This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Basics of amino acids, Subject name: Biochemistry
Which of the following amino acids can be phosphorylated? 
Select and explain. 
A. Cysteine, B. Leucine, C. Methionine, D. Serine
D: Serine
Answer: D: Serine 
Explanation: 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.