Insulin signalling Flashcards
What is reciprocal regulation?
Compare and contrast glycogen synthase and glycogen phosphatase, its stimulators/inhibitors in liver and muscle tissue, and activity based on phosphorylation, and what induces that phosphorylation.
Reciprocal regulation: mechanism in which two hormones or signals have opposing effects on a particular physiological process. In the case of glucagon and insulin, they play crucial roles in regulating blood glucose levels.
Glycogen synthase: glucose -> glycogen (glycogenesis)
Dephosphorylated = active
Phosphorylated = inactive
Liver:
+insulin, G6P
-glucagon, epinephrine, Ca2+
Msc:
+insulin, G6P
-epinephrine
Glycogen phosphatase: glycogen -> glucose (glycogenolysis)
Dephosphorylated = inactive
Phosphorylated = active
Liver:
+glucagon, epinephrine, Ca2+
-glucose, insulin, G6P, ATP
Msc:
+epinephrine, AMP, Ca2+
-insulin, G6P, ATP
How does the insulin receptor (IR) work?
-Ras-independent pathway
-Ras-dependent pathway
Explain the two pathways and their goals.
How is it structured?
Insulin receptor comprised of 2 alpha subunits and 2 beta subunits.
a: Extracellular insulin binding domain, makes contact with only the bloodstream, binds to insulin
b: also part of extracellular insulin binding domain, but also has transmembrane spanning domain with a receptor tyrosine kinase that makes contact with the cytoplasm.
Ras-independent signalling:
1. Insulin binds to alpha subunit triggering a conformational change of IR.
2. Autophosphorylation of RTK
3. Recruitment of different proteins to fulfill different functions.
-IRS1: binds to phosphorylated RTK, recruiting other proteins.
-PI-3K: docks with IRS1, is phosphorylated and activates PKB (promotes GLUT4 movement to membrane for glucose uptake).
4.PKB activates PP1 (amplification of signal = allows fast cell response), and inhibits GS3K (phosphorylates GS to inactivate it)
5. Alterations in protein and enzyme activity to increase glucose uptake and glycogen synthesis (ex: glycogen synthase activation, GLUT4 increase)
Ras-dependent signalling: cell proliferation pathway, activates metabolic pathways for building blocks.
1. Insulin binds to alpha subunit triggering a conformational change of IR.
2. Autophosphorylation of RTK
3. Recruitment of different proteins to fulfill different functions.
-IRS1: binds to phosphorylated RTK, recruiting other proteins.
-Grb-2: docks with IRS1, is phosphorylated and activates Ras.
4. Ras phosphorylates a protein causing alterations in gene transcription to increase glucose uptake and glycogen synthesis (ex: increased transcription of glucokinase)
How does insulin binding to the IR activate glycogenesis and inhibit glycogenolysis through reciprocal regulation?
What exactly is reciprocal regulation in this instance?
Activation of glycogenesis:
1. Insulin binds to IR -> PKB -> PP1 activates (phosphorylates) glycogen synthase (phosphorylated = active)
2. glycogen synthase converts glucose -> glycogen.
Inhibition of glycogenolysis:
1. Insulin binds to IR -> PKB -> PP1 inhibits (phosphorylates) glycogen phosphorylase (phosphorylated = inactive)
2. inhibited glycogen phosphorylase does not convert glycogen -> glucose
Reciprocal regulation: PP1 activates glycogen synthase and inhibits glycogen phosphorylase
How does insulin binding to the IR activate glycolysis and inhibit gluconeogenesis through reciprocal regulation?
Glycolysis: diversion of glucose towards pyruvate production in fed conditions.
-hexokinase/glucokinase is activated by insulin
-pyruvate kinase is activated by insulin
-phosphofructokinase-1 is activated by F26BP (insulin)
Gluconeogenesis:
-F16BPhosphatase inhibited by F26BP (insulin)
What is Fructose-2,6-biphosphate? Is it metabolized in glycolysis?
How does it allosterically regulate glycolysis and gluconeogenesis?
What is the effect of F26BP on the vmax of PFK1 and FBPase1?
F26BP: key regulatory molecule in carbohydrate metabolism, particularly in the control of glycolysis and gluconeogenesis
-It is not metabolized by glycolysis, its stricly a signalling molecule generated by insulin.
