Flashcards in M&R Session 6 Deck (101):
How can intracellular signalling occur?
Secretion of signalling molecules
Where adhesion proteins in adjacent cells are in contact
How can signalling by secreted molecules be subclassified?
What is paracrine signalling?
Signal molecules act on adjacent cells
What is endocrine signalling?
Signal molecules enter the bloodstream and act on distal tissues
What is synoptic signalling?
Use of neurotransmitter as a signal molecule
How do hydrophilic signalling molecules work?
Bind to cell surface receptors
How do hydrophobic signalling molecules work?
Transported by carrier protein and bind to intracellular receptors in cytoplasm or nucleus
What is a receptor?
A molecule that recognises specifically a second molecule or family of molecules and in response brings about regulation of a cellular process
Describe an unbound receptor.
What is a ligand?
A molecule that binds specifically to a receptor site
What is an antagonist?
A ligand which binds with the receptor site but does not cause activation
Does an antagonist switch off a receptor?
No. It prevent agonist binding therefore prevents switch on
What is an agonist?
A ligand which binds and causes activation of a receptor
What are the roles of receptors in cellular physiology?
Signalling by hormones/local chemical mediators
Modulation of immune response
Release of intracellular calcium stores
Control of gene expression
Sorting if intracellular proteins
How does the binding affinity at receptor sites compare to at enzyme sites?
Generally much higher
What accounts for the difference in affinity b/w receptor and enzyme binding sites?
Ligands generally have to travel much further than substrates and allosteric regulators for enzymes therefore are more diluted
What is KD?
The concentration of ligand required to half fill all available receptors
What are they comparative ranges of affinity for receptor and enzyme binding sites?
Receptor = 10^-9 M to 10^-6 M
Enzyme = 10^-6 M to 10^-3 M
How are receptors classified?
Affinity to a series of antagonists
How are ACh receptors classified?
Muscarinic into M1-M5 determined by the antagonists for which they have the highest affinity for
Give the antagonists which the receptors M1-M3 have the highest affinity for respectively.
M1 = Pirenzipine
M2 = Gallomine
M3 = Hexahydrosiladiphenol
How does an acceptor differ to a receptor?
An acceptor operates in the absence of ligand and ligand binding alone produces no response
Give two examples of acceptor molecules, explaining why they are not receptors.
Dihydrofolate reductase - functions in absence of methotrexate
Sodium channel - modulated by binding of other chemicals and functions w/out anaesthetic
What are membrane-bound receptors w/integral ion channels also called?
Classical ligand-gated ion channels
Describe the structure of nAChR.
5 subunits put -ve residues at mouth of channel
Give 5 examples of membrane-bound receptors w/integral ion channels.
What ions do nAChR permit the passage of?
What is the function of gamma amino butyric acid receptors?
Gated chloride channels which allow membrane hyperpolarisation
What ion movement do glycine receptors control?
Give examples of glutamate receptors and state what ionic movement they control.
All allow gated calcium entry
What does inositol 1,4,5-triphosphate allow?
Gated released of calcium from the ER
Give three examples of non-classical ligand-gated ion channels.
ATP-sensitive potassium channel
Describe the structure of ATP-sensitive potassium channels.
A channel pore with an ATP molecule on the intracellular side between 2 transmembrane domains
How does the structure of a purinoceptor relate to that of an ATP-sensitive potassium channel?
The same except for the shape of the channel pore and the location of the ATP molecule on the extracellular side
Describe the structure of a ryanodine receptor.
Four transmembrane domains with calcium and ryanodine linked to the first domain
Give two examples of membrane-bound receptors w/integral enzyme activity.
Atrial natriuretic peptide receptor
Dimeric growth factor receptor
What are ANP receptors directly linked to?
Guanylyl cyclase (GTP --> cGMP)
What are dimeric growth factor receptors directly linked to?
Describe the mechanism of dimeric growth factor receptors.
Allow phosphorylation of tyrosine for growth
Give some examples of the resultant molecules from activation of dimeric growth factor receptors.
Epidermal growth factor
Platelet-derived growth factor
What is autophosphorylation?
Phosphorylation of the chain in the opposite subunit of a dimer - allows cross communication
How does autophosphorylation facilitate the function of a tyrosine kinase-linked receptor?
The multiple phosphorylated a.a. residues give a specific site for binding which is recognised by transducing proteins which are allosterically activated/phosphorylated to regulated an intracellular event
How are insulin receptors synthesised?
Alpha and beta chains are made together and cleaved extracellularly, held together in the membrane by sulphide bridges
Describe the structure of insulin receptors in the plasma membrane.
Two tyrosine kinase domains intracellularly
Two transmembrane domains joined by a disulphide link (beta chains)
Two insulin binding domains linked to the two transmembrane domains by disulphide links (alpha chains) extracellularly
Give three examples of membrane-bound receptors that signal through transducing proteins.
Coupled through GTP-binding regulatory proteins to enzymes/channels
Beta-adrenoreceptors activate the enzyme adenylyl cyclase via Gs
M2 stimulates potassium channel opening by Gi
Where are binding domains found in GPCRs?
W/in the plane of the bilayer
How many transmembrane receptors are there in membrane bound receptors that single through transducing proteins?
Where are both beta-adrenoreceptors and M2 receptors found?
In the heart
Describe the function of activating stimulatory/inhibitory receptors in receptors that use transducing proteins.
