L14 Flashcards
AC is inhibited and stimulated via G protein
True
what subunit is different in G protein that stimulates AC
alpha s – stimulates AC
alpha i – inhibits AC
what receptors are coupled to Gs proteins
b-adrenoceptors
what receptors are coupled to Gi proteins
a2 adrenoceptors
how does the cholera toxin work
Signalling via G proteins depends upon exchange of GDP for GTP
Active a subunit has GTP bound
Hydrolysis of GTP leads to inactivation
Cholera toxin (CTx) acts on as subunit and causes ADP-ribosylation
This prevents hydrolysis of GTP
Causes persistent activation of a subunit
In colon causes activation of PKA-dependent Cl- channels and “secretory diarrhoea”
what is lost from the body in cholera
Na
Cl
water
how does Pertusiss toxin work
Pertussis toxin acts on ai subunits
In this case locks subunit into inactive configuration
Prevents activation by receptors
Prevents inhibitory control over AC / PKA
Again leading to increased levels of cAMP and PKA
In airway leads to symptoms of whooping cough
what are the features of Gq
G proteins containing aq11 subunits allow hormones / neurotransmitters to activate amplifier enzyme Phospholipase C (PLC)
Underlies autonomic effects of acetylcholine (eg salivary secretion, bronchial smooth muscle contraction)
Histamine H1 receptor responses (G.I. smooth muscle contraction, allergies etc.)
Responses due to increased internal Ca2+
ACh also activates Muscarinic receptor “metabotropic receptor”
true
muscarinic receptors dont bind nicotine
False
Muscarinic receptors are Gq/Gi coupled receptors
true
what muscarinic receptors are Gq coupled
1,3, and 5
what muscarinic receptors are Gi coupled
2 and 4
Gq proteins second messengers are products of phosphoinositide breakdown
True
how does Gq work
Gq proteins stimulate phospholipase C (PLC)
PLC cleaves PIP2,a membrane phospholipid, into inositol 1,4,5-triphosphate (IP3) and diacylglycerol (DAG)
IP3 is the water soluble part of PIP2 and travels through the cytosol to stimulate
Calcium release from ER
DAG remains in the membrane (hydrophobic part) where it recruits Protein Kinase C (PKC)
IP3 is a second messenger that stimulates Calcium release from ER
True
how does IP3 stimulates Calcium release from ER
Since IP3 is hydrophilic it enters the cytoplasm
Binds to receptors on ER and promotes release of stored Ca2+
Also promotes Ca2+ influx from extracellular fluid by a mechanism that is less clear
Resultant increase in intracellular free Ca2+ promotes cellular responses
what are the features of calcium mediated Intracellular responses
Mediated by Ca2+ -binding proteins
Calmodullin (CaM) is the most important
Each CaM binds 4 Ca2+ ions
Ca2+-CaM complex activates PDE (the enzyme that degrades cAMP)
Importantly, Ca2+-CaM complex activates
CaM kinases (CaMKs)
CaMKs phosphorylate Serine and Threonine residues on a number of substrate proteins
CaMKs are involved in smooth-muscle contraction.
α1 adreno-receptor is a Gq coupled protein receptor which mediates vascular smooth muscle contraction by increasing intracellular free Ca2+ activating CAMKs (vasoconstriction)
what are the effects of DAG
DAG is hydrophobic and so remains in the plasma membrane
Presence of DAG increases the activity of Ca2+-dependent protein kinase
Evokes cellular responses by phosphorylating other proteins. The most important one is proteins kinase C (PKC)
PKCs can potentiate the effects of IP3
Mediates desensitization (cf b-ARK)
Regulates cell shape, cell proliferation and transcription factor activity
PKC also activate proteins regulating cell shape, cell proliferation and transcription factors
what are the effects of a1-adrenoceptor
on blood pressure
It causes vascular smooth muscle tone contraction (‘vasoconstriction’)
via Gq-PLC-IP3-CaMK
Makes blood pressure to increase
what are the effects of beta 2-adrenoceptor
on blood pressure
Cause relaxation of vascular smooth muscle (‘vasodilation’)
via Gs-cAMP-PKA
Makes blood pressure to decrease
just adrenergic receptors can cause smooth muscle contraction
false
what are the features of H1 histamine receptor
A Gq protein coupled pathways important in Asthma as it causes smooth muscle contraction in the airways (broncospasm)
give 2 receptors that are linked to a Gi protein
alpha 2 adrenergic receptor
M2 muscarinic receptor
what are the types of enzyme linked receptors
Receptor guanylyl cyclases
Receptor serine/threonine kinase
Receptor tyrosine-kinase
Tyrosine kinase-associated receptors
Receptor tyrosine phosphatase
describe Receptor Guanylyl Cyclase
Contain 2 Guanylyl Cyclase domains which convert GTP to cGMP. cGMP activates downstream kinases
what is the Mechanism of signalling of Receptor Guanylyl Cyclase
- Binding of ANP induces a conformational change in the receptor that causes receptor dimerization and activation.
