Tyrosine kinase signalling Tyrosine kinase structure dimeric transmembrane protein intrinsic kinase activity phosphorylated receptor can bind protein called insulin receptor substrate (IRS) is a scaffold protein Pathway hormone binds to receptor receptor is dimerized auto cross-phosphorylation of intracellular domain of the dimerized receptor IRS binds phosphorylated domian IRS is phosphorylated on SH2-domains several enzymes bind phosphorylated SH2-domians phosphatases pathway signaled for by insulin kinases phosphoinositol-3 (PI-3) kinase G proteins ras oncogene present in many cancer constitutive activation leads to constant growth signal Examples insulin, PDGF, EGF receptors sensitization via PPAR-gamma G protein signalling G-protein structure trimeric enzyme α, β, γ α-subunit has intrinsic GTPase activity acts as timer for shutting off the active form Pathway hormone binds to receptor:G-protein complex on cell membrane GDP bound to inactive α-subunit is exchanged for GTP α-subunit now active can be inhibitory (Gi) or stimulatory (Gs, Gq) is released from β, γ subunits can act as 2 different effector mechanisms cAMP pathway Gs activates adenyl cyclase normally, ATP → cAMP to increase [cAMP] cAMP can activate protein kinase A Cholera and E. coli toxins ADP-ribosylates subunit of Gs, which activates Gs ↑↑↑ [cAMP] Gi inhibits adenyl cyclase normally, stops ATP → cAMP to decreases [cAMP] Pertussis toxin ADP-ribosylates subunit of Gi, which inhibits Gi ↑↑↑ [cAMP] PIP2 pathway Gq activates phospholipase C PIP2 → IP3 + DAG DAG can activate protein kinase C IP3 can release Ca2+ from the ER Ca2+ then activates a number of enzymes including protein kinase C GTP eventually hydrolyzed to GDP and α-subunit becomes inactive Examples cAMP = glucagon, epinephrine (β, α2) PIP2 = vasopressin, epinephrine (α1) full list given in G-protein-linked 2nd messengers topic cGMP signalling Pathway hormone binds to receptor on cell membrane there is also a soluble receptor in the cytoplasm receptor has intrinsic guanylate cyclase activity GTP → cGMP cGMP activate protein kinase G protein kinase G mediates smooth muscle relaxation/vasodilation no G protein required Examples ANF → cell membrane receptor NO → diffuses across cell membrane and binds soluble receptor mechanism for pharmacologic nitrates (nitroprusside, nitroglycerine)