Signal Transduction Flashcards
Describe paracrine signalling and give an example.
Paracrine signalling (neurotransmitters) - signalling molecules only affect target cells in close proximity to releasing cells.
Contact-dependant - signal doesn’t travel at all, based on the contact between the signalling cell and target cell.
This is recognized by a receptor exposed on the membrane of the target cell. This is typical of antigen presenting cells.
Why do receptors have to be so specific in the way they bind?
Because receptors can bind at high affinity even if there is a low concentration of molecules.
What are cell surface receptors?
Receptors that binds ligands which are represented by hydrophilic signalling molecules. They will never be able to enter the cell so needs to bind to an extracellular receptor.
What are intracellular receptors?
Receptors that bind ligands which are hydrophobic because they can pass membranes without aid of extracellular receptors.
Why is the type of receptor also important in cell transduction?
Because the same type of signal can cause different cells to respond in different ways.
Therefore cell response not only depend on the signals, but also the type of receptor and intracellular mediator.
What are the 3 major classes of cell surface receptor proteins?
Ion-channel-coupled receptors
G-protein-coupled receptors
Enzyme-coupled-receptors
Describe how ion-channel-coupled receptors work.
Also called transmitter-gated ion channels or ionotropic receptors.
They are involved in rapid signalling, i.e. synaptic signalling which is mediated by a small number of neurotransmitters that can transiently open or close protein ion channel, the ion permeability of PM is changed by binding.
Describe how G-protein-coupled receptors work.
Coupled to a GTP binding protein (G protein) this is needed to mediate interaction between receptor and target protein.
An extracellular signal changes the conformation of GPCR, triggering its interaction with a G protein.
The activation of a G protein changes concentration of one or more small intracellular signalling proteins.
Describe how enzyme-coupled receptors work.
Also know as catalytic receptors, they are usually single-pass transmembrane receptors.
Has 2 sides - an extracellular face and internal face. The binding of a ligand to the extracellular face causes enzymatic activity on the internal face.
What are intracellular signalling molecules?
Intracellular signalling molecules can either be small molecules or other proteins. When they are small molecules, they are often called second messengers.
They are small and hydrophobic, therefore easily diffuses in the cytoplasm.
Other lipid based second messengers which can diffuse in the channels of the PM. Pass signal on by binding to and altering behaviour of selected signalling or effector proteins.
What are characteristics of GTP binding proteins?
- Switch between ‘on’ state when GTP is bound and ‘off’ state when GDP is bound.
- Can shut themselves off when hydrolysing GTP.
- 2 major types: Large trimeric GTP-binding proteins (G proteins) help relay signals from G-protein-coupled receptors that activate them.
- Small monomeric GTPase (monomeric GTP-binding proteins) help relay signals from many classes of cell-surface receptors.
How are GTP binding proteins regulated?
- GTPase-activating proteins (GAPs) - drives proteins into an ‘off’ state by increasing rate of hydrolysis of bound GTP.
- Guanine nucleotide exchange factors (GEFs) - activates GTP-binding protein by promoting release of bound GDP, which allows new GTP to bind.
How do cells avoid unwanted cross-talks and interference between signalling systems?
- High affinity and specificity
- Many downstream target proteins have the ability to ignore other signals.
Describe the structure of G-protein-coupled receptors.
- 1 polypeptide chain that threads back and forth seven times across the lipid bilayer.
- Cylindrical structure.
- Often deep ligand-binding site at centre.
- All use G-proteins to relay signal into the cell interior.
Describe the activation of G proteins.
Extracellular signal arrives.
By binding to the receptor, signal changes conformation of receptor and thus the G-protein, which can recognise and bind to the receptor.
This binding by alpha subunit opens AH domain where GDP was contained.
GDP released.
Beta and Gamma subunit dissociates and everything’s activated.
How do G-protein-coupled receptors regulate the concentration of cAMP?
When an extracellular signal arrives and binds to a receptor, this can be translated very quickly resulting in rapid increase in cAMP.
It needs to be synthesised and removed very quickly.
Adenylyl cyclase is the enzyme for synthesis, while cyclic AMP phosphodiesterases is for breakdown.
Most isoforms of adenylyl cyclase are regulated by Ca2+ and G proteins.
Extracellular signals activate GPCRs that are coupled to stimulatory G proteins (Gs). Activated alpha subunit of Gs binds and activates adenylyl cyclase.
Opposite signals act through different GPCRs can reduce cAMP levels by activating an inhibitory G protein (Gi), which inhibits adenylyl cyclase.
Give examples of toxins that can disrupt the regulation of cAMP.
Cholera toxin alters alpha subunit causing it to no longer hydrolyze bound GTP and stay active. Consequently high cAMP concentration causes severe diarrhea.
Pertussis toxin prevents the alpha subunit of Gi from interacting with receptors and keeping inactive state, therefore unable to regulate target proteins.
How does Cyclic-AMP-Dependent Protein Kinases (PKA) mediate the effects of cAMP?
In its inactive state, PKA consists of 2 catalytic subunits and 2 regulatory subunits.
Binding of cAMP to regulatory units changes protein conformation, causing these units to dissociate from complex.
Released catalytic subunits are activated. They can then migrate into the nucleus to phosphorylate transcripion regulatort genes. The catalytic subunits remain in the cytoplasm.
Describe the function of NO as a gaseous mediator for blood vessels.
NO in mammals have the function of relaxing smooth muscle in the wall of blood vessels.
Acetylcholine stimumlates NO synthesis by binding to a GPCR of an endothelial cell. This then triggers IP3+ synthesis and Ca2+ release.
This in turn stimulates NO synthase. The NO produced can pass through to membranes of neighbouring smooth muscle cells.
What are some other clinical applications for NO?
- Nitroglycerin used for angina (chest pain from inadequate blood flow to heart). Nitroglycerine is converted to NO and relaxes blood vessel muscles to reduce the workload of the heart.
- Viagra - NO binds in a reversible way to iron in the active site of guanylyl cyclase, stimulating synthesis of cGMP.
The normal turnover rate of cGMP is very high. Its regulated by degradation by phosphodiesterase. NO interacts with iron and therefore the enzyme, which inhibits the degradation of phosphodiesterase, thereby increasing the amount of cGMP in the penile blood vessels, keeping it relaxed and consequently the penis erect.
What are the differences between G-protein-coupled receptors and enzyme-coupled receptors?
Their cytosolic domain doesn’t associate with a G protein, but instead has either intrisic enzyme activity or associates with an enzyme.
GCPR has 7 transmembrane segments, each enzyme coupled receptor subunit has 1.
Describe the structure of protein kinases and how this affects their function.
They have an external part for ligand binding, which when activated by a ligand, leads to the sctication of the tyrosine kinase domain on the cytocylic side.
This means that the tyrosine side chains are phosphorylated, many times they are autophosphorylated. In turn this will create a docking site for other proteins that relay the signal.
How does protein kinases work?
Ligand binding causes the receptors to dimerize. This means that the 2 cytoplasmic kinase domains are brought together.
Trans-autophosphorylation (one kinase phosphorylatres the other) generates binding sites for other proteins that are then activated and relays the signal.
How does the epidermal growth factor receptor (EGFR) work?
Without ligands, they exist as inactive monomers.
2 ligand molecules are needed to promote dimerizations of external domains.
One kinase domain (activator) pushes the other (receiver) resulting in an activating conformational change in the receiver.
The receiver phosphorylates tyrosines in the C-terminal tails of both proteins, creating docking sites for intracellular molecules.
