GPCR signalling Flashcards
Chemical interactions in NA binding at adrenergic binding sites
The two aromatic hydroxy groups hydrogen bond with hydroxyl side chains in two serine residues in TM5
The aliphatic amine (ionised at physiological pH) co-ordinates with the carboxylate side chain in the asp residue in TM3
The beta-hydroxyl hydrogen bonds with the amide side chain in the asn residue of TM6
the aromatic ring stacks with the phenyl side chain from phe residue in TM6
mechanics and structure of G-protein coupling
On the IC face of GPCR. Upon agonist binding, there is rotation and movement of TM3 and TM6 that open the binding cavity for the G-protein. The size and shape of the cavity determine which G-protein binds.
Regions that interact with the G-protein are the 3rd ic loop, proximal end of C-terminus (4th IC loop) and the second IC loop.
G-protein structure
heterotrimeric: alpha, beta, and gamma subunits. beta and gamma are considered a single unit. The beta gamma subunit exists in multiple subtypes, but they are functionally interchangable.
Gai, Gai/o, Gaq, and Ga12 alpha subunits available.
G-protein cycle of agonist binding, dissociation, and reassociation
Agonist binds
GTP binds alpha subunit, alpha and betagamma subunits dissociate and IC signal cascade.
GTP bound to alpha subunit hydrolysed to GDP, allowing re-association with betagamma subunit. Then the cycle can begin again when agonist binds.
Effects of GPCR alpha subtypes on different tissue types
Gs: enhance cardiac muscle contraction, relax SM, enhance NT release.
Gi: inhibit cardiac contraction, inhibit NT release
Gq: contract SM, enhance NT release
G12: stimulate chemotaxis, induce shape change.
Gas signalling cacades and effects in different tissue
Stimulates cAMP production, which activates PKA.
PKA subunits dissociate to R and C subunits. C subunits can phosphorylate cardiac Ca channels (increases IC [Ca]), proton pumps (increase gastric acid), adipocyte hormone sensitive lipase (increases fat metabolism), increases glucose synthase, MLCK (to decrease SM contraction) and the epithelial Cl- channel (increasing water secretion).
E.g., B1-AR in cardiac muscle (increase contraction)
B2-AR in vascular smooth muscle (vasodilatation) and bronchial SM (bronchodilation)
B3-AR in adipocytes increase fat metabolism
Gai/o signalling cacades and effects in different tissue
Both Gai and betagamma subunits bind to K+ channels to stimulate K+ efflux. Decreases NT release, reduces heart contractility, and decreases [cAMP] by the inhibition of adenylyl cylase (consequently decreasing PKA activity).
E.g., CB1, mGluR2, GABAB, a2-AR, H3, 5HT1, D2, M2/4, all opioid receptors.
mAChRs in cardiac muscle are responsible for decreasing contractility, by increasing K+ efflux and reducing cAMP.
Gaq signalling cacades and effects in different tissue
Stimulates phospholipase C (PLC) to produce DAG and IP3.
IP3 increases intracellular Ca2+ release from the ER. This increases calcium-bound calmodulin increasing contractility of SM mediated by MLCK.
DAG activates PKC to phosphorylates proteins. Also phosphorylates parietal cell proton pumps to increase gastric acid secretion.
M1 and M3 receptors are Gq coupled. Contract ileum, bladder, iris ciliary and bronchial SM. M3 innervates salivary gland, increasing saliva secretion. M1 innervates sweat glands, increases sweat.
a1-AR innervation contracts iris dilator and dermal vascular SM. It also mobilises glycogen in the liver.
How do GPCRs secondary messengers recover to baseline
cAMP hydrolysed by PDE to 5’-AMP - also recovers ATP.
IP3 hydrolysed to inositol
DAG conjugated with CTP to form CDP-DAG
Ca2+ recovered to IC stores by SERCA, or effluxed to EC space by PMCA
