Intro to Pharmacology and Toxicology
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Growth factors promote cell growth and proliferation by inducing changes in metabolism, cytoskeleton, gene expression and the cell cycle. Growth factor receptors have a single transmembrane domain, and most have an intracelllular tyrosine kinase domain. These receptors are cystein-rich, both in the extracellular and intracellular domains, and dimerize or trimerize upon activation. Some examples of growth factors are epidermal growth factor (EGF), neuronal growth factor (NGF), transforming growth factor (TGF), the insulin receptor family and many organ-derived growth factors like platelet-derived grrowth factor (PDGF).
The PDGF receptor exists in three isoforms: AA, BB and AB. Each isoform is a dimer held together by disulfide bonds. AA will bind alpha-alpha. AB will bind alpha-apha or alpha-beta. BB will bind alpha-alpha, alpha-beta or beta-beta. The combination of ligands and receptors gives selectivity to the system. Each isoform leads to a different signaling patway.
The rexceptor dimers are activated by cross-phosphorylation when bound to ligand. There are two clases of phosphorylation sites: in the kinase domains and outside the kinase domains. p857 is critical for activation of the kinase domains. Phosphorylation outside the kinase domains occur at Tyr, forming docking sites for other molecules containing the SH2 and SH3 domains to attach. SH2 recognizes a 5-10 amino acid sequencewith Tyr-PO4. SH3 recognizes sequences of about 10 amino acids, rich in proline. Another sequence, the PH domain (Plextein Homologous) leads to binding of kinases, Tyr kinases and some phospholipases..
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