Lecture 6 part 1 Flashcards
What are the 2 types of intercellular signalling?
- Signalling by secreted molecules
- Signalling by plasma-membrane bound molecules
Difference between paracrine and autocrine
Paracrine signaling: a cell targets a nearby cell (one not attached by gap junctions). The image shows a signaling molecule produced by one cell diffusing a short distance to a neighboring cell. An example of paracrine signalling is neurotransmitter.
Autocrine signaling: a cell targets itself, releasing a signal that can bind to receptors on its own surface.
3 types of signalling by secreted molecules:
Describe the process of paracrine, endocrine and synaptic signalling by secreted molecules.
- Paracrine = signal molecule here is called a local mediator, local mediator is released from signalling cell into interstitial space, signalling molecule then recognised by target cells within the tissue
- Endocrine = (hormone) mediator released into blood stream and circulates around body to have effect on long distance targets.
- Synaptic = NT released into synaptic cleft to bind to receptors on post-synaptic membrane/target cell.
What are the signalling molecules used in paracrine, endocrine and synaptic signalling? Why does this classification no longer hold true?
- Paracrine = local chemical mediators.
- Endocrine = Hormones.
- Synaptic = Neurotransmitters
There can be overlap between categories, e.g.: NT’s can be hormones.
Cell-surface and intracellular receptors:
What type of ligands bind to cell surface receptors? What type of ligands bind to intracellular receptors?
- Hydrophilic (bind to cell surface receptors)
- Hydrophobic (bind to intracellular receptors, as they can pass hydrophobic bilayer)
What is the definition of a receptor?
A molecule that recognises a second molecule (ligand) which regulates a cellular process in response to binding In the unbound state, a receptor is functionally silent when a ligand is not bound to it.
What is a ligand and the 2 classes a ligand can be?
- A ligand is a molecule that binds specifically to a receptor site.
2 classes:
- Agonist if it produces activation of a receptor.
- Antagonist if it binds without causing activation, opposing the effects of agonist binding.
List some roles of receptors in cellular physiology
- Signalling by hormones/local chemical mediators
- Neurotransmission
- Cellular delivery
- Control of gene expression
- Cell adhesion
- Modulation of the immune response
- Sorting of intracellular proteins
- Release of intracellular calcium stores
How tightly does a ligand bind to it’s receptor site?
- Is the affinity for ligands higher for receptor or enzyme binding sites and why?
• Affinity of ligand binding at receptor sites is generally much higher than binding of substrates and allosteric regulators to enzyme sites
- Receptor have a higher affinity of ligand binding because ligands are very dilute as they are released from signalling tissue and diluted during their passage to the target tissue, so in low concentrations. Receptor therefore have to have a high affinity as ligands are in low concentrations
look at pic below - Kd is sort of equivalent to Km
How is a receptor classified and sub-classified?
- Classifieds according to the agonist that binds to it
- Sub-classified by antagonist that occupies the sub-type
E.g.: mAChR is classified by muscarine binding, and sub-classified (M1-M5) by the different antagonists that bind.
What is the difference between a receptor and acceptor?
Receptor = silent at rest, binding stimulates biological response. Acceptor = operate in absence of ligand, ligand binding alone wont produce response.
Example of acceptors
- Sodium channel (often called receptors to local anaethetic agents etc, but sodium channels can carry out their activity in the absence of these two modulators, so are acceptors).
What are the 4 different receptor types/mechanisms by which transduce a signal from a ligand? (list from fastest to slowest)
1) Membrane bound receptor w/integral ion channels (ionotropic receptors), this causes a change in membrane potential
2) Membrane-bound receptors w/integral enzyme activity (next fastest, as the conformational change is connected directly within the protein, to the enzyme activity - producing the intracellular message)
3) Membrane-bound receptors coupled to effectors through transducing proteins (GPCR’s) (slower as the transducing membrane may have to move within the membrane between activated receptor and effectors)
4) Intracellular receptors slowest - as need to have transciption and translation, can take up to 72 hrs for full response
- *First superfamily: Ligand-gated ion channels**
- What is the “model” for ligand-gated ion channel receptor (ionotropic receptor)?
- Example
- Describe its structure. Do all of these channels have this structure?
- 4 transmembrane domains, binding site at N-terminus) -see pic attached
- Example: The nAChR.
- Structure of nAChR:
It is a pentameric complex: 5 subunits coming together to form an integral ion channel. 2 of the subunits (alpha subunits) bind to th acetylcholine. Gate is opened upon binding of ACh, and negatively charged AA’s in pore select for cations to move through, causing DP. (The binding domain is in the N-terminus region.) - No, other ionotropic receptors have different structures.
Membrane-bound receptors with integral ion channels - only 4 in this family (don’t need to learn in all this detail?)
GABA - leads to polarisation of the synapse
2nd Superfamily: Membrane-bound receptors with integral enzyme activity
Describe the structure of membrane-bound receptors with integral enzyme activity
- *- Ligand-binding domain:** Extracellular to allow easy access for ligands. Strong affinity for specific ligands, allows different ligands that bind to the same receptor to evoke particular cellular responses
- Transmembrane domain: Contains a series of hydrophobic amino acids.
- Cytosolic ‘active’ enzyme domain: The intracellular domain of the receptor itself is an enzyme or interacts with an enzyme
Membrane-bound receptors with integral enzyme activity - explain how this works
- Ligand binds to ligand binding site.
- This causes two RTK’s (receptor tyrosine kinase) to then ‘come together’ and act together. RTK’s coming together forms a ‘cross-linked dimer’
- RTK’s need to act in pairs because cross-linking activates the tyrosine kinase activity.
- Now the tyrosine is active it binds with a phosphate.
- Each RTK in the dimer, phosphorylates the tyrosine on the other RTK (in reality, there are actually multiple RTK’S). The process of one phosphorylating the other is called cross-phosphorylation.
It happens like this: The tyrosine cause ATP -> ADP + Pi. The tyrosine molecules then ‘picks up’ this ‘free floating’ phosphate group. Each one of the tyrosine picks up a phosphate group. (Remember, one RTK will phosphorylate the other one.) - Once cross phosphorylated, two pathways can occur (next slide)
Membrane-bound receptors with integral enzyme activity (2 pathways that occur)
1st pathway:
Following the phosphorylation, the tyrosine residues can be recognised by specific binding sites on enzymes. The enzymes that binds might be activated directly by binding or the enzyme binds and is phosphorylated itself by the tyrosine. This results in an activated phosphorylated enzyme.
2nd pathway: MORE COMMON
Transducers have a src homology domain that recognises the phosphotyrosine. This domain binds to the phosphorylated tyrosine, the transducer is then phosphorylated. This transducer acts as a docking site for multiple different enzymes. The transducers activate the enzymes by then going on to phosphorylate the enzyme. Advantage of phosphorylating a transducer:
Phosphorylating a transducer (rather than an enzyme straight away) allows different enzymes from different pathways to be activated by this transducer. Therefore, increasing a hormone in the blood, can lead to a variety of responses.
Membrane-bound receptors with integral enzyme activity - groups of receptors that are this type
- ANP
- Growth factors like insulin, epidermal growth factor (EGF), platelet-derived growth factor (PDGF)
