Insulin Action And Signalling Flashcards
What is the purpose of blood glucose homeostasis
To maintain blood glucose at approx 90mg/dl
What are the 2 main functions of the pancreas
Exocrine function
Endocrine function
Describe the exocrine function of the pancreas
Release digestive enzymes through ducal structures directly into the gut to help digestion
What is the endocrine function of the pancreas
Maintain blood glucose levels via the pancreatic islets
What are the 3 type of cells in the islets
Beta cells; release insulin
Alpha cells; release glucagon
Delta cells; release somatostatin
Why is important that the pancreatic islets have a good capillary network
So that they can sense changes in blood glucose rapidly and can release hormones directly into the blood stream
What inhibits insulin release
Adrenaline and somatostatin
What stimulates the release of insulin
Glucose (major regulator)
Gut hormones
Amino acids
What inhibits glucagon release
Insulin
Gut hormones
Glucose
What stimulates the release of glucagon
Cortisol
Adrenaline
Amino acids
Describe the process that insulin is formed
Pre-pro insulin -> pro insulin -> insulin + c peptide
How is insulin stored prior to release
Within secretory granules and stored as a hexatrimeric complex
Describe the structure of pro insulin
Folded in the ER to form the A chain and D chain
Disulphide binds between the A and D chains and within the A chain
Describe the cleavage of pro insulin to insulin
Cleaved at position 31 and 65 to end up with insulin (A and B chain connected via disulphide bonds) and C peptide
Why is C peptide a better diagnostic tool than insulin
Is a stable molecule with a longer half life than insulin
Will indicate if a patient can produce their own insulin
What happens when glucose enters a beta cell
Glucose converted to glucose-6-phosphate via glucokinase
Glucose 6-p undergo glycolysis -> pyruvate
Pyruvate undergoes Krebs cycle
= Production of ATP
What happens if the blood glucose is at a normal level
Not much glucose 6-p = low ATP levels
K-ATP channels channel remain open = no depolarisation = no release of insulin
What happens if there’s an increase in blood glucose levels
More glucose enters cell = more glucose 6-p
More pyruvate = higher ATP:ADP
K-ATP channel closes to the membrane potential depolarises
Ca2+ channel opens allowing an influx of Ca+ into cell
= release of insulin
Describe the structure of glycogen
Branched polymer of glucose w 14 and 1-6 glycosidic bonds
What is the first line of defence for declining blood glucose levels
Hepatic glycogen is degraded between meals
Why can’t muscle cells maintain blood glucose levels
It can’t release glucose back into the bloodstream
Only acts a store for its own tissue
How does the brain respond to glucose and insulin in the post prandial state
Doesn’t respond to insulin but has a high demand for glucose
How do muscle cells respond to insulin and glucose in the post prandial state
Wont respond to glucose unless insulin present
Will then take glucose -> glycogen via glycolysis and oxidation
How do adipose tissues respond to insulin and glucose in the post prandial state
Take in glucose and convert into triglycerides and it also signals for the tissue to take any fat in from circulation
How does the liver respond to insulin and glucose in the post prandial state
Converts glucose -> glycogen and convert surplus glucose/mono acids to lipids
Fat translocates out in lipoproteins which are taken up by adipocytes
What happens to excess glucose after the glycogen stores have been used up during the postprandial state
Glucose > pyruvate -> Krebs cycle -> citrate -> fatty acids
Modulated by insulin and is dependent on excess glucose conc
Why does insulin modulate the conversion of glucose -> fatty acids
Increases the activity of pyruvate dehydrogenase and increases the txn of a fatty acid synthase and acetyl CoA carboxylase
Drives the pathway to fatty acids
How does the muscles respond to insulin and glucose in the post absorptive state
Convert proteins into amino acids and release them into the blood stream
How does adipose tissues respond to insulin and glucose in the post absorptive state
Convert triglycerides -> fatty acids and glycerol and release fatty acids are released into the blood
Muscle cells take up fatty acids and metabolise them for fuel instead of glucose
How does the liver respond to insulin and glucose in the post absorptive state
Converts glycogen -> glucose and releases into blood
Amino acids -> glucose via gluconeogenesis
Fatty acids -> energy for its own metabolism
Fatty acids -> ketone bodies for the brain
Why can glucose not be taken into the cell in the absence cells of insulin
GLUT4 is associated w vesicles in the cytoplasm not at the plasma membrane in the absence of insulin
Describe what happens when insulin binds to the membrane of a muscle cell
GLUT4 translocation to the plasma membrane to bring glucose into the cel
How does insulin increase the formation of glycogen
Activating glycogen synthase = dephosphorylation
Why do muscle cells express hexakinase 1/2
Rapid metabolism of glucose in the cell
Why does high glucose levels activate pyruvate dehydrogenase
More conversion of pyruvate -> acetyl CoA
How does the glucose -> triglycerides in the adipose tissues
Glucose metabolised to pyruvate
Insulin increases the txn of fatty acids cid synthase and acetyl CoA carboxylase
Forms triglycerides
Why do the adipose tissues convert glucose -> triglycerides
There is no glycogen storage only fat
Why does insulin switch of hormone sensitive lip abase when glucose levels are high
Inhibit the degradation of triglycerides -> fatty acids acids that could be converted into energy
