Receptor Mechanisms I and II
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What is an antagonist
Something that interacts with the binding site of a natural ligand
How do antagonists affect the receptor
Prevents agonist from interacting with the receptor
State the 4 main types of receptors
Ligand gated ion channel G protein coupled Tyrosine kinase linked Nuclear/steroid
Order the main 4 types of receptor by cellular response speed (fast - slow)
Ligand gated G protein coupled Tyrosine kinase linked Nuclear
Why do nuclear receptors produce the slowest responses
Altering gene transcription
Name the four main types of ligand gated ion channels
Nicotinic cholinoceptor - Acetylcholine 5HT3 - 5-HT GABAA - Gamma amino butyric acid (GABA) Glycine - Glycine
Describe the structure of ligand gated ion channels
5 similar proteins (Pentamer) Much diversity in subunit composition Four transmembrane domains in each protein - 4 x 5 = 20 total in a receptor.
What happens when specific molecule binds to receptor
Molecular kink, big central pore to allow influx of ions down electrochemical gradient.
Describe nicotine cholinoreceptor receptor action
Acetylcholine released, two molecules bind to alpha subunits. Influx of sodium down electrochemical gradient. Excitatory junction potential. Takes membrane potential to a threshold where voltage gated ion channels open
Describe myasthesia gravis
Muscle nicotine AChRs become degraded and immune cells target the alpha-1 subunit. The antibodies degrade the protein and receptors disappear. A chemical is nothing without its receptor Symptoms - Muscle weakness, fatigue, difficulty swallowing/talking
Describe the electrical recordings of membrane potential of a person with myaesthesia gravis against a normal person
At rest (normal) - small depolarisations MEPPs - miniature end plate potentials. Motor nerves releasing acetylcholine, bind to receptors for small depolarisations. Normal hyperpolarisation by acetylcholine (EJP achieved) At rest (Myaesthesia Gravis) - Not much happens, no mepps and EJP not attained
How many proteins are G coupled receptors made up of
One, coiled up to make 3D structure
Compare speed and duration of ligand gated ion channels and GPCR responses
Ligand gated - Fast, shorter lasting GPCR - Slow, longer lasting
How many transmembrane domains in GPCRs/How many times does the protein pass through the receptor
7
Describe GPCR at rest then receptor action
Alpha subunit binds GDP can exchange for GTP Broken down into 4 subunits - Alpha s, i, q and 12 Protein interacts with G proteins. Receptor activated, beta and gamma interacting with enzyme, products produce cellular response. Cyclical repetition.
Describe GPCR action
Agonist binds, alpha has GTP dissociates, interacts with enzymes.
What do each of the subunits of the alpha subunits do
Alpha S - turns on adenylate cyclase Alpha I - turns down adenylate cyclase Alpha Q - activates phospholipase C
What enzyme does the alpha subunit contain and how is this significant
GTPase - Removes phosphate groups. GTP hydrolysed to GDP. Alpha subunit then has increased affinity for beta gamma subunits than the enzyme, causing it to rebind (cyclical process)
What does adenylate cyclase do when activated
Creates cAMP from ATP. cAMP converts inactive protein kinase A to activated protein kinase A. This phosphorylates other proteins. Level of cAMP reduced by phosphodiesterases (PDEs). They are potential targets to modify post receptor signalling.
What does alpha GQ/11 do
Increases phospholipase C activity - Works on a phospholipid in our cell membrane. Produces signal molecules - PIP2 - Phosphatidylinositol bisphosphate Ip3 Inositol trisphosphate
What does phospholipase C do to PIP2
PIP2 cleaved to Ip3 and diacylglycerol (DAG)
What does IP3 do
Interacts with IP3 receptor on organelles, to release calcium
What does DAG do
Activates protein kinase C