Energy Sensing (AMPK) Flashcards
Energy Sensing: The AMPK Pathway
- acetyl-CoA carboxylase (ACC) is a key enzyme involved in fatty acid synthesis:
- HMG-CoA reductase (HMGR) is the key regulatory enzyme of cholesterol synthesis
- ACC and HMGR are both activated by AMP-activated protein kinase (AMPK) named so after its allosteric activator AMP
Regulation of AMPK by phosphorylation:
- The Thr172 residue of AMPK is phosphorylated by an upstream kinase in a protein kinase cascade
- This Thr172 is within the activation loop between the highly conserved “DFG” and “APE” motifs
- Antibodies recognising phospho-threonine are now used as markers of AMPK activation
Regulation of AMPK by adenine nucleotides:
Binding of AMP to AMPK causes:
- Allosteric Activation
- Promoted Phosphorylation (by upstream kinase)
- Inhibited Dephosphorylation (by protein phosphatase)
These 3 effects all promote activation of AMPK, and all 3 effects are antagonised by the binding of ATP
Thus AMPK is an energy sensing kinase that is activated by a fall in cellular energy (Remember an increase in AMP:ATP ratio = fall in cellular energy)
Identification of AMPKs upstream kinase:
- Tests performed in yeast
- Only one found with the ability to activate AMPK
- Human kinase closet to this yeast kinase = Liver Kinase B1 (LKB1)
- LKB1 is a tumour supressor, a gene that causes cancer when lost due to mutations
An alternate upstream kinase:
CaMKK2/CaMKKbeta = Calmodulin-dependent kinase
AMPK is activated by Ca2+ rather than AMP (In this HeLA (i.e. immortal) cell pathway)
Agents that increase cytosolic Ca2+ often trigger ATP-requiring processes (e.g. secretin), so this mechanism may anticipate a demand for ATP even before it has occurred.
AMPK: Activation in muscle during exercise
- ATP consumption increases greatly during exercise
- AMPK is activated in muscle within seconds of starting exercise (Though to be due to the increase in AMP:ATP ratio caused by exercise)
AMPK: Activation by anti-diabetic drugs
Type 2 Diabetes Mellitus is most commonly treated by the drug “metformin”
Metformin activates AMPK by causing mild inhibition of mitochondrial ATP synthesis
Downstream Targets of AMPK: Glucose Uptake and Glycolysis
- Fusion of GLUT4 with the plasma membrane causes an increase in glucose uptake during exercise
- This fusion requires G proteins of the Rab family to be in their active GTP-bound state
- However the Rab-GAP TBC1D1 keeps Rabs in their inactive state (bound to vesicles)
- AMPK phosphorylates TBC1D1, causing its dissociation, converting Rabs to their active state
- In heart muscle, AMPK also enhances glycolysis by activating synthesis of fructose-2,6-bisphosphate, an allosteric activator of the glycolytic enzyme phosphofructokinase:
Downstream Targets of AMPK: mTOR and protein synthesis
AMPK inhibits protein synthesis by switching off the mTORC1 pathway via 2 effects:
- Phosphorylation of Raptor
- Phosphorylation of TSC2
GAP activity of TSC2 converts Rheb to its inactive GDP form
Without Rheb-GTP, mTORC1 is inactive
Therefore it AMPK inhibits protein synthesis and cell growth
Downstream Targets of AMPK: Fatty Acid Oxidation
- fatty acid oxidation generates a large amount of ATP and occurs in mitochondria
Downstream Targets of AMPK: Fatty Acid Oxidation
- fatty acid oxidation generates a large amount of ATP and occurs in mitochondria
- mitochondrial uptake of fatty acids occurs via two isoforms of carnitine:palmitoyl-CoA transferases (CPTs)
1. one on outside of the inner membrane (CPT1)
2. and the other on the inside of the inner membrane (CPT2)
Downstream Targets of AMPK: Fatty Acid Oxidation
- fatty acid oxidation generates a large amount of ATP and occurs in mitochondria
- mitochondrial uptake of fatty acids occurs via two isoforms of carnitine:palmitoyl-CoA transferases (CPTs)
1. one on outside of the inner membrane (CPT1)
2. and the other on the inside of the inner membrane (CPT2) - AMPK inhibits fatty acid synthesis, and at the same time it relieves inhibition of CPT1 and thus activates fatty acid oxidation, causing a switch from anabolism to catabolism
