Session 6 Flashcards
Compare and contrast receptor binding sites and the active sites and regulatory sites of enzymes
Similarities - specific, specificity governed by shape of binding cleft, have specific effects, reversible, induce a conformational change
Differences - ligands bind with higher affinity, ligand is not modified chemically
Define and discuss the terms receptor, ligand and acceptor
Receptor - a molecule that recognises specifically a ligand or family of ligands, brings about regulation of a cellular process
Ligand - any small molecule that binds specifically to receptor site, may produce an activation (agonist) but unable to elicit the maximum response possible (partial agonist) or no activation (antagonist)
Acceptor - operates in the absence of any signalling molecule, basic function can occur without the interaction of a ligand
Discuss why signal transduction is necessary for some signalling molecules
Hydrophobic signalling molecules can pass through plasma membrane whereas hydrophilic molecules can’t - signal transduction is necessary - presence of an extracellular receptor at the cell surface
- Integral ion channels - agonist binding to a ligand gated ion channel causes a conformational change allowing ions to pass through down an electrochemical gradient e.g. nAChR, RyanodineR
- Integral enzyme activity - agonist binding causes a conformational change which activates an intrinsic enzyme activity e.g. tyrosine kinase autophosphorylates and transduces the message into an intracellular chemical event
- Transducing proteins - 7TMDR couple to effector molecules via GPCRs. Separate GPCRs can act to both stimulate and inhibit the effector to produce a measured effect = integrated signalling
Discuss intracellular receptors
Hydrophobic ligands e.g. steroid hormones, cortisol, oestrogen, testosterone and T3/T4 can pass through the plasma membrane and bind to receptors inside the cell.
In their resting state they are bound to heatshock or chaperone proteins. The activated receptor dissociates from the stabilising protein and translocates into the nucleus, where it binds to control regions of DNA to regulate gene expression.
The action of intracellular receptors are relatively slow.
Explain amplification in cellular signalling
By stimulating the activity of an enzyme, the binding of a chemical signal molecule to a single receptor can cause the modification of hundreds or thousands of substrate molecules. An enzymatic cascade can produce further amplification.
Give an example of how responses to different receptors can lead to cellular activation or inhibition
Cardiac pacemaker cells:
Noradrenaline –> b-adrenoceptors –> increased heart rate
Acetylcholine –> M2 muscarinic receptors –> decreased heart rate
Hepatocytes:
Insulin –> B-cells –> glycogen synthesis
Glucagon –> a-cells –> glycogen lysis
Describe the processes of phagocytosis, pinocytosis and receptor mediated endocytosis (RME)
Phagocytosis - cell extends pseudopods which permit further receptor interactions and membrane invagination and particle internalisation via a ‘membrane zippering’ mechanism. Internalised phagosomes fuse with lysosomes to form phagolysosomes in which particle material is degraded
Pinocytosis - the invagination of the plasma membrane to form a lipid vesicle. This permits uptake of impermeable extracellular solutes and retrieval of plasma membrane. Subdivided into two forms: fluid phase, receptor mediated endocytosis
RME - specific binding of molecules to cell surface receptors permits the selective uptake of substances into the cell
Describe the uptake of cholesterol as an example of receptor mediated endocytosis (RME)
LDLs consist of a core of cholesterol molecules esterified to fatty acid surrounded by a lipid monolayer containing phospholipids, cholesterol and apoprotein B.
LDL receptors recognise apoprotein B.
These receptors are localised in clusters over clathrin coated pits. These pits form spontaneously and clathrin spontaneously forms cages.
When LDL binds to the receptors the pit invaginates to form coated vesicles. The vesicles are uncoated in a process that requires ATP. The vesicles fuse with endosomes.
The pH of the endosomes is lower than the cytoplasm (5.5-6.0). At this pH the LDL receptor has a low affinity for the LDL particle and the two dissociate. The endosome is also known as the Compartment for the Uncurling of Receptor and Ligand (CURL)
The LDL receptor is recycled to the plasma membrane while the LDL fuses with a lysosome where the cholesterol is hydrolysed from the esters and released into the cell
Describe the mutations affecting LDL receptors in hypercholesteraemia
Non functioning receptor - mutation to LDL binding site
Receptor binding normal - deletion of c terminal cytoplasmic domain, prevents the interaction between receptor and clathrin coat - LDL receptors distributed over whole surface instead of being concentrated
Receptor deficiency - prevents expression of LDL receptor
Describe the uptake of Fe3+ ions by transferrin as an example of RME
Two Fe3+ ions bind to apoptransferrin to form transferrin in the circulation. Transferrin binds to the transferrin receptor and is internalised. Upon reaching the acidic endosome, the fe3+ ions are released from the transferrin, but at this pH the apoptransferrin remains associated. The complex is sorted in the CURL for recycling back to the plasma membrane where at pH 7.4 the apoptransferrin dissociates from the receptor again.
Describe the uptake of occupied insulin receptors
Insulin receptors only congregate over clathrin coated pits when their agonist binds. Insulin binding causes a conformational change in the receptor that allows it to be recognised by the pit.
In the endosome, insulin remains bound to the receptor and the complex is targeted to the lysosomes for degradation.
This allows for reduction in the number of receptors at the cell surface membrane, desensitising the cell to a continued presence of high insulin.
Explain transcytosis
Some ligands that remain bound to their receptor may be transported across the cell.
E.g. maternal immunoglobulins to the foetus via the placenta, transfer of IgA from the circulation to bile in the liver (receptor is cleaved resulting in the release of Ig with a bound ‘secretory component’ derived from the receptor)
Compare and contrast the four modes of RME
LDL - receptor recycled, ligand degraded, metabolite uptake
Transferrin - receptor recycled, ligand recycled, metabolite uptake
Insulin - receptor degraded, ligand degraded, receptor down-regulation
Maternal IgG - receptor transported, ligand transported, transfer of large molecules across cell
Describe how membrane enveloped viruses and toxins take advantage of RME
Adventitious binding at plasma membrane. Once at the endosome the acidic pH allows the virus to fuse with the endosomal membrane, releasing the viral RNA into the cell where it can be translated and replicated by the host to form new viral particles e.g. cholera and diphtheria toxins bind to GM1 ganglioside
