AMPK

• the AMP:ATP ratio is sensed by AMPK which is a kinase of 3 subunits

  • alpha -> catalytic subunit, kinase domain
  • beta -> regulatory subunit
  • gamma -> regulatory subunit that binds AMP/ATP with 4 different nucleotide binding domains
  • 2 bind AMP/ADP with high affinity
  • 2 bind ATP with high affinity

  
• gamma subunit 4 binding sites on AMPK

  • position 1 -> binds ATP with high affinity
  • position 2 -> generally doesn't bind anything unless extreme conditions
  • position 3 -> can bind ATP/ADP/AMP with high affinity -> competition site!!!
  • position 4 -> binds AMP/ADP with high affinity

  
• AMPK in low ATP levels

  • AMP/ADP binds sites 3/4 -> conformational change that exposes Thr172 on alpha subunit/kinase domain which gets phosphorylated by kinases LKB1 and CaMKKB -> AMP bound AMPK is not a good substrate for phosphatase so phosphorylated AMPK will remain active until ATP levels are high again

  
• AMPK in high ATP levels

  • ATP binds sites 1/3 -> ATP bound AMPK is a good substrate for phosphatase so it will dephosphorylate AMPK at Thr172 on alpha kinase domain -> conformation change back to normal such that it can't be phosphorylated -> dephosphorylated AMPK is inactive

  
• active AMPK has 2 major outcomes

  • reduce/inhibit ATP consumption
  • increase catabolism, energy production

  
• adenylyl kinase (AK) is a kinase that can do 2 reactions

  • take 1 ATP and 1 AMP to make 2 ADP
  • take 2 ADP to make 1 ATP and 1 AMP

  the direction it goes in depends on what's in the cell

  • low ATP levels = HIGH ADP levels thus AK can quickly make ATP without entering glycolysis in ADP rich conditions
  • AMP accumulates which stimulates AMPK to make more ATP as well!

  
• AMPK controls many cellular processes

  • Acetyl CoA carboxylase to control fatty acid metabolism
  • glucose uptake into cells
  • cell migration
• acetyl CoA is a precursor for fatty acid biosynthesis

  • acetyl CoA (2C) + carbonate + ATP -> malonyl CoA (3C) achieved by ACC enzyme which makes a longer fatty acid chain which can be later used for energy or building biomass
  • ACC works in ATP dependent manner (thus consumes ATP, AMPK will prevent this) so it's most active when high nutrient and energy is available (anabolic states)

  
• in low ATP conditions

  • active AMPK (AMP/ADP bound) phosphorylates ACC which inactivates it which will prevent fatty acid synthesis so that it stops consuming ATP for fatty acid synthesis

  
• in absence of insulin (controls glucose uptake), active AMPK inhibits the endocytosis of glucose transporters (GLUT1 in all cells, GLUT4 in muscle cells) by phosphorylating alpha arrestin (promotes GLUT endocytosis) which inactivates it so there can be more glucose uptake into the cell and increase ATP production

  
• active AMPK slows cell migration/energy taxing processes

  • integrin proteins link the cell to the ECM, cell migration needs the membrane to traffic integrins from cell posterior to anterior
  • AMPK reduces rate of integrin membrane traffic and thus cell migration since it's such an energetically taxing process

  
• diabetes refers to a defect in insulin production (type 1) or action (type 2) that results in increased blood glucose

  • AMPK increases glucose consumption and metabolism by skeletal muscle cells by upregulating GLUT in cells (by phosphorylating alpha arrestin to stop endocytosis of GLUT
• AMPK and cancer

  • AMPK on
  • pro tumour -> generate ATP to help cancer survival
  • anti tumour -> can halt cancer cell growth by cell cycle arrest, p53 does this
  • AMPK off
  • pro tumour -> generate biomass to help cancer cell growth
  • anti tumour -> prevents metabolic adaption in cancer