F6P -> F26BP by PFK2
+insulin
F26BP -> F6P by FBPase2
+glucagon
F26BP then +PFK1 and FBPase1
F6P -> F16BP by PFK1
+AMP, +F26BP
-ATP, -citrate
F16BP -> F6P by FBPase1
-AMP, -F26BP
Effect:
PFK1: vmax obtained quicker in presence of F26BP and slower without F26BP
FBPase1: vmax obtained slower in presence of F26BP and quicker without F26BP
What is a bifunctional enzyme? Name one and the way its influenced by insulin and glucagon.
Bifunctional enzyme: enzyme that possesses two distinct catalytic activities within a single polypeptide chain or protein complex
Ex: PFK2/FBPase2 is a bifunctional enzyme
PFK2 catalytic site catalyzes F6P -> F26BP
FBPase2 catalytic site catalyzes F26BP -> F6P
Insulin: in fed state, activates PP1 through signalling which dephosphorylates PFK2, activating it to increase glycolysis
Glucagon: in fasting/exercise state, activates PKA through signalling which phosphorylates PFK2, inhibiting it and allowing FBPase2 to increase gluconeogenesis.
How does insulin binding to the IR activate lipogenesis thanks to glycolysis?
In adipocytes, activation of glycolysis combines with lipogenesis gene activation to allow glucose usage for lipogenesis.
In adipocytes, insulin binds to IR, which activate SREBP (sterol response element binding protein), which binds to SRE gene.
Alongside, insulin binding also activates glycolysis, of which its metabolites (X5P, AcetylCoA, G6P) activate ChREBP (carbohydrate response element binding protein), which binds to ChORE gene.
Both SREBP and ChREBP binding induces gene transcription of enzymes involved in FA synthesis (lipogenesis)
-ACLY for citrate -> Acetyl-CoA
-ACC for AcetylCoa -> MalonylCoA
-Fatty acid synthase
-FA desaturase
(other enzymes for acetyl-CoA -> Palmitate -> FA)
As ATP is utilized, there’s an accumulation of AMP. How does the cell regulate its fuel upon this accumulation?
What enzyme contributes to the accumulation of AMP?
How is this process a positive feedback loop?
Is AMPK active in fed conditions?
AMPK: AMP kinase.
-acts as the cell’s fuel gauge.
-high AMP activates AMPK, which activates ATP producing pathways (catabolic pathways) and reciprocally deactivates ATP consuming pathways (anabolic pathways).
-Phosphorylates AcetylCoA carboxylase, decreasing malonylCoA production (decreases lipogenesis)
AK: adenylate kinase
2 ADP -> AMP + ATP
-causes accumulation of AMP upon excessive ATP use (exercise)
Positive feedback loop:
ATP is consumed = ADP + Pi
ADP -> AMP by AK
High AMP activates AMPK
AMPK activates ATP production
ATP is consumed = etc…
No, AMPK activates catabolic pathways to produce ATP. Meaning, there is no abundance of ATP available from being in a fed state.
What is Acetyl-CoA carboxylase (ACC)?
Where does lipogenesis occur? How does acetylCoA exit the mitochondria to enter into lipogenesis?
Describe the pathway for lipogenesis in a fed state from glucose to palmitate.
What is an allosteric activator of ACC?
AcetylCoA carboxylase is the rate limiting enzyme involved in lipogenesis.
Lipogenesis occurs in the cell cytosol.
Citrate from CAC exits mitochondria through citrate transporter.
In the cytoplasm, ACLY converts citrate -> acetylCoA and oxaloacetate.
AcetylCoa is utilized in lipogenesis by ACC
Oxaloacetate is converted to malate and transported back into the mitochondria for CAC.
Glucose -> pyruvate (glycolysis)->enters mitochondria -> acetylCoA -> citrate (CAC) -> citrate transporter into cytoplasm -> AcetylCoA (AcetylCoa carboxylase) -> malonylCoA -> Palmitate (lipogenesis).
Citrate is an allosteric activator of ACC.
How do we know ACC is fully active?
How is ACC activated?
when it is polymerized, and can be observed in the cell.
Insulin is an allosteric activator for dephosphorylation of ACC (phosphorylated by AMPK). PP1 activation will dephosphorylate ACC to give it a monomeric form (low activity)
Citrate will promote polymerization of ACC (Allosteric activator), giving it a polymeric form (high activity)
