GPCRs Flashcards
What are the 4 types of cell signalling?
Paracrine signalling
Autocrine signalling
Endocrine signalling
Direct Contact
What are autocrine signals?
Hormones produced by a cell that will act on itself
Paracrine signals
produced by a cell and target nearby cells
Endocrine cells
go to target cells that are further away
Give examples of common endocrine signalling
- hormones secreted into bloodstream
- cytokines released at injury site
What properties can ligands (signalling molecules) possess?
Hydrophobicity (repel water)
Hydrophilicity (stays in water)
How are hydrophobic signalling molecules brought into cell?
They are brought into target cell via carrier proteins as they cannot freely float in Extracellular space
What happens to hydrophobic signalling molecules when they a re inside the cell?
they can diffuse across CSM and bind receptor proteins in target cell (either nucleus or cytoplasm)
How do hydrophilic signalling molecules act once inside the cell?
- they are hydrophilic - can float freely in extracellular space
- however, they are unable to cross CSM - therefore they must bind to receptors called transmembrane receptors
Describe the structure of transmembrane proteins?
extracellular end: binds to ligand
intracellular end: triggers signalling pathway inside the cell once ligand is bound
What are the three stages of cell signalling?
- reception - receptors bind to the ligand
- transduction - receptor protein undergoes conformational change to activate 2nd messenger (intracellular molecule)
- cell response - cell’s response to signal
What are the main classes of receptors?
1) GPCRs
2) enzyme coupled receptors
3) ion channel receptors
What are GPCRs?
7-pass transmembrane receptors which are coupled with intracellular end called guanine-binding nucleotide protein (G-protein)
Describe the structure of G proteins?
- α, β, γ subunits
- α and γ subunits are anchored to CSM
- they keep the G protein right next to the receptor
- when G protein is inactive it is bound to GDP - 3 subunits stay together
- when G protein is active it is bound to GTP
What happens when the G protein activates?
- A signalling molecular binds to GPCR
- GDP dissociates from α-subunit
- GTP binds to α-subunit instead
- the α-subunit and GTP dissociate from β and β, γ subunits (still bound to GPCR)
- α-subunit is free to inhibit and stimulate other proteins inside the cell
- α-subunit turns GTP into GDP
- 3 subunits associate again
- G protein switches off
What are the 3 types of G proteins?
Gs, Gi, Gq
What happens when Gq proteins are activated?
- They activate phospholipase C in CSM
- Phospholipase C cleaves Phosphatidylinositol 4,5-bisphosphate (PIP2) (lipid) → inositol triphosphate (IP3) and DAG (diacylglycerol)
What effect does inositol triphosphate (IP3) have in the cell?
- by diffusing into ER
- This opens Ca2+ channels
- in ER there is ↑ [Ca2+]
- Ca2+ moves out of ER and into cytoplasm via Ca2+ ATPase pumps on ER membrane
- cytoplasm electrical charge of cytoplasm changes → depolarisation occurs
What effect does DAG have on the cell?
- DAG is attached to CSM,
- binds to protein kinase C (PKC),
- PKC phosphorylates target proteins (adds phosphoryl group groups to them)
- this occurs when ↑[Ca2+]i due to activity of IP3
What effector protein does Gs protein activate?
stimulates enzyme adenylyl cyclase
What happens when adenylyl cyclase is activated?
this converts Adenosine Triphosphate (ATP) into Cyclic Adenosine Monophosphate (cAMP) by removing 2 phosphate molecules
What effect does cAMP have on the cell?
- cAMP binds to regulatory subunit of enzymes protein kinase A (PKA)
- (pKA has 2 subunits: regulatory and catalytic)
- cAMP binds to the regulatory subunit of PKA
- this allows the catalytic subunit to dissociate and is free to phosphorylate target proteins to trigger cellular response
How do Gi proteins work?
- Also acts on Adenylate cyclase however has an inhibitory effect
- negative feedback of Gs protein
- useful when inactivating cells
↓ adenylyl cyclase, which catalyses the conversion of ATP to cyclic AMP
cAMP doesn’t act on PKA
this doesn’t phosphorylate other cellular proteins
What are enzyme-coupled receptors?
consist 2 domains
receptor: extracellular domains which has receptor activity
enzyme: intracellular domain which has intrinsic enzyme activity
the enzyme domain is usually protein kinase which phosphorylates the receptor
what are 3 types of enzyme coupled receptors?
- receptor tyrosine kinases
- tyrosine kinase-associated receptors - NO INTRINSIC ENZYME ACTIVITY
- receptor serine/threonine kinase
How do ion channel receptors work to trigger a cellular response?
- allows ions to flow down a gradient to change electrical charge inside the cell triggering a cellular response
What happens once G protein is activated (8 marks)
Upon prostanoid binding, a GDP-bound inactive Gsα,β,γ complex is recruited to the agonist-bound IP receptor
This interaction catalyses GDP release from the Gsα subunit
allowing GTP to bind
and activate Gs
Active GTP-bound Gsα dissociates from Gβγ
GTP-bound Gsα then binds
and activates an adenylyl cyclase effector protein to
to generate cyclic AMP from ATP
What protein does cAMP activate and what effect does this have on muscle contraction? (8 marks)
cyclic AMP activates cyclic AMP-dependent protein kinase (PKA)
a multifunctional Ser/Thr-directed protein kinase
PKA substrate called MYPT = myosin light chain phosphatase becomes activated
MYPT dephosphorylates myosin light chain (MLC), reducing its ability to interact with actin
PKA also phosphorylates and inhibits myosin light chain kinase (MLCK)
which phosphorylates and
activates MLC
These two events combine to inhibit smooth muscle cell contraction
give examples of second messengers
cyclic AMP
inositol triphosphate (IP3)
diacylglycerol (DAG)
what is the structure enzyme-coupled receptors?
- extracellular ligand binding domain
- intracellular catalytic domain which increases catalytic activity of receptor
Summarise Gs activation?
Gs activates adenylate cyclase → adenylate cyclase removes phosphate from ATP to produce cyclic AMP → cAMP bidns to regulatory subunit of protein kinase A → causes catalytic subunit to dissociate and phosphorylate target proteins
Summarise Gq activation
Gq activates Phospholipase C → this cleaves phosphatidylinositol 4,5 bisphosphate (PIP2) into inositol triphosphate (IP3) and diacylglyceral (DAG) (2nd messengers):
IP3: acts on ER to open Ca2+ channels and ↑ [Ca2+] in cytoplasm, this changes the charge → depolarisation occurs
DAG: in CSM binds to protein kinase C which phosphorylates target proteins
Summarise receptor tyrosine kinase activity
when ligand binds → they phosphorylate their own tyrosine residues → conformational change creates binding site for other signalling proteins
