13 A. Types of Cell Signaling Proper signaling generally depends on a signal molecule being degraded rapidly
14 A. Types of Cell Signaling Types of Signals based on Where they BindTo an intracellular receptor: e.g. Steroid hormonesTo a cell-surface receptor: G-protein linked, enzyme-linked, or ion channel-linked
15 B. Intracellular Receptor Signaling: Steroid Hormones Overview of Steroid Hormone Action:Steroid hormone (small, hydrophobic) is secreted by glandular cells and released into bloodstream (usually transported via shuttle proteins)The hormone enters the cytoplasm of cellsIn the cytoplasm of target cells, the hormone binds to steroid hormone receptor protein
16 B. Intracellular Receptor Signaling: Steroid Hormones Overview of Steroid (cont.):The steroid-receptor conplex is translocated into the nucleus, where it activates the transcription of the primary response genesTranscription of the primary response genes, followed by translation, results in the production of primary response proteins
17 B. Intracellular Receptor Signaling: Steroid Hormones Overview of Steroid (cont.):The primary response proteins usually inhibit further transcription of their own genes, and they may activate transcription of secondary response genes.
21 C. Cell Surface Receptors: Types Three known classes of cell surface receptorsIon channel-linked, G-protein-linked, Enzyme-linkedAn activated cell-surface receptor triggers a phosphorylation cascade
25 D. Cell Surface Receptors: G-protein linked receptors Largest family of cell surface receptorsDifferent ones respond to a wide variety of mediators including different hormones, neurotransmitters, local mediatorsExamples: Receptors to epinephrine, acetylcholine, serotonin
27 B. G-linked Receptors What do G-protein receptors do? G-protein receptors activate trimeric G-proteinActivated G-protein alters the cellular concentration of a “second messenger”: usually cyclic AMP or Ca2+The second messenger activates a protein kinase enzymeThe protein kinase phosphorylates another enzyme and alters its activity
32 B. G-linked Receptors Trimeric G-proteins disassemble when activated Three chains: a, b, and ga chain binds and hydrolyzes GTPb & g chains form a tight complex bg that anchors G-protein to the plasma membrane
33 B. G-linked Receptors Trimeric G-proteins disassemble when activated Inactive G-protein has a bound GDPWhen activated: GDP dissociates, new GTP is boundThis causes a to dissociate from bga binds to adenylate cyclase, altering its activityGs protein stimulates activates adenylate cyclase, Gi inhibits it
36 B. G-linked Receptors Action of epinephrine on glycogen metabolism Epinephrine (adrenaline) is released from the adrenal gland in times of stress. It has two overall actions on glycogen metabolism: it inhibits glycogen synthesis, and it promotes glycogen breakdownThis is mediated by a protein phosphorylation cascade
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