GPCR 1 - cAMP Flashcards
(21 cards)
GPCR structure
1 polypeptide chain with 7 transmembrane passes
cyto loop C3 and C-terminus interact with G protein
GPCR superfamily
- important sub-families
largest family of cell surface receptors
yeast mating factor
rhodopsin receptor
olfactory receptors
GPCR ligands
functional:
- hormones
- neurotransmitters
biochemical:
- proteins
- small peptides
- amino acid derivatives
signal transduction
ligand binds to receptor
- > conformational change
- > recruitment of trimeric G protein
trimeric G-protein structure
alpha, beta, gamma subunits
alpha
- bound to GDP
- GTPase activity
lipid amino acid side chains anchor a + g subunits to PM
G proteins
- stimulating or inhibiting examples
Gs alpha
stimulates adenylyl cyclase
(+ cAMP = 2nd messenger)
Gi alpha
inhibits adenylyl cyclase
(- cAMP = 2nd messenger)
activation of G protein by GPCR
- receptor activated by ligand
- conformation change in Ga subunit
- > releases GDP - uptake of GTP (as more abundant)
- conformation change
- >G protein dissociates from receptor AND a subunit dissociates from b/g complex
what does the active a/GTP subunit do?
binds specific target enzymes
–> activates them
enzyme remains active while bound to a/GTP
-> propagates signal
why is the a/GTP only transiently associated with the target enzyme?
interaction activates GTPase activity of subunit (GAP)
-> after short delay…
a subunit hydrolyses GTP to GDP
= restores inactive subunit conformation
-> release from target enzyme
= restores resting state
- unless signal still present
regulation of G protein
GTPase stimulated by:
- interaction with substrates
- RGS proteins
(Regulator of G protein Signalling)
Gs signalling
- regulates cystolic cAMP
- adrenaline activates GPCR
- activated Gs alpha subunit
- activates adenylyl cyclase
- converts ATP into cAMP
- cAMP-phosphodiesterase hydrolyses cAMP into 5AMP
cell responses to increased [cAMP]
- cell type specific
in each cell type
cAMP elicits same characteristic response
1 signal can affect multiple cell types
- sometimes using different receptors
cell responses to increased [cAMP]
- examples
muscle
- adrenaline
= glycogen breakdown
kidney
- vasopression
= water resorption
increased [cAMP] activates protein kinase A
- features
PKA = serine/thronine kinase
substrates are cell type specific
catalytic subunits can translocate to nucleus + activate TFs
increased [cAMP] activates protein kinase A
- process
2 PKA regulatory units each bind x2 cAMP
(-> 4 molecules required for conformation change)
- > releases catalytic subunit
- > active
- > phosphorylate their target proteins
rapid response to adrenaline for energy mobilisation
- stimulates glycogen breakdown
- cAMP activate PKA
- phosphorylates phosphorylase kinase (active)
- phosphorylates glycogen phosphorylase (active)
- breaks down glycogen
rapid response to adrenaline for energy mobilisation
- inhibits glycogen synthesis
Glycogen synthase kinase
-> phosphorylates glycogen synthase
= inactive
slower cAMP response
e. g. CREB-mediated gene expression
- process
- why is this slower?
PKA activates CREB in nucleus
- > CREB + CBP (co-factor) bind to CRE motif in DNA
- > transcribes and translates DNA
(CRE = cAMP Response Element)
slower as not stimulating enzymes already present in cytoplasm
PKA activity is limited by serine/threonine phosphatases
Type I
= dephosphorylates many substrates of PKA
Type IIA
= broad specificity, most common
Type IIB (calcineurin)
= most abundant in brain,
Ca2+ activated
Type IIC
= not common, unrelated to others
Type I phosphatases
desphosphorylate PKA substrates
e.g. glycogen synthase + P -> activate
inhibited by PKA-dependent mechanism
PKA-dependent mechanism to inhibit Type I phosphatases
inactive phosphatase inhibitor
- > activated by PKA (phosphorylates)
- > inhibits protein phosphatase I