Receptor binding causes dissociation of GTP from alpha-s/i which acts in the target enzyme to stimulate/inhibit it
Describe the mechanism of action of beta-adrenoreceptors.
Adrenaline binds--> G-protein subunit dissociation (alpha and beta move away)--> Gs-alpha binds to target enzyme adenylyl cyclase whilst it is combined with a new molecule of GTP --> cAMP produced
What changes between intracellular receptors?
Their primary structure
What do all hydrophobic signals use?
Similarity of sequence of intracellular receptors
Describe the mechanism of intracellular receptor activation.
Binding of hormone --> huge conformational change --> inhibitory protein complex removed --> DNA binding site exposed
What is the DNA binding site of an intracellular receptor also known as?
What are intracellular receptors silent at rest?
The DNA site is blocked
Describe the mechanism of amplification.
Single, v. low concentration signal molecule binds in one receptor --> small number of GPCR activated --> cAMP --> PKA --> emzyme --> products
What is the magnitude of amplication by receptors?
Compare the activation and inhibition of receptors in hepatocytes.
Insulin stimulates glycogenesis
Glucagon stimulates glycogenolysis
How is heart rate increased in cardiac pacemaker cells?
Noradrenaline binds to beta-1 adrenoreceptors
How is heart rate decreased in cardiac pacemaker cells?
ACh binds to M2 muscarinic receptors
How is the plasma membrane trafficked to its final position?
From the ER to the CSM via the exocytic secretory pathway
What prevents the CSM being excessively ruffled?
Endocytosis of the membrane
What is the pathway of vesicular transport of membrane?
Donor organelle --> membrane vesicle --> trafficked to destination --> fuse w/recipient organelle
What is the function of pinocytosis?
Permits uptake of extracellular so lutes
How does pinocytosis take place?
Invagination of the CSM to form a vesicle
What is the function of phagocytosis?
Internalisation of particulate matter
What is the mechanism of phagocytosis?
Receptors sequentially bind to 'zip up' around engulfed material
Selective internalisation of molecules into cell by binding to specific cell surface receptors
What is cholesterol uptake an example of?
Describe the structure of LDLs
Core of esterified cholesterol
Contain apoprotein B
Core covered by a phospholipid and cholesterol monolayer
What do triskelions comprise of?
3 clathrin heavy chains
3 clathrin light chains
Always arranged in the same way
What allows the coat structure to form vesicles instead of tubes and sheets?
The hexagon and pentagon structure
Describe the mechanism of clathrin coating.
Clathrin coated pits form spontaneously --> protein coats prevent vesicle binding so must be removed --> uncoated by an ATP-dependent uncoating protein
How do clathrin coated pits form spontaneously?
Association of more and more triskelions on cell face
What happens to the clathrin triskelions removed by the uncoating protein?
Recycled back to newly forming clathrin coated pits
What two mutations can occur which affect LDL receptor in hypercholesteroleamia?
Non functional receptor
Receptor binding normal but no internalisation
What happens when there is no internalisation of LDL receptors?
LDL receptors are over the whole cell surface, not localised into pits
Deletion of C-terminal cytoplasmic domain prevents interaction w/clathrin coat
What is the fate of the ligand and receptor in cholesterol uptake?
What is CURL?
Compartment of Uncoupling of Receptor and Ligand
What is the affect of the low pH in the endosome?
It decreases the affinity of the receptor for LDL
What molecule allows uptake of ferric ions?
Describe the uptake of ferric ions.
Ferrotransferrin binds to transferrin receptor in coated pits --> coated vesicles --> pH in vesicle decreases due to hydrogen-ATPase --> ferric ions released from CURL --> apotransferrin released by CURL in pH 5 vesicle --> neutral pH at cell surface causes dissociation of apotransferrin from receptor
Why is it useful for the ligand to be recycled in ferric ion uptake?
It is a large molecule so you don't want to use lots of energy having to make it
What is the fate of the ligand and receptor in ferric ion uptake?
What is the fate of the ligand and receptor in the endocytosis of insulin?
What happens if insulin is high for a long time?
Receptors are removed to prevent overreaction but then it takes a few hours to synthesise replacements
How does chronic hyperglycaemia lead to T2DM?
Vicious cycle arises as the body works v.hard to make enough receptors --> beta-cells give up --> T2DM
What can cause insulin deficiency?
What can cause insulin resistance?
What is the fate of the ligand and receptor in endocytosis of immunoglobulin?
Describe the endocytosis of immunoglobulin.
Binds to receptor in coated pits --> coated vesicle --> uncoated vesicle --> endosome --> transfer vesicle --> bile canaliculi
What is the endocytosis of immunoglobulin an example of?
Transcytosis of a large molecule across a cell
How is IgA released?
Proteolytic cleavage of the receptor --> see small part of the ligand w/receptor
How do membrane-enveloped viruses take advantage of receptor mediated endocytosis?
Fortuitous association w/cell receptors
Unfolding hydrophobic domains in membrane fusion proteins in response to the acidic pH of the endosome
Insert membrane fusion proteins into endosome membrane --> membrane fusion and release of genomic RNA into cytosol
Use host machinery to replicate RNA and capsid proteins
How do cholera and diphtheria toxins act?
Bind to GM1 ganglioside which is incorporated into the vesicle
Do cholera and diphtheria bind to a receptor?
Name three coat proteins.
What are COP?
Family of coat proteins for vesicles from different organelles
What shapes are COP made up of?
Squares and triangles
Where is COPI from?