Glycolysis - Payoff Phase

3619

Summary

Coming soon...

Key Points

  • Glycolysis - Payoff Phase
    • Etymology 
      • Glyco- = glucose = sugar
      • -lysis = to cut/split
    • Summary
      • Glycolysis is a metabolic pathway breaks down glucose to produce energy
        • Other carbohydrates are metabolized by the pathway as well
      • Occurs in the cytosol of the cell
        • part of anaerobic respiration
          • Can function with or without the presence of oxygen
        • Ancient pathway
          • highly conserved across different forms of life
          • Present in aerobic and anaerobic organisms
      • two phases
        • Investment phase
          • 2 ATP are ‘invested’ into glucose metabolism
        • Payoff phase
          • 4 ATP and 2 NADH are produced
        • Net payoff: 2 ATP and 2 NADH (per glucose)
    • Reaction Steps
      • Glyceraldehyde-3-phosphate (G3P) + Pi → [glyceraldehyde-3-phosphate dehydrogenase] 1,3-Bisphosphoglycerate
        • For every 1 molecule of glucose that enters glycolysis, 2 G3P enter the payoff phase
          • Matter is conserved
            • Glucose has  a 6 carbon chain and G3P has a 3 carbon chain
        • Reduces NAD+ to NADH
          • Redox reaction: G3P is oxidized
        • Redox reaction is exergonic, fueling the endergonic phosphorylation of G3P
        • Product 1,3-bisphosphoglyercate is unstable
      • 1,3-Bisphosphoglycerate + ADP → [phosphoglycerate kinase] 3-phosphoglycerate + ATP
        • Forms 1 ATP
          • substrate-level phosphorylation
          • energy from the exergonic redox reaction that was ‘stored’ in the phosphate bond formation is now used to transfer a phosphate group to ATP
      • 3-phosphoglycerate → [Mutase] 2-phosphoglycerate
        • Mutase is an isomerase (isomerization reaction)
        • This isomerization reaction makes the molecule less stable (think higher energy)
      • 2-phosphoglycerate → [Enolase] Phosphoenolpyruvate
      • Phosphoenolpyruvate + ADP → [Pyruvate Kinase] Pyruvate + ATP
        • Forms 1 ATP 
          • substrate-level phosphorylation
        • Pyruvate is the final product of glycolysis
          • has multiple possible metabolic fates, including TCA cycle
        • Irreversible
    • Regulation
      • Regulation is based on energy need and blood glucose levels
      • Pyruvate Kinase reaction is a regulated step
        • PFK is still the overall rate-limiting step for glycolysis
      • Glycolysis speeds up (activation)
        • High blood glucose levels
          • Insulin
            • Covalent modification
              • Triggers cascade that dephosphorylates pyruvate kinase
            • Gene expression
              • Insulin → upregulation
          • F1,6BP (feed forward activation)
      • Glycolysis slows down (inhibition)
        • Low blood glucose levels
          • Glucagon
            • Covalent modification
              • Triggers cascade that phosphorylates pyruvate kinase
            • Gene expression
              • Glucagon → downregulation
          • Alanine
        • Low energy need
          • ATP