Kinase molecule, computer model. Kinases are enzymes that catalyse the transfer of phosphate groups from a high-energy phosphate-containing molecule (such as ATP or ADP) to a substrate (such as a protein or lipid). This process is known as phosphorylation. The largest group of kinases are protein kinases. Up to 518 different types have been identified in humans and are used to transmit signals and control complex processes in cells. In this model the molecule's shape (secondary structure) is seen, comprising alpha helices (coils), beta sheets (arrows) and linking regions (lines). Also seen is a molecule of the inhibitor 5-bromo-indirubin (centre right). Protein kinase inhibitors are a type of enzyme inhibitor that specifically block the action of a protein kinase.