Week 4 (Spring): G-Coupled Protein Receptors Flashcards
GPCR profile
- One of the oldest signal transducers that has been present in many organisms
- An important drug target on cell membranes
- Estimated to be >1000 GPCRs in human genome with potential for therapeutic utility
GPCR mutants/mutation examples
- Incapable of binding to ligand
- Incapable of generating normal signals
- Cnstitutive signal generation
- No/Inappropriate expression
- Chemokine CCR5 receptor mutant found to poorly bind to HIV (beneficial mutant)
GPCR families
- Classified based on aa sequence
- Class A: Rhodopsin-like
- Class B: Secretin-like
- Class C: Metabotrophic gluatamate/pheromone
- Class D: Fungal pheromone
- Class E: cAMP receptors (Dictyostelium-like)
Importance in drug therapy/ as a therapeutic target
- Estimated tobe >1000 GPCRs in human genome with potential therapeutic utility
- Associated with many different conditions + diseases
- can be targeted by:
+ Opiates
+ Cannabinoids
+ Hallucinogens
+ Cocaine
Examples of GPCR mutants/mutations
- Incapable to bind to its respective ligand
- Incapable of generating normal signals
- Constitutive signal generation
- Not/inappropriate expression
- However questionably beneficial mutants have been found
+ ie Chemokine CCR5 receptor mutant, failing to bind to HIV, but can be detrimental to learning ability
The G protein
- Known as heterotrimeric guanine nucleotide-binding protein
- Composed of the alpha, beta and gamma subunits
Alpha subunit of the G protein
- Related to small proteins such as Ras
- Guanine nucleotide binding proteins that can bind to GDP + GTP
- Has GTPase activity
- 39-52 kDa in size, for comparison Ras is 21kDa
- 4 G alpha subfamilies based on aa sequence + effectors
The 4 G alpha subfamilies
- G-as = Activates adenylate cyclase
- G-ai = Inhibits adenylate cyclase
- G-aq = Activates phospholipase C
- Ga12 - Na/H exchange, cytoskeleton
- Over 20 mammalian alpha subunits are possible with splice variants
Beta + gamma subunit action
- The alpha subunit signals by dissociation from beta + gamma subunit, specifically when the alpha subunit undergoes conformation change upon GDP/GTP exchange
- However Beta/Gamma subunits may not always separate from G-alpha subunits upon activation, instead it too may go undergo conformational change
- Could account for the fact that some beta-gamma dependent signals seen to also be dependent on the type of G-alpha they were associated with prior to activation
Beta/Gamma subunit
- Beta subunit is tightly bound to the gamma unit + only functions as a complex together
- The beta subunit is similar to the g-alpha subunit, in that it modulates effector activity
- All beta subunits are ~36kDa with 2 subfamilies
- The gamma subunits are 7-8kDa with 4 subfamilies
G-proteins are distinguished by their effectors + a variety of receptors that activate them
G protein-Effector-2nd messenger-receptor
- S- Stimulates AdCyc, Raises cAMP, B-adrenergic
- af- Stimulates AdCyc, Raises cAMP, Odorant
- i- Inhibits AdCyc + Opens K+ channels, Lowers cAMP, increases membrane potential, Somastatin
- o- Closes Ca2+ channels, Lowers membrane potential, M2 ACh
- t- Stimulates cGMP PDE, Lowers cGMP, Rhodopsin
- q- Activates phospholipase CB, Raises Ins 1, -4, -5, -P3, DAG, m1 ACh
Signaling via cAMP
- Adenylate cyclase
+ Each transmembrane domain is made up of 6 membrane spinning helices
+ Catalytic domains are present in duplicate activated
+ Activated by G-as - A major target of cAMP in cells is protein kinase A (PKA), with >500 substrate of PKA
- Activation of G-ai prevents stimulation of Gas
Activation of PLCB by G-aq
- A signaling molecule is activated but it swtiches off the things that switched it on, which is a relationship that is shared by PLCB by Gaq
- All 4 Gaq subunits activate PLCB isoenzymes, but no PLC delta/gamma, with 4 PLCB isoenzymes
