Receptors And Signal Transduction - Done Flashcards
What is the role of receptors?
To provide a communication system between cells, tissues and organs - essential in a complex organism
What is the difference between the sympathetic and parasympathetic nervous systems?
Parasympathetic - normal body function, cranial region
Sympathetic - fight or flight, spinal region
Describe the structure of a typical nerve cell
What’s the gap between neutrons and target cells?
100Å
What are the secondary effects of a neurotransmitter binding?
Binding leads to either:
A flow of ions across the cell membrane
Switching enzymes within the cell on/off - protein kinases
Give an example of a prostaglandin
Alprostadil
Give an example of a neuropeptide
B - endorphin
How do hormones travel such great distances?
They are released into the circulatory system by glands
Do chemical messengers that bind to receptors undergo a chemical reaction?
No - different to enzymes
What 2 processes happen after an induced fit at a receptor?
Signal amplification and signal transduction must then occur to produce an observable biological effect
What are the 3 types of membrane bound receptors involved in triggering signal transduction? How fast do they work?
Ion channel receptor - milliseconds
G-protein coupled receptors - seconds
Kinase-linked receptors - minutes
How many protein subunits doe an ion channel have?
5 protein subunits
How does signal amplification work in ion channels?
Relatively mall number of neurotransmitter molecules - opens a few ion channels - several thousand ions mobilised
What’s the difference in subunits between ion channels l and ll?
Ion channel l has
2 alpha
1 beta
1 gamma
1 sigma
Ion channel ll
3 alpha
2 beta
What receptor are the l and ll ion channels controlled by?
Ion channel l - nicotinic cholinergic receptor
Ion channel ll - glycine receptor
Where are the binding sites located on ion channel l & ll?
Ion channel l - 2 binding sites on alpha subunits
Ion channel ll - 3 binding sites on alpha subunits
How many loops/chains does the alpha subunit have, intracellular or extra-cellular?
1 lengthy extracellular N-terminus chain
1 lengthy C-terminus extracellular chain
1 extracellular loop
1 intracellular loop
1 variable intracellular loop
Where is Transmembrane receptor subunit 2 always facing?
TM2 faces the central pore of the ion channel
How does the structure of the TM2 subunit contribute to opening/closing of the ion channel?
TM2 is a kinked alpha helix, so one from each protein subunit changes conformation on binding to allow ions to pass through
How do ligand gated and voltage gated ion channels differ?
Ligand gated - controlled by a chemical messenger
Voltage gated - sensitive to the potential difference that exists across a cell membrane - membrane potential
- important drug targets for local anaesthetics
- opens when membrane is depolarised.
What % of marketed drugs act at G-protein coupled receptors and how are they activated?
30%
Activated by hormones and slow-acting neurotransmitters
-response time measured in seconds
Describe how the general mechanism in g-protein coupled receptors
1 - resting state - inner binding site is closed
2 - once messenger binds, receptor changes conformation and G-protein binds to newly opened inner site - allosteric
3 - G-protein becomes destabilised and fragments into monomer and dimer.
How many transmembrane domains does G-protein coupled receptors?
7
What chain is external in G-protein coupled receptors N or C - terminus?
N - terminus
What ligands bind to G-protein coupled receptors?
Monoamines - dopamine, histamine, noradrenaline, acetylcholine
Nucleotides
Lipids
Hormones
Glutamate
Ca2+
Where are the G-protein coupled receptor ligand’ binding sites?
What are the three classes of G-protein coupled sub receptors?
Class A - rhodopsin like receptors
Class B - secretin like receptors
Class C - metabotrophic glutamate like and pheromone receptors
What class of G-protein coupled sub receptors is the most important in drug discovery /development?
Class A - rhodopsin like
Define divergent evolution
Various receptor subtypes diverge from a common evolutionary branch - eg. Dopamine subtypes D2,D3 and D4
Define convergent evolution
Receptor subtypes found in separate branches - eg. D1A, D1B and D5 are convergent to D2, D3 and D4
Why is greater similarity of receptors a problem for medicinal chemists?
Possible selectivity issue
Design of selective drugs is paramount to reduce side effects
What receptor subtypes are found in heart muscle?
Beta1 adrenergic receptors
What receptor subtypes are found in fat cells?
Beta3 adrenergic receptors
What receptor subtypes are found in bronchial muscle?
Alpha1 and Beta2 adrenergic receptors
What receptor subtypes are found in the GI-tract?
Alpha1, Alpha2 & Beta2 adrenergic receptors
Why do kinase-linked receptors not require G-proteins?
Because they activate enzymes directly
What’s the key kinase-linked receptor type?
Tyrosine kinase receptors are important receptors
What cofactor is required for kinase linked receptors?
ATP - to provide necessary phosphate group
How does signal amplification occur in kinase linked receptors?
Active site remains open for as long as messenger molecule is bound - several phosphorylation reactions take place
What ligands activate kinase linked receptors?
Polypeptide hormones, growth factors and cytokines
What can a loss of function of kinase linked receptors cause?
Developmental defects or hormone resistance
What can an overexpression of kinase linked receptors?
Malignant growth disorders
Describe the structure of kinase-linked receptors
What reaction does tyrosine kinase catalyse?
Phosphorylation of tyrosine residue
Describe how the epidermal growth factor (EGF) receptor works
What’s the purpose of phosphorylation of tyrosine residues?
Phosphorylated regions act as binding sites for further proteins and enzymes - resulting in activation of signalling proteins and enzymes
- message carried into the cell
How does the insulin receptor work?
Receptor already exists as a tetramer
Insulin binds (opening active site) and then phosphorylation occurs
How does the growth hormone receptor work?
