G-protein coupled receptors Flashcards
GPCRs
Largest famiky of related proteins known to exist.
Structure conserved thoughout eukaryotes.
Bind diverse stimuli, have many functions.
>30% prescribed drugs act on GPCRs.
Common core domain of 7 trasmembrane alpha helices connected by 3 intracellular and 3 extracellular loops.
Have an extracellular N terminus and intracellular C terminus.
Intracellular loops are involved in G-protein coupling and regulation of signalling.
Helices form a cavity which is largely non-polar with a few critical polar residues which are responsible for Specific high affinity ligand interactions.
There are cysteine residues on extracellular loops 1 & 2, they from a disulphide link which is important for receptor stability and hekical packaging.
Proline residues in helices 6 & 7 introduce kinks to the alpha helices, these are pivots for the conformational change which occurs when agonist binds.
GPCR signalling pathways have 3 components
- Receptor
- G-protein
- Effector
G-protein plus effector translate agonist binding event into the generation of a second messenger or appropriate cell response.
GPCRs typically activate multiple G-proteins.
Design of GPCR signalking pathways facilitates amplification.
GPCRs can be subdivided into 5 families
Family 1 (a,b,c) Family 2 Family 3 Family 4 Family 5
Each family is distinguishable by mechanism of receptor activation by agonist.
Family 1a
Small charged molecule agonists (adrenaline).
Ligand binds in cavity in common core region.
Family 1b
Short peptide agonists (thrombin/chemokines)
Peptide ligand binds to N terminal domain and residues in extracellular loop, C terminus of peptides also bind cavity in common core.
Family 1c
Glycoprotein hormones (TSH) Ligand binds to very large N terminal domain then contacts extracellular groups to activate.
G-proteins
Control activity of effectors after activation by GPCRs.
Heterotrimeric - 3 different subjnits (alpha, beta, gamma)
Attached to the membrane by lipid modifications on alpha and gamma subunits.
Alpha subunit can bind GDP or GTP and possesses GTPase activity (critical for function)
Each G-protein is classified by the identity of the alpha subunit
Gs
Stimulatory
Increases adenylate cyclase and therefore increases Ca channel opening.
Gi
Inhibitory
Decreases adenylate cyclase, increases K channel opening
Gq
Increases phospholipase C
Gt
Increases cGMP phosphodiesterase
Activation of multiple signalling pathways
Necessary to initiate complex bilogical phenomena such as angiogenesis.
Cyclic AMP pathway
Adenylate cyclase converts ATP to cAMP. This activates cAMP dependent protein kinase (PKA). PKA adds a phosphate group to serine or threonine residues on target proteins giving phosphorylated substartes with altered activity.
PKA
Structure - two regulatory R subunits (which bind cAMP, 2 molecules per subunit) and two catalytic C subunits which are responsible for kinase activity when active.
Binding of 4 cAMP molecules releases active catalytic subunits. Positively co-operative binding enzures high sensitivity to cAMP.
PKA type 1 - located in the cytosol
PKA type 2 - tethered to specific subcellular locations via R subunit interaction with specific localisation proteins (AKAPs - A kinase anchoring proteins)
PKA-regulated function in the liver
In response to infreased cAMP by glucagon -
Phosphorylation and activation of protein kinase
Phosphorylation and inactivation of glycogen synthase.
Leads to breakdown of liver glycogen to glucose which is released into blood for tissue uptake.
