Hormone Mediated Cell Transduction Flashcards
What are the receptor classes?
Tyrosine Kinase Receptors - they are enzymatically active and undergo autophosphorylation
G-protein coupled receptors - they activate downstream signalling
What are the potential pathways for the receptor classes?
TKR 1. Ras -> MAPK -> Effector 2. PI3K -> PIDK -> Effectors GPR 1. cAMP -> PKA -> Effectors 2. DAG -> PKC -> Effectors 3. IP3 -> Ca2+ -> Effectors This shows amplification of signalling
Describe tyrosin kinase receptors?
They have to interact with the extracellular environment because their ligands are hydrophilic - therefore they need a receptor
TKR are transmembrane proteins but they communicate throughout the cytoplasm
When they bind to their ligand they become dimeric = active = inducing trans-autophosphorylation (this is ligand induced dimerization)
The active form of the receptor communicates with the intracellular environment and switches on pathways
How does trans-autophosphorylation work within the tyrosin kinase receptor?
Three tyrosine residues are phosphorylated and the activation loop changes its conformation - the N-terminal lobe undergoes nearly 21° rotation relative to the C-terminal lobe
This forms part of the substrate recognition site
What happens with the TKR after activation?
The intracellular domain of the RTK, when the tyrosine residues become phosphorylated they serve as docking ports for many proteins in the cytoplasm
The proteins in the cytoplasm recognises the phosphotyrosine residues because the proteins have a small domain structure within their structure = Src Homology 2 (SH2) domain
What is the role of Src Homology 2 (SH2)?
Small globular domain found in many proteins
It specifically recognises phosphotyrosine, and has a high affinity for them
Once the receptor is active it then recruits many proteins to it, to recognise the receptor
This forms multi-protein complexes at the receptor
These complexes transmit discrete signals to different pathways within the cell
Give an example of a kinase cascade activation after the TKR was activated?
TKR activate the G-protein Ras - which is a growth factor
Ras is a GTPase that catalyses the hydrolysis of GTP
There is a cascade of protien kinases until reaching transcriptional factors
This shows amplification from a small signal
How are protein kinase pathways within cells kept separate?
Scaffold proteins bind some or all of the component protein kinases to ensure that they only interact with one another - this prevents a crostalk
They also properly orient and allosterically activate some kinases to target specific subcellular locations
What is special about some molecules and their receptors?
Cytokines, interferons and T-cell receptors, don’t respond to ligand binding by autophosphorylation, so the TKR change conformation in a way that activates their associated nonreceptor tyrosine kinases (NRTKs)
Describe nonreceptor tyrosine kinases?
The belong to the Src family and have a SH2, SH3 and tyrosin kinase domain
- Tyr 527 is dephosphorylated and SH2/SH3 domains bind to target peptides - this relaxes the conformational constraints on the TK domain = TK active site cleft open
- The exposed phospho-Tyr 416 forms a salt bridge with Arg 409 - leads to structural reorganisation of the activation loop
- The rupture of the Glu 310–Arg 409 salt bridge frees helix C to assume its active orientation which allows Glu 310 to form its catalytically important salt bridge to Lys 295 - activating the Src PTK activity
What are TKR a target for?
Anticancer drugs
What needs to be done to tyrosin kinase receptors after activation?
Intracellular signals must be “turned off” after the system has delivered its message so that the system can transmit future messages
Protein phosphatases -hydrolyse the phosphoryl groups attached to Ser, Thr, or Tyr side chains
Protein tyrosin phosphatases (PTPs) - dephosphorylate tyrosine
Describe G-proteins receptors?
Heterotrimeric G-proteins with 3 subunits: alpha, beta and gamma
They are soluble cytoplasmic proteins - anchored to membrane via fatty acids (during post translational modification)
The G-protein coupled receptors contain 7 transmembrane helices
How is the G-protein swithced between active and inactive forms?
Normally in an inactive form but when a G-protien coupled receptor is activated the G-protein binds GTP
There is an exchange of GDP to GTP on one of the alpha subunits of the trimer, dissociating into 3 monomers
Eventually GTP hydrolysis leads to switching the protein back to the inactive form
This pathway is therefore on a timer as the protein is only active for as long as it takes for GTP to be hydrolysed
The proteins are aided by GAPs (exchange factors) - they bind to the G-protein and help in driving the reaction
What are the three major components of the signal transduction system of G-proteins?
G-protein-coupled receptors (GPCRs) - they bind a corresponding agonist extracellularly, inducing a confomational change intracellularly Heterotrimeric G proteins - anchored to the cytoplasmic side and activated by GPCRs Adenylate cyclase (AC) - a transmembrane enzyme that is activated/inhibited by activated heterotrimeric G proteins
What does adenylate cyclase do?
It catalyses the synthesis of cAMP from ATP
cAMP is a second messenger model
What are some second messengers?
cAMP, IP3 and Ca2+
What does cAMP do?
