Phosphorylation is the addition of the phosphate monoester to a macromolecule, catalyzed by a particular kinase enzyme. With respect to the proteins, particular amino acid side chains (serine, threonine, tyrosine) are subject to the phosphorylation catalyzed by protein kinases; altering the phosphorylation status of a protein might have dramatic effects on its biological properties, and is a common cellular control mechanism. With respect to the DNA, 5' ends should be phosphorylated for ligation.