (Ser): Contains a hydroxyl group ($-OH$).
• Threonine (Thr): Also contains a hydroxyl group ($-OH$).
• Tyrosine (Tyr): Contains a phenolic hydroxyl group ($-OH$).

These residues are phosphorylated by enzymes called kinases.

Proline's Inability to Undergo Phosphorylation

Proline (Pro) is structurally unique among the common amino acids. It possesses an *imino* group, a secondary amine where the side chain cycles back to the alpha-amino group. Crucially, Proline lacks a hydroxyl group ($-OH$).

Because Proline does not have the necessary hydroxyl functional group, it cannot be directly phosphorylated by the kinases that modify Serine, Threonine, and Tyrosine. Therefore, Proline does NOT undergo phosphorylation.