metabotropic receptors Flashcards
what time frame do G-protein coupled receptors work in?
seconds
what is a G-protein coupled receptor?
when a ligand binds to them they signal by activating G-proteins in the cell
G-proteins activate second messenger pathways
G-proteins lead to downstream effects of receptor
what is an example of G-protein coupled receptors?
muscarinic ACh receptors
GPCR structure
- 7 transmembrane (TM) domains
- 3 intracellular, 3 extracellular loops
- N-terminus extracellular
- intracellular phosphorylation sites
- 3rd intracellular loop & C-terminus
interacts with G-protein - ligand binding in TM domains or
N-terminal domain - most are monomeric, some dimeric
example of a sensory GPCR
rhodopsin
examples of neurotransmitter GPCRs
cholinergic
dopaminergic
GABAb
G-protein = GTPAse
when G-protien binds to GDP it is inactive (off state) however with stimulus it binds to GTP and forms active (on state) G-protien to regulate ion channels
because of intrisnic GTP hydrolysis activity it will hydrolyse its GTP into GDP (off state)
heterotrimeric (3 subunits, alpha beta and gamma) G-proteins
G-proteins that couple to GPCRs, the alpha subunit has GTPase activity, not the beta or gamma
the role of heterotrimeric G proteins in signal transduction
information transfer= signal to receptor to G protein to effectors
- ligand bind to receptor which causes conformational change in rceeptor which increases its affinity to G-protein so it can bind
- also conformational change in G-protein so GDP is exhanged for GTP (active in GTP-bound state)
- alpha subunit and beta-gamma subunit diffuse apart and independently activate intracellular signalling pathways
alpha subunit in GTP-bound active state activate downstream pathways as well as beta-gamma
over time alpha subunit hydrolyses GTP to GDP and lose affinity for target
alpha and beta-gamma subunits come back together to form trimeric G-protein
resting state in heterotrimeric G proteins
alpha subunit is bound to GDP (off state)
target proteins also not being regulated
what are 4 examples of effector systems for G proteins?
adenylate cyclase
phospholipase C
potassium channels
calcium channels
role of adenylate cyclase
catalyses the production of second messenger cyclic AMP (cAMP)
role of phopholipase C
catalyses the production of IP3 and diacyl glycerol (DAG) from PIP2 (second messengers)
role of potassium channels
G-proteins regulate membrane potential
role of calcium channels
G-proteins allow calcium ions to enter the neuron (increase intracellular calcium)
adenylate cyclase activity
- neurotransmitter binds to receptor
- activates G protein (active alpha subunit)
- activates enzyme adenylate cyclase which uses ATP to produce cAMP
- produces second messenger with cAMP
- cAMP activates protein kinase A (phosphorylates other proteins)
- protein kinase A phosphorylates K+ channel
- causes reduction in potasssium influx
phospholipase C activity
- alpha subunit in GTP-bound state regulates phospholipase C
- phospholipase C cleaves a membranre phosopholipid called PIP2 into IP3 (cytosolic) and DAG (stays in plasma membrane)
- these products are important second messengers that activates certain pathways
- IP3 binds to calcium channels on the surface of ER= calcium flows out into cytosol which can activate calcium-sensitive proteins (e.g protein kinase C that phosphorylates multiple downstream targets)
- DAG also has to bind to protein kinase C to avtivate it as well as increase in calcium concentration
ion channel activity
active alpha subunit binds to receptors so regulates ion activity
alteration in electrical properties of the cell
advantages of G proteins in signal transduction
- simple amplification
- signal diversification
signal amplification
a single receptor may activate several alpha-subunits (activate lots of G-proteins)
- depends on factors such as how long ligand remains bound
signal diversification
- receptors can interact with more than one type of G-protein
(there are >20 different variants of alpha-subunit which all have different targets) - G-proteins can regulate more than one effector
-eg. Gi and Go can inhibit adenylate cyclase, open K+ channels and close Ca2+-channels - both alpha and beta-gamma subunits can regulate target proteins (downstream of the effector you get multiple effects)
examples of intracellular second messengers
3’, 5’-cylic AMP
calcium ions
1,2-diacylglycerol (DAG)
inositol 1,4,5-trsphosphate (IP3)
3’,5’-Cyclic AMP (cAMP)
receptor= protein kinase A
action= protein phosphorylation
calcium ion
receptor= calmodulin and other calcium binding proteins
many actions like activating calcium dependent kinases
1,2-Diacylglycerol (DAG)
receptor= protein kinase C
action= protein phosphorylation and activating protein kinase C
inositol 1,4,5-trisphosphate (IP3
receptor= Ca-release channel (IP3 receptor)
action= calcium release
presynaptic neuromodulation- intracellular signalling following GPCR
facilitator neuron regulates how excitable a presynapse is
1. neuron releases 5HT, DA, Ach,
peptides, etc
2. GPCR activates effector system on postsynaptic membrane
3. increased production of 2nd
messenger from active G proteins (e.g adenylate cyclase to cAMP)
4. intracellular “receptor” (eg protein
kinase which inhibits potassium channels)
5. phosphorylates K+ channel to close it
6. VSCC allow Ca2+ into the presynaptic neuron (sense depolarisation) which primes release of neurotransmitter as it is already excitable
7. glutamate is released
8. glutamate receptors depolarise postsynaptic neuron
essentially through GPCR signalling you are effecting how active the presynapse is (more)
postsynaptic modulation- intracellular signalling following GCPR (e.g cerebellar long term depression)
- presynaptic neuron releases glutamate
- mGluR (GPCR) activates effector system (phospholipase C)
- increased production of 2nd messengers
(DAG and IP3) - intracellular receptors (protein Kinase C and IP3
receptor) - PKC phosphorylates AMPA receptor
- IP3 receptor mediates Ca2+ release
- AMPAR phosphorylation plus other Ca2+- dependent processes cause AMPAR endocytosis (changing how synapse responds in future by removeing AMPAR receptors from postsynaptic membrane)
- this reduces synaptic strength
GPCR heterodimers
orginally dimers were thought to be homodimers (2 copies of same protein subunit)
however those working on metabotropic GABAb receptors found them to be heterodimers (2 different GPCR that come together to form the receptor) as a subunit on its own it doesnt work
this means there is a wide variety of heterodimeric G protein couples receptors in the brain e.g glutamate and serotonin subunit so binds to 2 different ligands
- activation of heterodimer causes a different effect than its two subunits=expands receptor diversity signalling
