Lecture 12- GPCRs Flashcards
What is signal transduction?
Receptors needed for cells to respond to extracellular signalling molecules (hormones, NT, GF)
receptors can be
intramembrane (majority) or intracellular
how many types of GPCR identified in human genome
> 800 (>2% of identified genes)
structure of GPCRs
- Single peptide sequence (300-1200 amino acids)
- 7 transmembrane structures
- N terminal outside the cell
- C terminal faces inside the cell
N terminal
outside the cell- ligand binding domain
c terminal
faces inside the cell- Gprotein
where do ligands bind on GOCR
two different sites:
- binding site on N-terminal region
- bidding site in transmembrane
each receptor subtype is specific for
one chemical ligand
ligand binding activates the receptor which directly or indirectly
brings about change in the cellular activity
percentage of prescription drugs that exert their therapeutic effects directly or indirectly at the GPCRs
40%
agonists
bind and activate
• Leading to intracellular signal transduction
example of a GPCR agonist
salbutamol (B adrenoreceptor agonists)
morphine (u-opioid receptor agonist)
antagonists
bind and block the effect of agonist on the receptor
example of a GPCR antagonist
cardiovascular (hypertension) B-adrenoreceptorsantagonists
what can GPCRs respond to
1) Sensory GPCRs- light, odours and taste
2) Ions
3) NT
4) Peptide (glucagon, insulin, adrenaline) and non-peptide hormones
5) Large glycoproteins- TSH
G protein is made up of three different proteins
Alpha and Beta/Gamma(single unit)
activation of the GPCR facilities activation of G protein on the inside of the membrane..
- When adrenaline binds the GPCR changes its conformation- initiation of cascade
- Conformational change attracts G protein, which is activated
a. Alpha Part of G-protein bound to GDP - G protein activated- exchanges GDP for GTP
- Once GTP is bound to alpha subunit it dissociates from the beta and gamma subunit- leaving a free alpha-GTP and free B/Y subunit which both have effector role
Termination of G-protein signalling
- GTPase hydrolyses the GTP that’s bound to the alpha submit releasing Pi returning alpha to Alpha-GDP state- no effector function
- Alpha-GDP ready to be reactivated by GPCR
Where is the G protein (Alpha and beta-gamma subunit) found
inserted in the cell membrane uses plasma membrane as a monorail to move around in the plane of the membrane
types of G protein
GalphaS
GalphaI
GalphaQ
human genome encodes how many Ga proteins
20
human genome encodes how many GB proteins
5
human genome encodes how many GY proteins
12+
how many Ga-BY combinations
over 1000
G protein diversity means that
an extracellular signal, working via a specific GPCR, will activate a single, or small sub-population of G proteins and effectors in the cell to bring about a specific cellular response.
alpha1 receptor
Q
- stimulate phospholipase C
- increase IP3 and DAG
alpha 2 receptor
I
- decrease adenylyl cyclase activity
- decrease cAMP
beta 1 receptor
S
- increase adenylyl cyclase activity
- increase cAMP
beta 2 receptor
S
- increase adenylyl cyclase activity
- increase cAMP
M1 receptor
Q
- stimulate phospholipase C
- increase IP3 and DAG
M2 receptor
I
- decrease adenylyl cyclase activity
- decrease cAMP
M3 receptor
Q
- stimulate phospholipase C
- increase IP3 and DAG
M4 receptor
Q
- stimulate phospholipase C
- increase IP3 and DAG
interfering with G-protein function
target the GTPase enzyme
which toxins are used to study GPCR-G- protein signaling
vibrio cholera
pertussis toxin (Ptx)
vibrio cholera
covalent modification of αs subunit
Stops the αs from turning itself off
• Can undergo GDP for GTP exchange
• Can signal
• But cannot turn itself off via GTPase
Causes sustained αs mediated signal- causing overactivation of adenylyl cylase- increase in [cAMP]
High [cAMP] opens water channels- releasing water and electrolytes into the lumen of the gut- diarrhea and dehydration
Pertussis toxin (hooping cough)
o Toxin covalently modifies αi subunit
Prevents alpha subunit releasing GDP for GTP exchange
