Session 7 Flashcards
How do ligand gated ion channels work?
Ligand binding ‘gates’ the channel to allow ions to move into or out of the cell (e.g Na+ in, K+ out)
What are the super families of cell-surface receptor?
Ligand-gated (receptor operated) ion channels
Receptors with intrinsic enzymatic activity
G protein coupled receptors
How do receptors with intrinsic enzymatic activity work?
Ligand binding activates an enzymes activity that phosphorylates the receptor itself and other substrates –> signalling scaffold leading to signalling cascade
How do G proteins-coupled receptors work?
Agonists - bind to receptor and activate it
Antagonists - bind to receptor but do not activate it
Give some examples of therapeutic application of drugs targeting GPCRs
CNS - depression, schizophrenia, psychosis, Parkinson’s, migraine
CVS - hypertension, congestive heart failure, cardia arrythmia
Respiratory - asthma, COPD
GI - acid reflux, gastric ulcer, nausea
Others - chronic pain, glaucoma, rhinitis, motion sickness, anaphylaxis
Broadly speaking what can GPCRs respond to?
Ions (H+, Ca2+)
Neurotransmitters (e.g. acetylcholine, glutamate)
Peptide & non-peptide hormones (e.g. glucagon, adrenaline)
Large glycoproteins (e.g. TSH)
What is the basic structure of GPCRs?
Single polypeptide chain (300-1200 aa), 7 transmembrane spanning regions, extracellular N-terminal, intracellular C-terminal
Which regions of GPCRs can be responsible for ligand binding?
Can be formed by 2/3 of the transmembrane domains
Can be the N-terminal region
How many subunits are there in GPCRs?
What are they called?
Three.
They are called, alpha, beta and gamma subunits
What is the significance of the GPCR-G protein interaction?
It activates the G protein by causing GTP to be exchanged for GDP on the G protein alpha subunit
What happens when a G protein is activated?
The alpha subunit dissociates from the beta-gamma complex. They then interact with effector proteins. These interactions last until the alpha subunit GTPase activity hydrolyses GTP to GDP, at which point the alpha subunit and the beta-gamma subunits then reform an inactive heteromeric complex.
What effect does nor/adrenaline have on GPCRs?
Effects beta-adrenoreceptor and Gs alpha. Effector = activation of adenylyl cyclase
Effects alpha2-adrenoreceptor and Gi alpha. Works to inhibit adenylyl cyclase
What effect does light have on GPCRs?
Acts on rhodopsin, Gt alpha and activates cyclic GMP phosphodiesterase
What effect does ACh have on GPCRs?
Acts on M2/M4 muscarinic receptors and Gi alpha. Works to inhibit adenylyl cyclase
Acts on M1/M3 receptors and Gq alpha. Works to activate phospholipase C
What effect does pertussis toxin have on GPCRs?
Pertussis toxin contains an enzyme (ADP-riobsyl transferase) activity which specifically modifies (and inactivates) Gi-type proteins, “uncoupling” receptor-effector linkage.
What effect does cholera toxin have on GPCRs?
Cholera toxin contains ADP-ribosyl transferase enzyme activity which specifically modifies Gs type proteins, in this case leading to irreversible activation
Give examples of enzyme effectors
Adenylyl cyclase catalyses conversion of ATP –> cAMP
Phospholipase C catalyses PIP2 –> IP3 + DAG
PI3K catalyses PIP2 –> PIP3
cGMP phosphodiesterase catalyses cGMP –> 5’-GMP
Give examples of effectors that are ions
Voltage operated Ca2+ channels (VOCCs)
G-protein regulated inwardly rectifying K+ channels (GIRKs)
Gs coupled receptors
Gi coupled receptors
What is the function of phospholipase C?
Catalyses cleavage of membrane phospholipid (PIP2) into 2 second messengers (IP3 & DAG). Very important for Ca2+ regulation intracellularly
Give an example of signal amplification
Adrenaline binding to cell surface beta adrenoreceptors causing a relatively big cellular response
Give a general overview of how a ligand binding to a GPCR can cause signal amplification
Ligand binds to receptor, G protein activated, adenylyl cyclase activated (enzyme/ion), cAMP activated, enzymes (e.g. PKA) activated.
What is inotropy?
Inotrope is an agent that alters the force or energy of muscular contractions. Negatively inotropic agents weaken the force of muscular contractions. Positively inotropic agents increase the strength of muscular contraction.
What causes positive inotropy of the heart?
How?
Adrenaline (blood borne) and noradrenaline (sympathetic NS). They interact with ventricular beta1-adrenoreceptors to increase force of contraction
How does smooth muscle contract?
Sympathetically released NA can interact with vascular smooth muscle alpha1-adrenoceptors to cause vasoconstriction.
Parasympathetically released ACh can interact with bronchiolar SM M3-muscarinic receptors to cause bronchoconstriction.
How is neurotransmitter release modulated?
Pre-synaptic GPCRs. G beta and gamma subunits inhibit specific types of VOCCs, reducing Ca2+ influx and neurotransmitter release.
What is diversity?
Range of stimuli, receptors, G proteins and effectors
What is specificity?
Ligand-receptor interactions, specific G-protein alpha subunits (beta and gamma) recruitment, coupled to particular effector pathways
What is amplification?
When small changes elicit significant changes in cellular behaviour
Describe the process of deactivation
Binding of agonist molecule weakens and dissociation is more likely, stimulation of GTPase activity, enzymatic activities in cell, enzymatic cascades are activated downstream of 2nd messenger/PK activation are opposed by activities which act to reverse the second messenger/PK effect.
