Superfamily 2 Flashcards
What is the molecular structure of G protein coupled receptors?
Single polypeptide chain
Typically 400-500 amino acids long
7 transmembrane spanning alpha helices
Ligand binding site embedded in membrane surface
Long 3rd cytoplasmic loop interacts with G protein
What are the features of the intracellular C-terminal?
Phosphorylation (involved in receptor desensitisation)
Palmitoylation to anchor in lipid membrane
Interacts with scaffolding proteins to orientate in membrane and localise micro-domains
Interacts with other enzymes such as tyrosine kinases to influence signalling (upon agonist binding)
Give some interesting facts about G protein coupled receptors
Largest superfamily
1-3% of total genome
Targets for >30% of all drugs
>900 genes
Give some examples of G Protein coupled receptors
Muscarinic cholinoceptor
Adrenoceptors
Histamine, dopamine, serotonin (amine receptors)
Endothelin and angiotensin (peptide mediators)
Prostaglandin receptors
Rhodopsin and olfactory chemoreceptors
What are the three subgroups of G protein coupled receptors?
Group A
Group B
Group C
Describe Group A GPCR
Rhodopsin / beta2 adrenoceptor like
Short N terminal
Binding site embedded in TM alpha helices
Includes eicosanoid, purine and amine receptors
Describe Group B GPCR
Glucagon/ VIP/ CT receptor like
Large N terminal
Several cys-cys bridges
Many peptide receptors
Describe Group C GPCR
Metabotropic neurotransmitter / calcium receptors
Very large N terminal
GABA A, pheromone and taste receptors
Describe heterotrimeric G proteins
Consist of 3 subunits
Alpha, beta and gamma
Describe small G proteins
5 subfamilies
RAS, Rho, ARF, Rab, Ran
Molecular weight 20-30kDa
Regulate wide variety of cellular processes
What do heterotrimeric G proteins do?
Communicate between receptor and effector enzymes or ion channels
Require GTP for activation
What is the mechanism of action for GPCR?
Agonist binding destabilises intra molecular interactions
Alpha helixes TM3/TM6 move apart during activation
Expose cytoplasmic sequences that facilitates G protein interaction
2nd cytoplasmic loop confers efficiency of G protein activation
3rd cytoplasmic loop confers specificity of G protein interaction
What are the basic steps of GPCR?
Agonist
Receptor
G protein
Channel/ enzyme
Describe what stimulatory G proteins do
Induce a conformational change in the target to a more active state
To enhance activity
Describe what inhibitory G proteins do
Induce a conformational change in the target to a less active state
To reduce activity
What are the properties of G protein alpha subunits?
Different receptor interaction sites
Different target interaction sites
Similar GTP/ GDP site and GTPase activity
Variable sensitivity to bacterial toxins
What effectors are regulated by G proteins?
Adenylate cyclase
Phospholipase C-beta
Voltage gated potassium channels
Voltage gated calcium channels
What are the differences between Galpha proteins?
GTP affinity
GTPase kinetics
Regulation
Localisation
What are the four different types of Galpha proteins?
G alpha (s/ olf)
G alpha (I/ o/ t/ Z)
G alpha (q / 11)
G alpha (12 / 13)
What does the G alpha (s/olf) do?
Increase adenylate cyclase
Increase calcium channels
What does G alpha (I/o/t/z) do?
Decreases adenylate cyclase
Increases potassium channels
Decreases calcium channels
Increases cGMP-specific PDE
What does G alpha (q/11) do?
Increases phospholipase C-beta and others
What does G alpha (12/13) do?
Increase chloride channels
Regulates protein Ras, Rho activity
Explain the example of cholera on GPCR
‘Faulty off switch’
Arg amino acid impairs GTPase activity
Uncontrollable increased activity of Gs/ cyclic AMP
Increased transluminal movement of water and electrolytes
Severe diarrhoea
Explain the example of whooping cough on GPCR
Pertussis toxin Gi family
C terminal cys residue prevents transduction between receptor and effector
Stabilises G protein in inactive state
Decreases activity of some Gi, Go, Gt
Immobilises cilia prompting cough
Interferes with signalling in phagocytes and their ability to counter infection
What is the clamshell model?
Agonist receptor induces changes in G protein confirmation
Exposes G alpha and G beta gamma interfaces
Act as scaffolds to direct interaction with respective effectors
G alpha remains closely associated with G beta gamma
Not interchangeable between other G alpha subunits
Describe some of the features of regulators of G protein signalling proteins
Largely inhibitory
Cytosolic proteins adjacent to membrane G proteins
Roughly 20- 25kDa
Bind directly to activated G alpha subunits
Interact with C terminal of receptor and with specific domains of effector
What is the mechanism of action of regulating proteins?
GTPase activating proteins - increases the inactivation rate of G alpha, promotes heterotrimer reformation so also blocks G beta gamma signalling
Competitive inhibitor of G alpha effector or G beta gamma effector interactions
Impedes GDP releases/ prevents GTP binding/ G alpha activation