GH binds and dimerisation occurs
2 kinases bind - form tetramer - and activates kinase enzymes
Phosphorylation occurs
What are the general principles of intracellular receptors?
50 members of this group directly regulate gene transcription
- nuclear hormone receptors or nuclear transcription factors
Response time - hours or days
Chemical messengers must be hydrophobic and must cross cell membrane
Example - steroids and steroid receptors
Describe how a messenger triggers or inhibits the start of transcription
Messenger passes through surface membrane
Binds to a receptor and dimerises
Co-activator protein binds to dimer - then transcription is either triggered or inhibited, affecting the eventual synthesis of a protein
What role do zinc ions have in intracellular receptors?
They stabilise and determine the conformation of the DNA binding region
Describe the signal transduction pathway for Gs - protein coupled receptor
1 - Alpha subunit binding pocket binds guanosine diphosphate (GDP)
2 - Neurotransmitter binds receptor - opens intracellular binding site
3 - G-protein now binds and changes shape - GDP released
What does the alpha subunit + or - activate?
cGMP phosphodiesterase
What do alpha s and i subunits activate?
Adenylate cyclase
What does the alpha q subunit activate?
Phospholipase C
What does the alpha o subunit activate?
Ca2+ ion channels
What does the alpha i subunit activate?
K+ ion channels
What pathway follows once adenylate cyclase is activated?
Adenylate cyclase —> cAMP —> PKA
What two pathways follow after phospholipase C is activated?
Phospholipase C —> IP3 —> Ca2+
Phospholipase C —> DAG —> PKC
Describe the process by which adenylate cyclase is activated
1 - alpha subunit binds to adenylate cyclase
2 - binding opens the active site and ATP is converted to cAMP - signal transduction
3 - GTP in alpha subunit hydrolysed to GDP by alpha subunit
4 - GDP causes a change in shape of alpha subunit - weaker binding to enzyme so subunit departs - closing adenylate cyclase active site.
5 - alpha s subunit recombines with Beta-gamma dimer to reform Gs protein
What’s the benefit of the adenylate cyclase active site staying open as long as ås subunit is bound?
Several hundred ATP converted before ås - GTP is deactivated
- signal amplification
Cyclic AMP becomes secondary messenger - enters cell cytoplasm with message
What is protein kinase A?
A serine-threonine kinase
What activates PKA?
Cyclic AMP - need ATP
What does PKA do?
Catalyses phosphorylation of serine and threonine residues on protein substrates, turning the enzyme on
Phosphate provided by ATP
PKA subunit C phosphorylates 3 enzymes, what are those enzymes?
Glycogen synthase - inactivates, stops glycogen production
Phosphorylase kinase - activates
Inhibitor - activates inhibitor of phosphtase which stop Phos-a going back to Phos-b
What does phosphorylase kinase cause?
It phosphorylates phosphorylase kinase b to phosphorylase a
Converts glycogen to glucose-1-phosphate
What causes glycogen metabolism?
Triggered by adrenaline in liver cells
ås subunit binds adenylate cyclase - produces cAMP from ATP
cAMP activates PKA…
What is the coordinated effect?
Activation of glycogen metabolism & inhibition of inhibition of glycogen synthesis
How does the Gi-protein differ to the Gs-protein?
åi subunit inhibits adenylate cyclase
Adenylate cyclase is under dual control - depends on dominant alpha subunit
What are the key points to do with signalling cascade involving cAMP?
Explains signal amplification through the molecular ‘relay runner’ - G-protein
Explains how the message delivered reaches enzymes within the cell
Give the roles of the beta-gamma dimer
Can activate adenylate cyclase
Regulation by dimer becomes more important where more receptors are activated
Higher conc of dimer are required for same effect as monomer
Control other enzymes
What is phosphorylation involved in?
Key in the (de)activation of enzymes
Also involved in desensitisation of G-protein linked receptor
Where does the åq subunit come from?
A Gq-protein - same splitting mechanism as Gs and Gi
What does the åq subunit do?
It activates / deactivates phospholipase C (membrane bound enzyme)
-reaction catalysed for as long as åq subunit is bound, signal amplification
Brake and accelerator effect
Describe the Gq-protein effect on phospholipase C (PLC)
1 - åq subunit with GTP bound binds to PLC
2 - PIP2 bound splits into IP3 and DG, phosphate is lost from GTP
3 - GDP formed, reaction stops as åq subunit moves away / unbinds
What properties does Diacylglycerol have?
DG is hydrophobic so remains in cell membrane when formed
What does DG activate? What happens next?
Protein kinase C (PKC)
PKC then moves to cell membrane from cytoplasm and catalyses phosphorylation of serine and threonine residues of enzymes
What effects does Diacylglycerol induce?
Tumour propagation
inflammatory response
Contraction or relaxation of smooth muscle, etc.
How does IP3 differ to DG?
IP3 enters the cytoplasm as its hydrophilic
What is the action of IP3?
IP3 activates calcium stores, activating protein kinases through Ca2+ ions and calmodulin with Ca2+ bound
What happens to IP3 and DG once their tasks have been completed?
They are recombined through a complex synthesis - cannot be linked directly
What do lithium salts treat and how is this achieved?
Used to treat manic depression
Interfere with the complex synthesis for IP3 and DG recombination
What’s the benefit of phosphorylating kinase-linked receptors?
Each phosphorylated region can bind to a specific signalling protein or enzyme
What does Guanylate cyclase do and why’s it special?
It opens Na+ channels in kidney
Special as it acts as both a receptor and enzyme
What biological effect does Ca2+ cause?
Smooth muscle and cardiac muscle contraction
What biological effect does PKC cause?
Tumour propagation & inflammatory response
What biological effect does GAP have?
GTPase-activating proteins
