G protein coupled receptors Flashcards
Percent of genome that encodes the GPCR?
3-4% (2/3 of GCPR are oderant receptors, some are orphan)
Location of GPCR?
Plasma membrane
How generally does GPCR transmit signals?
Conformational binding of GTP and hydrolysis of the protein complex
Ligands of GPCR?
Light, cations, small molecules (amines, AA, peptides, lipids, sugars), large proteins
General functions of GPCR?
Organismal homeostasis, embryonic development, learning and memory
GPCR have what general structure?
7 pass membrane protein in trimeric form.
Function of epinephrine
Mediates response to stress by supplying energy for rapid movements of muscle. Liver and adipose liberate glucose and FA, heart increases contraction rate
What is metoprolol?
Beta blocker that blocks beta1-andregenic receptor as an antagonist to treat cardiac arrhythmia and angina.
What is albuterol?
A agonist for Beta2-adrenergic receptors in treatment of asthma by stimulating the bronchioles of the lung to open by relaxing the smooth muscles in the lining
Beta-adrenergic receptors are what type of receptor?
GPCR
Describe the mechanism of GPCR activation
GDP trimer in inactive form, signal comes in and causes release of GDP and uptake of GTP (GTP is x10 concentration in cytosol than GDP), alpha unit with GTP is activated and separates from the activated beta/gamma complex.
GPCR interact with what two nucleotides?
GTP and GDP
Describe step by step mechanism of activation of GPCR
Binding of hormone changes conformation of protein receptor, activated receptor binds to Galpha unit, conformational change in Galpha losing GDP, GTP binds to Galpha and causes dissociation from Gbg, hormone dissociates causing GTP to be hydrolyzed and returned to trimeric inactive form
Describe the 4 modes of activated GPCR action
Regulation by Galpha-ATP, Gbg, both on two separate proteins, both on same protein
Major functions mediated by GPCR
Organismal homeostasis, embryonic development, learning and memory
T or F: 90% of GPCR are expressed in the brain
T
Three types of GPCR we care about
Gs, Gi, and Gq
Effector, 2nd messenger, and example receptor of Gs
Adenylyl cyclase, cAMP increased, B-adrenergic receptor, glucagon, serotonin, vasporessin
Effector, 2nd messenger, and examples of Gi
Adenylyl cyclase, potassium channel (activates Gbg effector), cAMP decrease, change in membrane potential, alpha1-andrenergic receptor, muscarinic acetylcholine receptor
Effector, 2nd messenger, and example of Gq
Phospholipase C, IP3 and DAG increased, alpha2-adrenergic receptor
Describe action of acetylcholine on muscarinic receptor in heart and potassium channels
GPCR activated and Gbg unit activates K channel to make cytosol more negative to reduce frequency of heart beat
Describe the two major G protein signal cascades
Gs to Adenylyl cyclase to cAMP
Gq to phospholipase C-beta to IP3 to Ca+2 to PKC
to DAG to PKC
Describe the cross-talk of Gs and Gi in terms of regulation
Stimulatory hormone like epinephrine, glucagon, ACTH activate Gs to activate Adenylyl cyclase and inhibitory hormone like PGE1 and adenosine to activate GPCR to deactivate adenylyl cyclase
Cholera toxin does what?
Blocks the inactivation of Gs which leads to diarrhea and dehydration (block in hydrolysis of GTP on Galpha)
Pertussis toxin does what?
Prevents the release of GDP from Gi which prevents Gi from turning off adenylyl cyclase
cAMP activates among others, what effector?
PKA (or cAPK) - cAMP dependent protein kinase (activated by Gs for example)
PKA activates what transcription factor? What does it do?
CREB (cAMP responsive element binding protein) by phosphorylating it, recruits HATs and other coactivators to start Tx (CREB binds to CRE)
Glucagon and epinephrine do what to glycogen?
Stimulate its break-down via cAMP
Insulin is the opposite in terms of body effect (glycogenolysis, gluconeogenesis, ketogenesis, lipolysis) as ?
