14 | GPCR functions Flashcards
how is adenylyl cyclase regulated
there are receptors for inhibitory hormone and stimulatory hormone (of the effector enzyme, adenylyl cyclase)
each of these receptors have their respective G-alpha-s (stimulatory) and G-alpha-i (inhibitory)
what does adenylyl cyclase do?
it is an effector enzyme that catalyzes formation of cyclic AMP (cAMP)
describe the activation of adenylyl cyclase in cholera toxin
cholera toxin enters enterocytes of small intestine and then internalizes
cholera toxin maintains G-alpha-s subunit in active state (GTP)
this leads to mass activation of Adenylyl Cyclase, which then results in a massive increase in protein cAMP levels
this leads to increase levels of hyperactivity CFTR (cystic fibrosis transmembrane conductance regulator) ion channel
when activated, Cl- pumped out & Na+ follows → H2O loss to intestine due to osmotic pressure (H2O leaves with the ions)
overall result = massive diarrhea
describe the activation of adenylyl cyclase in pertussis toxin
Bordetella pertussis toxin enters ciliated epithelial cells in lungs
pertussis toxin maintains G-alpha-i subunit in inactive state (GDP) – so only G-alpha-s is active
leads to mass activation of Adenylyl Cyclase and subsequent mass increase in cAMP levels
which leads to increase activity in ion pumps
overall result = mucous secretion and electrolyte/ H2O accumulation in lungs
describe cAMP activation of protein kinase A (PKA)
PKA has 2 catalytic domains and 2 regulatory domains - a tetramers
when inactivated (normal conditions), the regulatory subunits of PKA will be bound to the catalytic domains
but when there’s increased cAMP, cAMP will bind to the regulatory subunits’s B cleft, causing conformational changes… allowing the 2nd cAMP to bind to the A cleft… once all 4 sites bound, the regulatory subunits will release the catalytic subunits
cAMP binds to the R subunits in a coop fashion i.e. binding the 1st cAMP to CNB-B lowers the Kd for binding of the 2nd cAMP molecule to CNB-A.
what does cAMP activate?
activates/binds to protein kinase A
what does cGMP activate? what does it do?
activates/binds to protein kinase G and opens cation channels in rod cells
what does DAG do?
activates protein kinase C
what does IP3 do?
opens Ca2+ channels in the endoplasmic reticulum
describe the regulation of glycogen metabolism by cAMP
epinephrine is released and it will bind to GPCRs → activation of G-alpha-s subunits → activation of effector protein, adenylate cyclase → increased cAMP → activation of PKA (protein kinase A) → activates GPK (glycogen phophrylase kinase) → phosphorylates GP (glycogen phosphorylase) → catalyzes the reaction from glycogen to glucose
at the same time, inhibit glycogen synthase to prevent the reverse reaction
PKA also inhibits phosphoprotein phosphatase which inactivates proteins responsible for glycogen breakdown
when we no longer need energy, we remove the residual phosphorylations by activating (removing PKA from) phosphoprotein phosphatase we inhibit glycogen phosphorylase kinase and glycogen phosphorylase kinase
also removes inhibitory group from glycogen synthase so we can go back to forming glycogen stores
how are genes regulated by PKA
Genes regulated by PKA (which is from the increase in cAMP) contains a specific nucleotide sequence (TGACGTCA) known as the cAMP response element (CRE)
causes the activation of the CREB transcription factor (CRE-binding protein) by phosphorylation (of CREB) in nucleus
how are memories formed? (describe the pathway involving CREB)
neurotransmitters released into the post synaptic neuron → causing Ca2+ channels to open and an influx of calcium → calcium will bind to calmodulin which activates a kinase called CaMKII → phosphorylates and activates adenyl cyclase → increased production of cAMP → activates PKA → goes into the nucleus and phosphorylates CREB → binds to CRE elements found within certain genes in the nucleus → more gene expression and certain proteins
one of the protein will help promote the mobilization of more neurotransmitter receptors to the post synaptic membrane AND encode for structural remodelling of the post synaptic membrane – both of these now make the post synaptic neuron more sensitive
repeated exposure to this stimuli stabilizes the results – memory formation!!
what role does phospholipase play?
Phospholipase C (PLC) cleaves PIP2 into free DAG and IP3
—— PLC (PLC-beta) is an effector which is activated by certain G-alpha subunits
IP3 can also trigger release of calcium from endoplasmic reticulum to cytoplasm
describe the IP3 and DAF signalling pathway and Ca2+
when the alpha subunit is activated, it will bind to phospholipase C
phospholipase C will then cleave off IP3 from DAG … DAG is still very hydrophobic so it is associated with the membrane
IP3 will bind to a channel found on the endoplasmic reticulum, called an IP3-gated Ca2+ channel – when bound, it will open, and release Ca2+ from the ER to the cytoplasm
this Ca2+ can bind to calmodulin but also protein kinase C (PKC)
for PKC to be fully activated it needs to also bind to DAG
