Protein kinase

In biochemistry, protein kinases are "a family of enzymes that catalyze the conversion of ATP and a protein to ADP and a phosphoprotein."

In signal transduction, cell surface receptors may activate second messenger systems such as adenyl cyclase-cyclic AMP and cyclic GMP which then may activate protein kinases which then affect downstream targets (see figure).

Examples of protein kinases include G-protein-coupled receptor kinases.