Part II: Receptors, Ion Channels and Signalling Flashcards
What are GPCR involved in
Senses, autonomic function
GPCR structure
Single polypeptide in an anticlockwise bundle, 7 (α-helices) transmembrane spanning domains, extracellular N terminus, intracellular C terminus and 3 extra (ECL) and intracellular loops (ICL)
Another name for GPCR
7 transmembrane domain receptors
5 families of GPCR and info abt them
1) Family A “Rhodopsin” = most common. 2) Family B “Secretin”: Some peptide hormones. 3) Family C “Glutamate”: Glutamate + GABA neurotrans in brain. Calcium + some taste receptors. 4) “Frizzled”: Receptors in development, bitter taste receptors. 5) “Adhesion”: Cell-cell adhesion and motility
How does the rhodopsin (A) family bind ligands
Mostly small molecs: Binding site within transmembrane domain bundle. Medium sized ligand sites involve bundle and receptor ECL domains + N terminus. Largest = N-terminus built to generate a binding site
How does Glutamate (C) family bind to ligands
To small molecules but have a large N terminus called Venus Fly Trap (VFT) domain. It closes around the ligand to activate the recptor.
Types of bonds on binding site
Ionic: +ve charge NH3+ with COO- amino acid, Hydrogen bonds, Hydrophobic bonds with benzene rings
What is receptor mutagenesis
Change amino acid you think is important on binding site by genetic engineering. Eg change OH group to CH3. Then run competition binding study to find if it made a difference.
How can GPCR structures be seen/visualised
X ray crystallography
What is an allosteric modulator
Binds to a separate site from agonist orthosteric site. Changes nature of response to orthosteric agonist. Can alter orthosteric ligands affinity (ability to bind) / acitvate receptor/both
What are positive/negative allosteric modulators
Positive: enhances agonist effect. Negative: inhibits agonist effect
What is co-operativity for allosteric modulators
Orthosteric + allosteric ligands influence each other. Positive coop for PAMs (ennhances agonist effect) + negative for NAMs
How can an allosteric modulator prevent the dissociation of agonist
Bind above binding bocket so its locked in. Enhances the affinity
Benefits of allosteric modulation
Increased selectivity: Agonist needs to be present otherwise no effect. Allosteric sites may be unique to receptor subtype. Controlled response: Max effect is limited. Reduces risk of receptor over stimulation
Conformational change of GPCR during activation of drug+receptor
Extracellular domain with drug moves inward. Intracellular moves away from each other allowing effectors such as G proteins to be recruited
What are heterotrimeric G proteins
GPCRs activate them. G proteins made of 3 subunits where at least 1 is different from the other 2
What are the G protein subunits
Gα: Binds guanine nucleotides (GDP/GTP). Gβ + Gγ: Always form a dimer (Gβγ)
Where are Gα and Gβγ located
On intracellular side of plasma membrane (using lipid anchors)
What is the G protein cycle
1) When GPCR inactive, Gα binds GTP and forms a heterotrimeric complex with Gβγ. 2) Activated GPCR catalyses exchange of GDP for GTP on Gα by conformational change. 3) Gα-GTP & Gβγ separate from each other. 4) Gα-GTP & Gβγ activate effector proteins along membrane. 5) GTPase activity of Gα converts bount GTP back to GDP. 5) Gα-GDP reassociates with Gβγ.
3 main Gα classes
Gsα: stimulates adenylyl cyclase whcih increases cyclic AMP. Giα: inhibits adenylyl cylcase - decreases cyclic AMP eg smooth muscle contaction, inhibits transmitter release. Gqα: stimulates phospholipase C - enhanced transmitter release, increases intracellular Ca2+, smooth muscle contraction
ATP -> cAMP -> response -> break down cycle
ATP + adenylyl cyclase enzyme -> cAMP. Protein kinase A (PK A) forms an inactive tetramer of 2 regulatory + 2 catalytic subunits. cAMP binds to regulatory -> dissociates tetramer, releases active catalytic subunits. PK A phosphorylates target protein -> response. cAMP broken down by phosphodiesterase -> AMP -> ATP
cAMP pathway in Gs coupled receptors
Gsα-GTP releases from acitvated receptor. Activates adenylyl cyclase which generates cAMP from ATP. cAMP binds to regulatory subunits on Portein kinase A which releases activates catalytic. They phosphorylate target proteins
cAMP pathway in Gi coupled receptors
ATP is stopped from converting to cAMP bc adenylyl cyclase is inhibited
Calcium pathway by Gq couple receptors
Gqα-GTP activates phospholipase C (PL C) enzyme. This hydrolyses plasma membrane lipid (PIP2). This releases 2 intracellular messengers: IP3 = diffusable out into cytoplasm & DAG = fatty acid tails kept in membrane. IP3 releases Ca2+ from intracellular stores in endoplasmic reticulum into cytoplasm. DAG activates protein kinase C (PK C) which goes to own target for phosphorylation.