Adenylate cyclase converts ATP to cAMP (9 different isoforms)
cAMP is a secondary messenger that activates protein kinase A (PKA)
It is produced in response to GPCR signalling
It activates PKA in the cells - this will go on to mediate changes within the cell e.g. Regulation of glycogen formation
It diffuses through the cell but only lasts for second
What is the overall equation of cAMP and protein kinase A?
R2C2 + 4cAMP ⇌ 2C + R2(cAMP)4
R - regulatory subunit
C - catalytic subunit
R competitively inhibits the C
What can receptors adapt to?
Desensitization
Adapting to long-term stimuli, when they respond to changes in stimulations rather than absolute values
How is cAMP second messenger activity limited?
Phosphodiesterases
They can be activated by a variety of agents, depending on its isoform
The PDEs provide a means for crosstalk between cAMP signalling systems and other systems
Describe the initiation of the second messenger model - the phosphoinositide pathway?
An agonist binds/activates a GPCR and GTP then activates phospholipase C
Phospholipase C breaks down PIP2 into IP3 and DAG
(inositol-1,4,5-trisphosphate) and (1,2-diacylglycerol)
Phospolipase C has 19 isoenzymes and some require Ca2+
What happens to IP3 after it was produced in the phosphoinositide pathway?
IP3 moves to the ER and interacts with a Ca2+ channel protein in order to open the channel and release Ca2+ into the cytosol
This can switch on many kinase proteins to phosphorylate many proteins within the cell
It is also used in: glucose mobilisation, muscle contraction and calmodulin
What is calmodulin?
It is a Ca2+ activated switch
It is a ubiquitous, eukaryotic Ca2+ binding protein, participating in many regulatory processes
The binding structure between calmodulin and Ca2+ is similiar in troponin
What is the structure of calmodulin?
EF hand
Put up your index finger and thumb on your right hand
Index - E helix
Thumb - F helix
Ca2+ binds at the bottom of the index finger
How does Ca2+ activate calmodulin?
Intrasteric mechanism:
It induces a conformational change, that exposes a buried methionine rich hydrophobic patch, which can then binds with high affinity to regulated protein kinases
These proteins are then phosphorylated = cellular response
What happens to DAG after formation in the phosphoinositide pathway?
DAG (lipid soluble) remains in the plasma membrane, in concert with Ca2+, activates protein kinase C,
This can then phosphorylate other proteins for activation
What is an example of amplification of a signal?
Regulating blood glucose levels
Fed state - high glucose levels
Fasted state - low glucose levels
Therefore hormones need to respond to these environmental conditions
The three areas that hormones can assess these levels are: the liver, muscles and adipose tissue (fat)
Increase in blood glucose level = release of insulin (pancreas) targetting all three tissues
Decrease in blood glucose level = release of glucagon (pancreas) and adrenaline (adrenal glands)
Why is there a requirement for glucose?
Brain cannot use fatty acids as an energy source
Oxidation of fatty acids cannot provide ATP fast enough for intense exercise
Very low blood glucose leads to coma and eventually death
Blood glucose needs to be kept in a narrow range
What is the role of the pancreas?
High blood glucose - this triggers an ATP dependent switch, causing the release of insulin from beta-cells
Low blood glucose - this triggers alpha-cells will release glucagon
Adrenalin stimulates glucagon release and inhibits insulin release
What does insulin affect in the different tissues?
This is carried out mainly in the liver:
Facilitates glucose entry into cells
Stimulates glycogenesis
Inhibits glycogenolysis and gluconeogenesis
In adipose tissue:
Promotes triglyceride synthesis
Inhibits lipolysis
In the muscle:
Promotes amino acid uptake and protein synthesis
Inhibits protein degradation
=Building storage molecules
What is the mechanism of action for insulin?
Insulin uses a tyrosine kinase pathway, after phosphorylation it switches on signalling
This can give rise to many different outcomes such as: DNA/RNA/Protein synthesis, glycogen synthesis and glucose transport
How is insulin used in regulation of glucose transport?
Insulin activates a transporter protein - GLUT4
GLUT4 is translocated to the plasma membrane upon activation of the GPCR
It’s job is to facilitate to movement of glucose out of the blood and into the cell
This transporter can also allow glucose back out and therefore the cell regulates the activity of hexokinase (moves it from the nucleus to the cytosol)
By phosphorylating the glucose to G6P, this maintains the concentration gradient and traps the glucose as it is now impermeable to the membrane
What is the overall outcome of insulin?
Glucose and free fatty acid release falls
Glucose use and uptake rise
Blood glucose levels fall
Excess sugars stored as glycogen and/or fat
Describe the glucagon receptor?
Coupled to G-protein that activates adenylate cyclase and PKA
Describe the adrenaline receptors?
This has two receptors that will recognise it: alpha and beta, which have different coupled G-Proteins that will recognise this
α-adrenergic receptors -> utilise IP3/Ca2+ pathway
β-adrenergic receptors -> utilise adenylate cyclase and PKA
Muscle cells express β-adrenergic receptors
Liver cells have both