- The Guanylyl cyclase activity of the receptor generates cGMP.
- Increased concentrations of cGMP activates other signalling molecules determining the response
- Example of response: Atrial natriuretic peptide (ANP) relax vascular smooth muscle and dilate blood vessels (vasodilation)
describe Receptor Serine/Threonine kinases
Contain Serine-Threonine kinase domains which phosphorylate target proteins (similar to PKA)
what is the Mechanism of signalling of Receptor Serine/Threonine kinases
- first messenger binds to receptor Type II
- Receptor Type I then binds forming a ternary complex with Type II and first messenger.
- Type II receptor phosphorylates Type I, activating the Ser-Thr Kinase activity of Type I
- Type I then phosphorylate target proteins (re SMAD proteins)
- Example of response: TGFbeta mediated cell proliferation
what do SMAD proteins do
regulate transcription of target genes
describe Receptor Tyrosine Kinases (RTKs)
Contain tyrosine kinase domains which phosphorylate themselves/other proteins.
what is the mechanisms of signaling of Receptor Tyrosine Kinases (RTKs)
- Binding of 2 molecules of insulin causes the receptor to dimerise.
- Once the receptors dimerise, then they use their cytoplasmic tyr kinase activity to phosphorylate each-other at multiple tyrosine residues creating “phosphotyrosine motifs”.
- These motifs recruit intracellular signalling molecules leading to the response
- Example of response: insulin mediated glucose uptake and storage in liver and muscles
give an example of RTKs signal transduction
MAP kinase signalling pathway
what happens in MAP kinase signalling pathway
RTKsactivateRas
Anadaptorproteindocksonaparticularphosphotyrosineontheactivatedreceptor
TheadaptorrecruitsaRasguaninenucleotideexchangefactor(Ras-GEF)thatstimulatesRastoexchangeitsboundGDPforGTP
TheactivatedRasproteincannowstimulateseveraldownstreamsignalingpathways
RasactivatesMAP-kinase kinase kinase
MAP-kinase kinase kinaseactivates MAP-kinase kinase
MAP-kinase kinase activates MAP-kinase
MAP-kinase can change protein activity or gene expression
how is Ras anchored to the inside of the plasma membrane
Rasproteincontainsacovalentlyattachedlipidgroup
describe Tyrosine kinase-associated receptors
The receptors do not contain kinase domains. Instead, tyrosine kinase proteins are associated non-covalently with the cytoplasmic domains
what is the mechanism of signaling of Tyrosine kinase-associated receptors
1) Binding of first messenger to the receptor, induce a conformational change that cause dimerization of the receptor
2) The dimerization causes activation of the associated Tyr kinases (E.G. JAK2).
3) These kinases then phosphorylate tyrosine residues on both themselves and the receptor, creating “phosphotyrosine motifs” (Like the Tyr-Kinase receptors).
4) These motifs recruit intracellular signalling molecules leading to the response (re STAT proteins)
5) Example of response: cytokines signalling pathways (IL-6 acute phase response)
describe Receptor tyrosine phosphatase
the receptors contain tyrosine phosphatase domains and remove Tyr residues to deposphorylate target proteins (For instance Tyr residues created by Tyr kinase receptors signaling)
what is the mechanism of signaling in Receptor tyrosine phosphatase
- 1st messenger binding to the receptor inducing a conformational change that activates the Tyr phosphatase activity of the receptor.
- Target proteins are dephosphorylated by Tyr phosphatase activity
- This causes phosphorylation of downstream proteins (for example Lck and Fyn)
- Response example: CD45 induces the maturation of lymphocytes via binding to this receptor
which receptors do not contain intrinsic enzymatic activity
Tyrosine kinase associated receptor
which receptors involve dephosphorylation of target protein rather than phosphorylation
receptor tyrosine phosphatase
what receptor increase cGMP
receptor guanylyl cyclase
