GPCRs 2 Flashcards
what is cAMP synthesised from?
ATP by plasma-membrane bound enzyme adenylyl cyclase
what is cAMP rapidly + continuously destroyed by?
phosphodiesterase (PDE) enzymes
hydrolyses cAMP to adenosine 5’ - monophosphate
what does the synthesis and degradation of cAMP involve?
- loss of 2 phosphates
- creation of cAMP
- formation of 5’-AMP
irreversible steps
phosphodiesterase enzymes (PDE)
at least 10 subtypes
- PDE 3, PDE 4 selective for camp
- PDE 5 selective for cGMP
inactivated weakly by drugs
- methylxanthines (caffeine)
- rolipram (discontinued) PDE 4 selective
- sildenafil (viagra) PDE 5 selective
- milrinone (treats heart failure)
what are PDE inhibitor used to treat?
cardiovascular + respiratory disease
what is the best example of the signal transduction role of cAMP?
beta-adrenoreceptor
stimulates G protein (GS) + activates AC
what does the beta adrenoreceptor do?
- initiates breakdown of glycogen + inhibition of ist synthesis in liver + skeletal muscle
- relaxation of smooth muscle e.g. bronchioles in treatment of asthma
switch on process
- adrenaline binds to beta-adrenoreceptor on cell -> adenylyl cyclase activates through Gs
- cAMP formed in cell by alphaS subunit
- exchanging GDPl for GTP
- causes alphaS to dissociate from beta-gamma + activate adenylyl cyclase
why is the lifetime of the active form of the alphaS short?
GTPase activity of alphaS = stimulated when alphaS binds to adenylyl cyclase
switch off process
- bound GTP hydrolyses to GDP
- inactivating alphaS + adenylyl cyclase
- alphaS reassociated with beta-gamma to reform inactive Gs
how does cAMP exert its effects in animal cells?
by activating enzyme protein kinase A (PKA)
changes cellular activity + generates response
what does PKA do?
catalyses transfer of terminal phosphate group of ATP to specific serine/threonine residues on certain proteins
- inactive PKA consists of complex of 2 catalytic subunits + 2 regulatory subunits that bind cAMP
- binding of cAMP alters conformation of regulatory subunits
- causes them to dissociate from complex
what are the actions of PKA?
phosphorylates 2 other enzymes
- first phosphorylase kinase which phosphorylates enzyme
- = glycogen phosphorylase - activating release of glucose residues from glycogen
stimulation of glycogen breakdown by cAMP in skeletal muscle
- inactive A-kinase -> activate A-kinase (via add of cAMP)
- inactive phosphorylase kinase -> active phosphorylase kinase via add of Phosphate
- inactive glycogen phosphorylase -> active glycogen phosphorylase via add of Phosphate
- glycogen -> glucose 1 phosphate
what is the second enzyme phosphorylated by PKA?
glycogen synthase
- performs final step in glycogen synthesis from glucose
- phosphorylation inhibits enzymes activity
- stopping glycogen synthesis
what 4 groups of protein are able to dephosphorylate proteins phosphorylated by PKA?
phosphatases I, IIA, IIB, IIC
I plays imp role in response to cAMP
- counteracts phosphorylations brought about in adrenaline-stimulated muscle cells
what is another target of PKA?
phosphatase inhibitor protein
- when protein phosphorylated by PKA
- binds to protein phosphatase I + inactivates it
what can an increase in cAMP levels do?
- stimulate glycogen breakdown
- inhibit glycogen synthesis
thus maximising amount of glucose available to cell
what enzymes catalyse the removal of phosphate groups from proteins making them inactive and available for reuse?
protein phosphatases
what is the purpose of the cascade reactions?
amplification
to get a big response
what are the actions of cAMP?
regulates many cellular processes
- enzymes involved in energy metabolism
- cell division + cell differentiation
- ion transport
- ion channels
- contractile proteins in smooth muscle
what does CREB stand for?
cyclic AMP response element binding protein
what is CREB?
- transcription factor activated by neuronal activity
- key regulator of pathway producing changes in synaptic connections (moves to nucleus + activates creb)
- forms new memories
- inhibitors of PDE4 enhance memory
muscarinic M2 receptor in heart?
- ACh causes dec in broth force + rate of heart beating
- couple to Gi protein
- causes dec prod of cAMP (alphai) (inhibits ad cyc) -> dec in contraction force
- opens K+ channels (beta-gamma subunit)
- causes hyperopolarisation of SA node cells (becomes neg)
- leads to dec in firing rate
- dec in heart rate
what are the actions of cAMP?
- interacts directly with pacemaker current channels (iF)
- activates PKA which phosphorylates calcium channels in cell membrane to inc open state
- this inc plateau current - Ica
- PKA phosphorylates phospholamban - regulatory protein associated with Ca2+-ATPase pump
- inc affinity of pumps for Ca2+ -> removing Ca2+ faster + shortens systole
- PKA phosphorylates delayed rectifier K+ channels -> inc depolarising iK
- terminates plateau earlier -> shortens AP
SHORTENS EVERYTHING DOWN SO HEART CAN SPEED UP
opposite to beta gamma effect
what does an inhibition of phosphate lead to?
inc in heart rate
more calcium into cell means…
more forced contractions
what does the autonomic nervous system modulate?
frequency of depolarisation of pacemaker
sympathetic stimulation
NA binds to beta 1 receptors on SA nodal membranes
parasympathetic stimulation
ACh binds to muscarinic receptors on nodal membranes
inc conductivity of K+ + dec conductivity of Ca2+
cholera
- caused by bacterium that secretes toxin Ctx
- acts on alpha subunit of Gs
- causes persistent activation of adenylyl cyclase
(toxin inhibits G protein) - big amount cAMP produced
- resulting in excessive secretion of fluid from GI epithelium
- —> chronic diarrhoea
pertussis toxin (Ptx)
- secreted by bacterium causing whooping cough
- acts on alpha subunit Gi
- causes ADP - ribosylation of Gi
- inactivates inhibitory G- protein
- prevents deactivation of adenylyl cyclase
- big amount cAMP produced
- inc mucous secretion interference with many cellular functions
