Catalytic domain of the Protein Tyrosine Kinase, Insulin ReceptorPTKs catalyze the transfer of the gamma-phosphoryl group from ATP to tyrosine (tyr) residues in protein substrates. InsR is a receptor PTK (RTK) that is composed of two alphabeta heterodimers. Binding of the insulin ligand to the extracellular alpha subunit activates the intracellular tyr kinase domain of the transmembrane beta subunit. Receptor activation leads to autophosphorylation, stimulating downstream kinase activities, which initiate signaling cascades and biological function. InsR signaling plays an important role in many cellular processes including glucose homeostasis, glycogen synthesis, lipid and protein metabolism, ion and amino acid transport, cell cycle and proliferation, cell differentiation, gene transcription, and nitric oxide synthesis. Insulin resistance, caused by abnormalities in InsR signaling, has been described in diabetes, hypertension, cardiovascular disease, metabolic syndrome, heart failure, and female infertility. The InsR subfamily is part of a larger superfamily that includes the catalytic domains of other kinases such as serine/threonine kinases, RIO kinases, aminoglycoside phosphotransferase, choline kinase, and phosphoinositide 3-kinase.