Desensitisation/tolerance of receptors definition
Sustained or repeated agonist exposure often leads to reduced
responses (all types of receptor)
Difference between desensitisation and tolerance
Tolerance is over a longer time-scale: days/weeks not s/mins
Mechanism of β-arrestin / desensitisation of GPCRs
- Uncoupling from G protein: GPCR phosphorylated by GPK kinase attached to βγ. β-arrestin binds to phosphorylated receptors - same place to block G protein. 2. Removal of receptor from cell surface. β-arrestin targets GPCRs for endocytosis (brings them into cell). 3. Internalised receptors degraded by lysosomes (downregulation - long term decrease in receptors) or recycled by dephosphorylation. Alterations in synthesis of new receptor proteins
What is downregulation
G prot coupled internalised receptors are degraded by lysosomes - decreases receptor number
What is endocytosis
When cell forms a vacuole (pore) to take in matter into cell
2 ways ions can pass the lipid bilayer
Transporters (conformational change to take them through it) or channel protein (pore formed - can passively diffuse or is regulated w gating)
How is active transport done through lipid bilayer
Using transporters
How are active transporters and ion channels complementary
Transporters create the conc gradient that drive ions through open ion channels thus generating electrical signals
2 type of membrane channels
Gap junctions & ion channels
Difference between cytosol and cytoplasm
Cytoplasm contains cytosol, organelles etc except the nucleus. Cytosol is the liquid component of cytoplasm with no organelles.
What are gap junctions
Cell-cell comunnications, large and permissive pores. They connect cytoplasms of 2 cells
What are ion channels/what they do
Connect the cytosol to extracellular space, narrow + highly selective pores, mainly inorganic ion passage. Not coupled to energy source
Are ion channels/transporters quicker
Ion channels - about 10^5 times quicker
What are active vs passive ion channels
Active: Gates can open or close. Passive: (leakage) always open, ions pass through contiuously
Ion channel components
- Transmembrane pore: Selectrivity for charge and size of ion
- Sensor switch for gating: Membrane potential sensor (voltage-gated ion chan) or neutrotransmitter binding site (ligand-gated ion chans)
- Regulation mechanisms: In built inactivation switches (open/closed/inactive). Modulation (G proteins, 2nd messengers, protein kinases that can change function of channels. Localisation (where it is)
2 ways membrane potential arises
Active electrogenic pumping and passive ion diffusion. Intracellular space negative relative to outside
What is passive ion diffusion
Largest contribution to electrical potential across membrane. K+ tends to leave due to conc gradient
What is electrogenic ion pumping
Involves Na+, K+, ATPase. Actively pumps 3 Na+ out against electrochemical gradient and 2 K+ in
Structure of K+ ion channel
2 sets of Outer helix connected to pore helix connected to inner helix. Betw pore and inner helix there is a selectivity loop with C=O. In between sets there is a vestibule with a pore on either side where ions travel through. It has a selectivity filter where C=O are on the other side.
Explain permeability for K+ but not Na+ in ion channel
Ions dehydrated in selectivity filter. Dehydration energy balanced by interaction of ions with carbonyl oxygens. Na+ is too small to interact with all oxygens so only enters at greater energetic expense.
3 (4) types of ion channel gating
Voltage-gated, ligand-gated (extra/intracellular ligand), mechanically gated
Voltage gated ion channels structure
4 subunits in a circle of 5 transmembrane domains + P loop + S6 transmembrane domain. S1 starts intracell with NH3+ and inactivating particle. P loop lines pore and provide selectivity filter. S6 ends with COO- (C terminal domain) intracellular. S4 contains positively charged amino acids.