Which one of the following amino acids in proteins does NOT undergo phosphorylation?

Proline (Pro) is the only common amino acid in proteins that does not undergo phosphorylation. Phosphorylation occurs on hydroxyl-containing amino acids: serine (Ser, A), threonine (Thr, B), and tyrosine (Tyr, D). These hydroxyl groups serve as nucleophilic targets for kinases. Proline has a secondary amine as its side chain (it's an imino acid that forms a cyclic structure with the backbone), lacking any hydroxyl group that could be phosphorylated. This unique structure also makes proline a helix breaker in protein secondary structure.