Is an enzyme that can transfer a phosphate group from a donor molecule (usually ATP) to an amino acid residue of a protein. The protein kinase mechanism is used in signal transduction for the regulation of enzymes: phosphorylation can activate (or inhibit) the activity of an enzyme. Although most protein kinases are specialized for a single kind of amino acid residue, some exhibit dual kinase activity (they can phosphorylate two different kinds of amino acid).