Session 7 - GPCRs and Intracellular Signalling Flashcards
What are the 4 basic structural features that all GPCRs share?
- single polypeptide
- 7 transmembrane domains (7TM) spanning regions
- Extracellular N- terminal
- Intracellular C - terminal
Where are the 2 regions of GPCRs that can be responsible for ligand binding
- (2,3 of) the transmembrane domains
- the N-terminal
Outline the steps in the action of a GPCR protein
1) The binding of ligand to a GPCR results in a conformational change that allow the binding of the inactive G protein (Gaby)
2) This binding of the G protein results in the exchange of its GDP by GTP and the separation of the alpha and Beta Gamma subunits. G protein now active
3) Alpha and Beta Gamma subunits go on to interact with downstream effectors of GTPase hydrolysing GTP and GDP on the alpha subunit
4) GTP hydrolysis (deactivation) results in the reformation of the heterotrimeric complex bringing the G protein back to the inactive state
When describing a G protein what is meant by a heterotrimeric complex?
A G protein is made up of 3 different subunits bound together to form a complex
What are the 3 most common G protein types and what are there effectors?
G Alpha S - stimulates Adenyl Cyclase
G Alpha I - inhibits Adenyl Cyclase
G Alpha Q - stimulates Phospholipase C
Using the pneumonic outline what each receptor is linked to and their effectors
Alpha 1 - Adrenoreceptor - Q - Stimulates Phospholipase C
Alpha 2 - Adrenoreceptor - I - Inhibits Adenyl Cyclase
Beta 1 and 2 - Adrenoreceptor - S - Stimulates Adenyl Cyclase
M1 - Muscarinic AChR - Q - Stimulates Phospholipase C
M2 - Muscarinic AChR - I - Inhibits Adenyl Cyclase
M3 - Muscarinic AChR - Q - Stimulates Phospholipase C
M4 - Muscarinic AChR - I - Inhibits Adenyl Cyclase
Give 2 examples of diseases which interfere with G protein formation.
Pertussis Toxin - The toxin interacts with the alpha(i) protein and covalently bonds to it (almost all alpha(i) in the cell will be modified). This prevents the GDP to GTP exchange and as such it isn’t activated. This ,as a result of it’s inhibitory action, mean adenyl cyclase activity is raised
Cholera Toxin - This toxin binds to alpha(s) but prevents termination of signalling by alpha(s) (ie. GTP can’t be hydrolysed). This over stimulates adenyl cyclase and downstream this over stimulates chloride channel and as such faeces is very watery.
What are the extracellular, cytoplasmic and ER/SR calcium concentrations?
- Extracellular = 1-2- x 10^-3
- Cytoplasmic = 1x10^-7
- ER/SR = 2-3x10^-4 (this acts as a sort of internal Ca2+ reserve)
Changes in intracellular [Ca2+] regulate what?
- Muscle contraction
- Neurotransmission/stimulus-secretion coupling
- Fertilisation
- Cell death (apoptosis/necrosis)
- Regulation of metabolism
- Learning and memory
How are Ca2+ gradients set up?
NCX = Na+/Ca2+ exchanger PAMCA/SERCA = Ca2+ ATPase
Mechanisms that increase [Ca2+] internal
1) “influx” Ca2+ movement across the plasma membrane
- Ligand gates ion channel
- Voltage gated Ca2+ channels
2) “release” movement out of the ER/SR
- IP3 receptors (IP3R)
- Ca2+ induced Ca2+ release (CICR = ryanodine receptors)
Explain how Adenyl cyclase acts as an effector enzyme for a second messenger
1) Adenyl Cyclase converts ATP to cyclic AMP
2) This is then released into the cytoplasm where it is acts on PKA (cyclic AMP dependant protein Kinase)
3) PKA releases its catalytic subunits which phosphorylate target proteins in the cell
Describe the structure of PKA and how this allows it to carry out it’s function
PKA has a regulatory unit and 2 inactive catalytic units (inactive when bound). Upon binding of cAMP 2 catalytic units are releases which phosphorylate target proteins
Explain how phospholipase C acts as an effector enzyme for second messenger
1) Phospholipase C converts PIP2 into IP3 and DAG
2a) IP3 goes on to open IP3 receptor channels on the SR/ER which releases Ca2+ into the cytosol. Some Ca2+ binds to PKC
2b) DAG (membrane bound 2nd messenger) stimulates PKC
What is the inotropy of the heart?
The force with which the heart contracts
