Lecture 4 - Exam 1 Flashcards
(19 cards)
What are the epinephrine (adrenaline) receptors and what are some functions?
beta adrenergic, convert glycogen to glucose on liver, stimulates triacylglycerols to FAs on fat cells, relaxes smooth muscle cells in intestine, increases heart rate, alpha adrenergic cause smooth muscle lining blood vessels to contract
What do all GPCR signaling pathways have?
7 transmembrane receptor, trimeric G protein, membrane bound effect protein, off mechanism, second messenger, agonists and antagonists
What is the structure of the 7 transmembrane receptor?
7 alpha helices with NH3+ facing exterior and COO- facing cytosol and C3 and C4 interacting with the G protein
What are the trimeric G protein subunits?
alpha, beta, gamma - beta and gamma also anchored together
What is an agonist and anatagonist?
agonists promote signaling, antagonists block signaling
Is activation of beta adregernic receptors all or nothing?
No - the agonist effects are increased by the side chain present
What is the general mechanism of GTPase switch in GPCRs?
7 transmembrane receptor recruits alpha subunit of trimeric G protein and GPCR acts as GEF to change shape of alpha and release GDP causing alpha to dissociate. The free alpha interacts with the effector protein that acts as GAP to replace GTP and process starts over
What are the effector proteins that are either activated or inactivated by trimeric G proteins?
Enzymes that generate second messengers (adenyl cyclase, phospholipase C), or ion channels
Explain the muscarinic acetylcholine GPCR of the heart (Galphai) mechanism.
Acetylcholine acts as the ligand that binds and activates the muscarinic acetylcholine receptor alpha activates and changes conformation to act as GEF and exchanges GDP for GTP and alpha dissociates, Beta gamma subunit bind potassium channel (the effector protein) and open it allow potasium to run out of cell and make inside of cell more negative and slow the heart rate (hyperpolarization)
What is the only exception to a seven transmembrane receptor structure for GPCR?
Rhodopsin has opsin (the 7 transmembrane portion) and retinal
Explain the activate rhodopsin GPCR in the eye (Galphat) mechanism.
When light strikes the GPCR - rhodopsin it activates so that the alpha subunit of the trimeric G protein (transducin) can bind and act as the GEF molecule. Phosphodiesterase (PDE) is inactivated by it’s own gamma subunits and the activated subunit of transducin can bind the gamma and effectively inhibit the inhibitor (gamma) to activate PDE (the effector protein) and allow for the degredation of cGMP to close the Na/Ca ion channel and brain registers as light (hyperpolarization) - takes 50 ms and resets. In the dark cGMP is always bound to ion channel and the channel is constantly on.
How can Galphat be turned off or reset?
RGS5 and Gbeta5 can act as GAP to inactive the Galphat so alpha can not bind to gamma on PDE
Additional phosphorylation to rhodopsin by rhodopsin kinase on the tail gives less signal and arrestin binding gives no activation at all
Guanylate cyclase increases the cGMP levels (restores them)
What is the distribution of transducin and arrestin in the rod cells of the eye in the dark and light?
In the dark all G proteins (transducins) are moved to outer segment where receptor is and arrestin the inhibitor is moved out of the outer segment to give sensitivity to signaling and little inhibition, but in the light the reverse is true to give maximum inhibition
What is the structure of protein kinase A and how is it activated?
PKA has 2 catalytic and 2 regulatory subunits, cAMP binds to the regulatory units and causes them to dissociate
What does PKA do and how is it localized to certain areas?
PKA is localized by A-Kinase-Associate protein (AKAP) and activated PKA leads to phosphorylation of nuclear CREB protein transcription factor that activates many genes, also results in inhibition of glycogen synthesis while stimulating glycogen breakdown - signals amplified by second messengers
How are the mechanisms of GPCR/cAMP/PKA pathway downregulated?
When GDP is replaced by GTP the affinity of the GPCR ligand decreases
Increased GTPase activity by GAP
Heterologous desensitization - feedback repression by PKA phosphorylation of all receptors it is close to
PDE hydrolyzing cAMP
BARK phosphorylating active receptor
Beta-arrestin binding to phosphorylated sites inhibits signaling or promotes endocytosis or scaffold for other cascades
What is a calmodulin?
A cytosolic protein that mediates many effects of of calcium in the cell
How does GPCR trigger elevations in cytosolic calcium?
Hormones that bind GPCR are connected to Gao or Gaq G proteins that activate phospholipase C which cleaves membrane PIP2 to yield IP3 and DAG. IP3 triggers opening of calcium channels and cytosolic calcium rises, activating PKC that moves to plasma membrane and is activated by DAG– calcium calmodulin complex regulates many different proteins
How do you activate IP3/DAG pathway through RTK?
Phospholipase c is bound to RTK through the SH2 domain