What do the adipose tissues do with triglycerides when glucose is high
Lipoprotein lipase converts triglycerides -> monacylglycerol hitch Cana be taken up into adipose tissues
Can be converted into triglycerides when needed
Why does the liver express GLUT2 instead of GLUT4
Has a higher affinity for glucose
Why does the liver express glucokinase
Conversion of glucose -> glucose 6-P when glucose levels are high
What effects does insulin have on the liver
Increases translation on glucokinase
Stimulates activity of glycogen synthase
Decreases activity of glycogen phosphorylase
Increases the txn of acetyl CoA carboxylase
What is property of RTK receptors
Has tyrosine kinase activity -> it can phosphorylate tyrosine residues to evoke a cascade of effects
Describe the PH domain
Pleckstrin homologous domain
Binds to phosphorylated inositol phospholipid in the plasma membrane
Why is the PH domain important
Locates signal proteins to the plasma membrane
Describe the PTB domain
Binds to phosphotyrosine P-Y residues
Describe the SH2 domain
SRC homologous 2 domains binds P-Y readies surrounded by unique protein sequences
Describe the SH3 domain
Src homology 3 domain
Binds specifically to proline rich regions
Describe the insulin receptor
Tyrosine kinase receptor and part of the insulin-IGF1R
Has 2xalpha and 2xbeta subunits
Describe the function of the IR alpha subunits
Sit on the membrane and bind insulin
Describe the function of IR beta subunits
Transmembrane and has tyrosine kinase activity
How do the 2 isoforms of IR happen
Alternative splicing
What happens when insulin binds to the alpha subunit of IR
Causes transphosphorylation and activation of beta subunits and its tyrosine activity
What happens when the beta subunit on the IR becomes phosphorylates
Phosphorylation of other tyrosine residues on the subunit
Exposes binding site for proteins w PTB and SH domains
What site is phosphorylated on the beta subunit in the IR
Activation loop
What happens at the juxtamembrane domain in the beta subunit on the IR
Phosphorylation of tyrosine residues creates binding site for PTB domain
What happens at the C-terminal domain in the beta subunit on the IR
Phosphorylation of tyrosine residues creates binding site for SH2 domain
What happens at the Ser/Thr sites on the beta subunit on the IR
Inhibition of receptor kinase activity when the sites have been phosphorylated
What are the 2 adapter proteins that sit below the IR
She and IRS1-4
Describe Shc
SH2 containing adapter proteins
Has 3 different domains; PTB, SH2 and CH1 domains
Binds to IR via PTB and SH2 domains
What is the purpose of CH1 domain on Shc
Phosphorylation of tyrosine kinase uncovers binding site for proteins w an SH3 domain e.g Grb2
What happens once Grb2 is bound to CH1
Conveys signal to sos which links to the MAP-kinase signalling pathway
What is the purpose of the insulin receptor substrate IRS
Essential for most of insulins biological actions in the liver muscles at adipose tissues
Describe the structure of IRS and their functions
PH domain; binds to a phospholipid in the plasma membrane
PTB; binds directly to PTB binding site on IR
Why doe the IRS have a lot of tyrosine residues
Leads to phosphorylation of many downstream sites once bound to IR
What sites act as positive regulators on the IRS
Y - phosphorylation sites
Docking protein w SH2 domain
Can bind GRB2, PI3K and SHP2
What sites act as negative regulators on IRS
S/T phosphorylation sits
Phosphorylated by stress kinases activated by lipids, inflammation or components of insulin signalling a
What happens after the IRS binds to the IR
Becomes phosphorylated by IR tyrosine kinase activity
Describe the insulin signalling pathways that lead to MAP-kinase pathway activation
IR -> IRS -> Grb2 -> SOS -> MAPK
IR -> Shc -> Grb2 -> SOS -> MAPK
How does Grb2 interact w SOS
Activated by binding to IRS or Shc allowing it to bind to SOS via SH3 domain
How does SOS act as a GDP/GTP exchange factor (GEE)
Exchanges GDP for GTP on RAS to activate it
Describe how RAS is activated and then activates ERK
RAS activated when its bound GTP is exchanged for GDP
RAS-GTP activates Raf
Raf activates MEK
MEK activates ERK
How does PI-3 kinase pathway lead to activation of AKT
Recruitment of IRS to IR beta
Recruitment of PI3K to IRS via SH2 domains and formation of PIP3
Recruitment of AKT and PDK1 by PIP3 to plasma membrane
Phosphorylation of AKT
How is PIP3 formed by recruitment of PIK3K
Activation of catalytic activity of PIP3K adds aa phosphate to inositol phospholipid in the plasma membrane
Describe the structure of PI3K
Regulatory subunit p85
Catalytic subunit p110
Describe the regulatory unit p85 on PI3K
Contains;
2 SH2 domains allowing it to bind to IRS
1 SH3 domain that allows it to bind to p110 subunit
Describe the catalytic subunit p110 on the PI3K
Activated by interaction w the p85 subunit
When active it allows the addition of the phosphate group to the inositol rings
Allows it to localise to the plasma membrane
What mediates the degradation of PIP3 signal
Phosphate and tension homologous deleted on chromosome 10 (PTEN)
Removes phosphate group from position 10
How does PIP3 dual phosphorylate AKT
PIP3 activates mTORC2 and PDK1
PDK1 phosphorylates AKT on Thr308
mTORC1 phosphorylates AKT on Ser473
What is the function of AKT once it has ben phosphorylated
Regulates proteins and and glycogen synthesis as well as GLUT4 translocation
What regulates the movement of storage vesicles containing GLUT4
AS160
How does AS160 regulate GLUT4 translocation
Maintains Rab in its GDP form
Rab requires to be bound to GTP to facilitate movement
How does the AS160 complex become inactive
Phosphorylation ok AKT -> phosphorylation of AS160 complex