Glucagon, epinephrine
T or F: PKA regulates glycogen metabolism in liver and muscle
T
cAMP in liver cells leads to what?
PKA activation leading to inhibition of glycogen synthesis and stimulation of glycogen breakdown (opposite when absent)
Describe the inositol phospholipid pathway
Phospholipase C-beta takes PIP2 to diacylglycerol (DAG, which activates PKC) and inositol 1,4,5-triphosphate (IP2, which releases Ca+2 from endoplasmic reticulum)
Describe mechanism of Gq protein signal cascade
Activation by ligand of GPCR, activation of phospholipase C, PIP2 taken to DAG and IP3, DAG remains in membrane, IP3 dissociates and stimulates release of Ca ions from ER which push PKC to the membrane where it meets DAG to become activated
What is Tubby??
Acitvated by PIP2 hydrolysis, binds to DNA via nuclear translocation to regulate eating behavior (knockout leads to obesity)
T or F: Ca+2 levels in the cell are 10000 less than EC fluid
T (very rapid response as a signaling agent)
What is calmodulin?
A Ca+2 binding protein which changes conformation upon binding with Ca+2. NOT AN ENZYME. Activated form binds and alters the catalytic activity of other enzymes like CaM kinase II)
Describe the action of Calmodulin on CaM Kinase II
Activated calmodulin with calcium binds to CaM kinase II which changes conformation and allows it to autophosphorylate becomeing fully active. Loss of calcium leads to loss of calmodulin and eventual loss of phosphate via a protein phosphatase
T or F: A fully active CaM kinase II upon losing Ca and Calmodulin becomes completely inactive
F (still 50-80%) until protein phosphatase removes the P group
CaM Kinase II calcium “memory” is important for what?
Memory function of brain!
What donates the P for the autophosphorylation of activated CaM Kinase II?
ATP
Gq mechanism leading to activation of CaMKII?
Gq to phospholipase C-beta to IP3 to Ca+2 to calmodulin to CaMKII (DAG is also there, this is the most direct route)
What is calcineuerin?
A serine/threonine phosphatase which is activated by both Ca bound calmodulin and Ca. Generally insensitve to Ca flux, activated by sustained Ca only. Blocked by cyclcosporin A.
Describe the role of Calmodulin in NO muscle response
Acetylcholine binds to GPCR in endothelial cells to phospholipase C to IP3 to Citrulline and NO. Calmodulin activates NO synthase NO is released to smooth muscle and interacts with NO receptor to take GTP to cGMP to activate PKG which relaxes smooth muscle cell (Vasodilation)
What is cardiac hypertrophy?
Adaptive response to cardiac disease resulting in an increase in cel size and up-regulation of fetal cardiac genes.
What is hypertrophic cardiomyopathy?
Occurs primarily in left ventricle and is characterized by myocellular disarray and increased fibrosis. Dilated cardiomyopathy can lead to cell death
What is familial hypertrophic cardiomyopathy?
Mutation in contractile proteins in sarcomeres. Occurs in 1/500. Leads to sudden cardiac arrest in young atheletes.
Describe the role of Ca in FHC
Mutation in sarcomere proteins leads to increase in intracellular calcium in order to maintain contractility and cardiac output. Increased Ca however leads to hypertrophic response of the cardiomyocytes
T or F: Inhibition of calcineurin can reverse cardiac hypertrophy
T
Describe 4 pathways of GPCR desensitization
Loss of affinity to hormone ligand, heterologous desensitization by secondary messenger regulated kinases (decreased adenylyl cylcase activity), Activation of GPCR kinases which lead to homologous densitization by interacting with Galpha, Beta-arrestin activation leading to receptor endocytosis
Homologous vs heterologous desensitization
Homologous involves GPCR kinase interaction with Galpha binding to downsteam effectors, heterologous involves secondary messengers leading to adenylyl cyclase deactivation.
GCPR phosphorylation leads to?
Activation of B-arrestin which endocytoses the receptor
