10. GPCRs Flashcards
how many TMS alpha helices in GPCRs
7
how many extra- and intracellular domain
what is each used for?
4 extracellular domains
- used to interact with stimulus
4 intracellular domains
- used to interact with G-proteins
GPCRs have an 8th alpha helix, where is this located
at the c-terminal end on the cytosolic side of the plasma membrane
what is the 8th alpa helix (H8) covalently bound to
a membrane phospholipid called palmitic acid
name 3 functions mediated by GPCRs
hormone action
hormone secretion
cell growth
what % of medicines exert their effects by influencing G-protein pathways
60%
give 2 examples of GPCRs
beta-adrenergic receptor
alpha adrenergic receptor
what hormone do beta-adrenergic receptors respond to
epinephrine (fight or flight hormone)
describe the structure of beta-adrenergic GPCRs
7 alpha helices that form a barrel structure
where on beta-adrenergic receptors does epinephrine bind
in the centre of the barrel
what happens when epinephrine binds to beta-adrenergic GPCRs
interacts with 15 specific amino acids on the TMS helices
= induces a conformational change - allows the receptor to activate the G-protein
the G-proteins involved in the beta-adrenergic pathway are what?
heterotrimeric
what does heterotrimeric mean?
composed of 3 different subunits: alpha, beta and gamma
which subunit on a g-protein can bind GDP or GTP
alpha subunit
what is the name of the GTP/GDP binding site on g-proteins
guanyl nucleotide binding site
describe the G-protein cycle
- inactive form = GDP is bound to the guanyl binding site
- epinephrine binds = conformational change - activating the G-protein
- GDP exchanged for GTP
- alpha subunit bound to GTP dissociates and carries out downstream signalling
- to switch off signalling GTP is hydrolysed back into GDP and alpha subunit recombines
- cycle continues
what does the alpha subunit-GTP complex bind to
adenylate cyclase
describe the structure of adenylate cyclase
12 TMS alpha helices
2 domains: 6 helices in each
each domain has a catalytic domain, what does this do?
converts ATP into cAMP
what does cAMP activate
protein kinase A
what does protein kinase A do?
increase lipid hydrolysis
decrease glycogen synthesis
protein kinase has 4 subunits,, what does each do?
2 catalytic subunits
2 regulatory subunits
what happens when cAMP binds to kinase A
cAMP binds to regulatory binding site causing conformational change - removes pseudo substrate from catalytic domains
= activates the protein kinase to phosphorylate downstream signalling intermediates
what do all g-protein alpha subunits have
intrinsic GTPase activity
what occurs as soon as GTP binds to guanyl nucleotide binding sites
hydrolysis immediately occurs by intrinsic GTPase activity
signal termination can occur in 3 ways, what are these?
- dissociation
- phosphorylation by GPCR kinase
- endocytosis
describe how signal termination can occur by phosphorylation by GPCR kinase
kinase enzymes can phosphorylate the cytosolic tail of the receptor
allows proteins called arrestin to bind, preventing the GPCR interacting with the G protein
how do inhibitory hormones inhibit g-proteins
inhibits adenylate cyclase = so no cAMP is produced
- downstream signalling is reduced
what do alpha-adrenergic receptors do
lead to activation of phospholipase C by the activated G-protein
what does phospholipase C do
hydrolyses PIP2 in half
this produces DAG and IP3
where is DAG produced from cleavage
in the membrane
where is IP3 produced from cleavage
in the cytosol
what does DAG do
activates protein kinase C = phosphorylation cascade
what does IP3 do?
releases calcium from intracellular stores
what can calcium do
act as a second messenger
