Cell Signaling 2 2020 PDF

Cell Signalling (2): the G-protein coupled receptor Stuart Cruickshank The disassembly of an activated G-Protein into two signaling components. Ground state Facilitated by RGS:...

Cell Signalling (2): the G-protein coupled receptor Stuart Cruickshank The disassembly of an activated G-Protein into two signaling components. Ground state Facilitated by RGS: (Regulator of G protein Signaling) Ga specific GAP Ligand Binding G protein activation Gα specific Gβγ specific targets targets  Signalling Pathways Some G proteins Signal by Regulating the Production of cAMP  G proteins and adenylate cyclase adenylate cyclase ATP cAMP adenylate cyclase (AC) is membrane-associated enzyme found in all eukaryotic cells if G protein stimulates AC called Gs if G protein inhibits AC called Gi cAMP can mediate diverse cellular responses cAMP can activate certain ion channels but most cAMP effects are through PKA  Cyclic AMP Second Messenger System Drug Adenylate Receptor cyclase G protein CELL’S RESPONSE The sequence of events by which Receptor binding activates cAMP signalling: 1. Initially Gα has bound GDP, and α, β, & γ subunits are complexed together. 2. Activation of a 7-helix receptor (GPCR ) causes a conformational change that is transmitted to the G protein. The nucleotide-binding site on Ga becomes more accessible to the cytosol, where [GTP] > [GDP]. Ga releases GDP & binds GTP (GDP-GTP exchange). 3. Substitution of GTP for GDP causes another conformational change in Ga. Ga-GTP dissociates from the inhibitory βγ complex & can now bind to and activate AC. 4. AC utilizes ATP resulting in cAMP and PPI 5. cAMP activates Protein Kinase A; catalyzes phosphorylation of various cellular proteins, altering their activity.  How does cAMP affect cellular processes? cAMP signalling involves phosphorylation of proteins activates cAMP-dependent protein kinase (protein kinase A, PKA) PKA Protein Protein P ATP ADP PKA can phosphorylate many different proteins PKA can activate enzymes or gene regulatory proteins  The activation of cAMP dependent protein kinase (PKA)  cAMP Responsive Element (CRE), CRE Binding Protein (CREB), and the CREB-binding protein (CBP) Hormone (First messenger) Hormone Receptor (GPCR ) Gαs (Relay Protein) Adenylate Cyclase (Amplifier) cAMP (Second messenger) PKA CREB CBP  Heart Contraction 1) β1 receptor linked via Gs protein – increased [cAMP], leads to stronger contraction (L-type Ca2+ channel activation via cAMP) 2) acetylcholine receptor linked via Gi protein – decreased [cAMP], leads to weaker contraction BUT also reduces heart rate** How does a Gi work? Gi limits AC activation and the βγ may activate K+ channels. Slows conduction via the AV node. The Fight or Flight Response Adrenaline (epinephrine) stimulates glycogen breakdown and inhibits glycogen synthesis via cAMP Glycogen: glucose polymer stored primarily in liver and muscle. Major carbohydrate store in body. Adrenaline plays major role in synthesis and degradation regulation. e.g. in muscle adrenaline, induces glycogen breakdown and inhibits glycogen synthesis, ultimately increasing available glucose for muscle cell. Binding of adrenaline to its receptor results in PKA activation. PKA phosphorylates glycogen phosphorylase kinase which catalyse glycogen-glucose PKA phosphorylates glycogen synthase to inactive state PKA can inactivate phosphoprotein phosphatase-1 As a consequence: [adrenaline] at 10-10M increase glucose from 5-8 mM Reversed when [adrenaline] falls. TERMINATION OF ADEN YLATE CYCLASE SIGNALS cAMP must be degraded rapidly so signal does not linger enzyme phosphodiesterase breaks cAMP to AMP phosphodiesterase cAMP AMP  How does coffee perk you up? active component is caffeine caffeine inhibits phosphodiesterase so cAMP levels remain high adrenalin’s effect on heart is prolonged – increasing heart rate & force – more O2 to brain & tissues gives a feeling of increased vitality and energy – a moderate “fight or flight” rush  Protein Phosphatases Make the effects of PKA and other Protein Kinases Transitory  Additional Information. Rang, Dale, Ritter and Moore. Chapter 3