Chem*3560
Lecture 34: G-protein coupled receptors
Epinephrine is recognized by two classes of plasma membrane receptors, which give different responses
in different tissues; namely α-adrenergic and β-adrenergic receptors. (Adrenergic is derived from
adrenaline, the alternative name for epinephrine.)
The β-adrenergic receptors are members of the "seven pass",
heptahelical or serpentine receptors family, proteins with
seven closely spaced transmembrane helices, and somewhat
variable length of connecting loops.
Animals have about 1000 different receptors of this type,
including those for taste and smell. Ligands range from
epinephrine, prostaglandins, peptide hormones including
glucagon, histamine etc.
Ligand binds in the central recess between helices causing a
slight angular displacement of the helices, which is dectectable
as a conformation change in the cytoplasmic domain. The
cytoplasmic face of the receptor binds a protein complex of
three components that contains tightly bound GDP - the
heterotrimeric G-protein. Hence the more general name for
this receptor class is the G-protein coupled receptor
(GPCR) family.
The largest subunit contains the bound GDP and is the α
subunit; it is also lipid anchored to the bilayer. There are
several variants of α including Gs α, Gi a, Golf (for olfactory
receptors).
The heterotrimer is completed by the Gβγ subunits, which
bind to each other very tightly. The small γ subunit is lipid
anchored by a farnesyl chain. In the absence of ligand bound
to the receptor, the bound GDP can't be released and the
G protein is inactive. Receptor with bound ligand acts as
guanosine exchange factor (GEF), allowing entry
of GTP (Lehninger p.449-450).
When Gsα binds GTP it changes its conformation,
and loses affinity for receptor and for Gβγ. Thus
the whole complex breaks up and Gs α
separates.
The Gs α -GTP complex stimulates adenylate cyclase
Gsα-GTP remains membrane associated because of its
lipid anchor. The presence of GTP increases its affinity
for another membrane associated protein, adenylate
cyclase, and when Gsα-GTP binds, adenylate cyclase
is stimulated to produce cyclic AMP (Lehninger
p.451). Gs α denotes the stimulatory G protein.
Other GPCR, for example the α2-adrenergic receptor
and opioid drug receptors, may release the closely
related Giα
α as a GTP complex. Giα is an inhibitor of
adenylate cyclase, and opposes the effect of Gsα..
Gsα
α and Giα
α hydrolyse GTP over a period of time
Gsα is a slow GTPase (turnover 3 hr–1 ), so that eventually Gsα
reverts to its inactive GDP state, and dissociates from adenylate
cyclase, shutting down its activity. Gsα
α -GDP then re-assembles as
a heterotrimeric complex with its GPCR until fresh ligand arrives to
bind to the receptor. However unlike the the Ras-GTP-Raf
complex, adenylate cyclase does not appear to have GTPase
accelerator protein (GAP) activity, and the decay of Gsα remains
slow. The GTPase activity is purely a mechanism to set a time limit
on the lifetime of active Gsα.
Giα also hydrolyses GTP very slowly, however some of its activation targets such as phospholipase Cβ
do act as GAPs.
Cyclic AMP phosphodiesterase turns off the cyclic AMP signal
Cyclic AMP is broken down by the enzyme 3',5'-cyclic nucleotide phosphodiesterase. This enzyme is
inhibited by caffeine, leading to cyclic AMP induced glucose release and high catabolic rate that is
sustained longer than usual.
Sildenafil (Viagra) is a selective inhibitor of the phosphodiesterase 5 isozyme. This isozyme targets
cyclic GMP for hydrolysis, and cyclic GMP relaxes vascular smooth muscle, increasing blood flow. By
inhibiting the phosphodiesterase, cyclic GMP levels stay high for longer.
Gβγ
βγ plays a secondary signalling role
When Gsα-GTP releases the Gβγ pair, they can act to modulate certain process, e.g. Gβγ activates
certain protein tyrosine kinases, as well as phospholipase Cβ and some ion selective channels.
Desensitization of GPCRs
The β-adrenergic receptor can be phosphorylated by the protein kinase A cascade, although not by
protein kinase A itself. When phosphorylated, the receptors are drawn off in endocytotic vesicles, and
temporarily unavailable to respond to epinephrine. Thus the response to epinephrine gradually
attenuates.
A caffeine "crash" is the result of sustained cyclic AMP levels following ingestion of caffeine. The short
term effect is a high induced by glucose release into the blood, but after sustained use or an excess of
caffeine, the β-adrenergic receptors are withdrawn from the plasma membrane and not available to
respond to a genuine signal that blood glucose level is low.
Cholera toxin blocks the GTPase action of Gs α
Cholera toxin is an enzyme that transfers ADP from NAD + and adds it to a key Arg side chain in
Gsα. This prevents GTP hydrolysis, so that the Gsα is left permanently in the stimulatory state.
In vivo, cholera toxin enters the body through intestinal cells, so it exerts its effects there. Normally,
cyclic AMP induces secretion of digestive fluids from the epithelium into the intestinal lumen. This
process goes totally out of control, so that the volume of fluid expelled becomes far more than the
intestinal system can handle.
Certain pathogenic strains of E. coli produce enterotoxins with similar but milder effects. If
contaminated food is heated, this denatures the toxin, but does not necessarily kill the organism.
Pertussis toxin blocks the ability of Giα to replace bound GDP with fresh GTP. The result is that Giα
no longer provides a counterbalance for Gsα, again allowing the adenylate cyclase to be excessively
stimulated. The difference in the diseases is due to the location in the body where each organism
strikes, lungs and bronchial passages for Pertussis (whoopin cough).