Which of the following amino acids can be phosphorylated?

The OH groups of serine, tyrosine, and threonine frequently serve as the points of covalent attachment for phosphoryl groups that regulate protein function Protein kinase enzymes catalyse the phosphorylation while protein phosphatase enzymes catalyse the dephosphorylation. Ceain protein kinases are specific to the amino acid e.g. tyrosine kinases specifically phosphorylate tyrosine residues.