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The G protein coupled receptor (How does the G protein coupled receptor…
The G protein coupled receptor
How does the G protein coupled receptor work?
structure
the receptor protein
transmembrane protein
7 transmembrane domains
G protein
trimeric
alpha, beta, gamma subunits
GDP / GTP
Signalling
signalling "switch"
common mechanism for inducing conformational changes in signalling molecules within a cell
mechanism
1) changes receptor conformation
allows interaction of the G protein to the receptor
causes release of GDP
replaced by GTP
a-subunit is activated
2) the active a/GTP subunit can associate and activate specific target proteins
target enzyme remains active while bound to a/GTP subunit
target protein will propagate signal
3) activation of GTPase activity
caused by interaction between a/GTP subunit and target enzyme
hydrolysis only occurs after a short delay
inactive a subunit conformation restored
release of target enzyme
How does activation of G-protein coupled receptors cause a cell response?
example 1: adenylyl cyclase
raise the level of intracellular small second messenger molecules
these diffuse widely to act on target proteins in various parts of the cell
adenylyl cyclase hydrolyses ADP to cAMP
reversal of signalling can occur
cAMP breakdown by PKA
PKA = AMP-dependent protein kinase
phosphorylation of cytoplasmic proteins
direct change of activity
fast change
< sec to mins
phosphorylation of transcription factor
alteration of gene expression
slow
mins to hours
formation of linear 5'-AMP
example 2: phospholipase C
increase levels of inositol phospholipid (IP3)
IP3 open Ca2+ channels on endoplasmic reticulum
activation of PKC
phosphorylation of cytoplasmic proteins
phosphorylation of transcription factors