Phosphorylation, the addition of a phosphate group, is an important chemical modification that helps cells to regulate protein function. During phosphorylation, protein kinases transfer the terminal phosphate group of ATP to specific amino acid side chains of a target protein. As a result, the target protein may change conformation, or the phosphorylated region can act as a novel binding site for other proteins. These changes can modify the function of a protein, completely activating or deactivating it. The phosphate group can be removed by the action of protein phosphatases, enzymes that catalyze the dephosphorylation of proteins. In this way, kinases and phosphatases can reversibly regulate the activity of certain proteins turning them on or off as and when required by the cell.