Phosphorylation—the addition of a phosphate group—is an important modification for cells to regulate protein function.
When a phosphate group is removed during ATP hydrolysis, enzymes called protein kinases can catalyze the covalent bonding of this group to certain amino acid side chains in a protein. As a result, the protein changes shape, or allows other proteins to bind to the phosphorylated regions.
These changes can modify the function of a protein, and may even completely activate or deactivate it. The phosphate group can be removed by the action of protein phosphatases—enzymes that catalyze dephosphorylation of proteins—essentially reversing the reaction.
By regulating phosphorylation in this way, the activity of certain proteins can be reversibly turned on, off, or modified as needed in the cell.