Which of the following does not undergo phosphorylation by protein kinases?

Phosphorylation occurs at the hydroxyl groups of serine, threonine, and tyrosine. A protein kinase is a kinase enzyme that modifies other proteins by chemically adding phosphate groups to them (phosphorylation). Phosphorylation usually results in a functional change of the target protein (substrate) by changing enzyme activity, cellular location, or association with other proteins. The chemical activity of a kinase involves transferring a phosphate group from a nucleoside triphosphate (usually ATP) and covalently attaching it to one of three amino acids that have a free hydroxyl group.Most kinases act on both serine and threonine, others act on tyrosine, and a number (dual specificity kinases) act on all three. There are also protein kinases that phosphorylate other amino acids, including histidine kinases that phosphorylate histidine residues.