Lecture 27 Flashcards
GPCRs
- largest family of cell surface receptors
- can bind to hormones, local mediators and neurotransmitters
- composed of a single polypeptide chain that spans the lipid bilayer 7 times
- ancient, can be found in prokaryotes
activation of a GPCR
- binding of an extracellular signal molecule to a GCPR causes it to change conformation causing the associated G-protein to decrease affinity for GDP
- GDP dissociates and is exchanged for GTP
- sometimes g-protein subunits dissociate (this doesn’t always happen) and are switched on
- this in turns activates trimeric G-protein which results in transmission of a signal
- g-protein then interact with target enzymes or ion channels
describe the structure of a G-protein
- composed of a, y, b subunits
- a and y are tethered to the plasma membrane by a short lipid tail
- when unstimulated, a is bound to GDP
what dictates the length of the response
- the amount of time the g-protein subunits are switched on
explain the switching off process
- the a subunit contain a GTPase activity which can hydrolyze GTP to for GDP
- the a subunits reassembles with the by complex and the protein returns to its original, inactive state
G-proteins and ion channels
Example: regulation of the heart
- acetylcholine binding to GCPRs of heart pacemaker cells activate G-protein Gi
- then the by subunit binds to K+ ion channels causing it to open
- channel closes when GTP is cleaved and the subunits reassembles with one another
the two most common enzyme targets include
- adenylyl cyclase which produces cAMP
- phosphlipase C which produces inositol triphosphate (IP3) and diacylglycerol (DAG)
Give examples of second messengers
- cAMP
- IP3
- DAG
cAMP signalling pathway
- the g-protein Gs is responsible for the activation of adenylyl cyclase which generates cAMP from ATP, releasing PPi
- cAMP phosphodiesterase converts cAMP into AMP using water
what role does caffeine play in cAMP signalling pathway
- it inhibit cAMP phosphodiesterase keeping cAMP levels high
cAMP- glycogen breakdown
- cAMP activate protein kinase A (PKA)
- PKA is normally inactivated by regulatory proteins
- when cAMP binds to PKA it releases regulatory proteins
- PKA can then phosphorylate other proteins (like glycogen phosphorylase in skeletal muscle)
- glycogen breakdown is an example of a relatively quick response
cAMP slow responses
- cAMP can also cause activation of gene expression - a relatively slow response
- PKA phosphorylates transcriptional regulators which can initiate transcription
what is cholera toxin
- a bacterial toxin that modifies the a subunit of the g-protein so that it can no longer hydrolyze GTP therefore the g-protein is always active
- result: in the intestine this can result in a prolonged outflow of Cl- and water (dehydration)
Inositol phospholipid pathway
- phospholipase C converts membrane phospholipids into diacylglycerol (DAG) which stays in the membrane and IP3 which is released into the cytosol
- IP3 binds to and opens Ca2+ channels embedded in the ER and free Ca2+ is released into the cytosol
- DAG recruits protein kinase C (PKC)
- activation of PKC requires the binding of Ca2+
- PKC phosphorylates many intracellular protein
Ca2+ signalling
- it is important in muscle contraction and secretion
- when Ca2+ channels open, Ca2+ rushes down its electrochemical gradient
What is the most common protein Ca2+ binds to
calmodulin
Calmodulin
- binds to four Ca2+ ions, inducing a conformational change allowing it to interact with proteins like Ca2+ / calmodulin dependent-protein kinases (CaM-kinases)
GPCR signalling- photoreceptors
- light activates GCPR known as rhodopsin
- rhodopsin activates g-protein known as transducin
- transducin activates cyclic GMP phosphodiesterase which reduces cGMP levels by converting it to GMP
- signalling is very fast despite the multiple steps
is vision possible in dim light? why?
- yes because photoreceptors can amplify signal so that vision is possible even in dim light
What does it mean when we say “photoreceptors are capable of adaption?”
- the adjustment of sensitivity
- allows you to see in bright sunlight
- strong light reduces Ca2+ concentrations which is required by signal amplification enzyme
