Medicine & USMLE

Glycolysis - Investment Phase

  1. Citric Acid Cycle (TCA, Krebs)
  2. Glycolysis - Investment Phase
  3. Glycolysis - Payoff Phase
  4. Pentose Phosphate Pathway - Oxidative Phase
  5. Pentose Phosphate Pathway - Non-Oxidative Phase
  6. Glycogenesis
  7. Glycogenolysis
  8. Gluconeogenesis
  9. Electron Transport Chain (ETC)


Glycolysis is a metabolic pathway that breaks down sugar molecules to produce high energy molecules, like ATP and NADH. It can be broken into two phases: an investment phase and a payout phase.

In the investment phase, energy is ‘invested’ in the form of ATP. In the first step, hexokinase catalyzes the transfer of a phosphate group from ATP to glucose. This step is an important point of regulation, as it’s an irreversible step that traps glucose in the cell.

Next, glucose-6-phosphate is isomerized to fructose-6-phosphate via phosphoglucoisomerase.

Next, PFK -- or phosphofructokinase -- catalyzes the transfer of another phosphate group from ATP molecule to produce fructose-1,6-bisphosphate. This step is irreversible and is often the most important point of regulation. 

Aldolase next splits fructose-1,6-bisphosphate into G3P and DHAP. G3P and DHAP are isomers that are interconverted by Triosephosphate Isomerase (TPI) because only G3P can proceed to the payout phase of Glycolysis.

Key Points

  • Glycolysis Investment 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 (investment phase)
      • Glucose → [hexokinase] Glucose-6-phosphate
        • Kinases catalyze the transfer of a phosphate group
          • a phosphate group is transferred from ATP to Glucose
            • Phosphate transferred to #6 carbon in glucose
        • Commits glucose to the pathway
          • Glucose-6-phosphate cannot diffuse out of the cell
        • powered by 1 ATP
        • Irreversible (regulatory site)
      • Glucose-6-phosphate → [Phosphoglucoisomerase] fructose-6-phosphate
        • Isomerases catalyze conversions between isomers
          • Glucose-6-phosphate and fructose-6-phosphate are isomers
      • Fructose-6-phosphate [phosphofructokinase (PFK)] → Fructose-1,6-bisphosphate
        • Kinases catalyze the transfer of a phosphate group
          • a phosphate group is transferred from ATP to fructose-6-phosphate (F6P)
            • Phosphate transferred to #1 carbon in F6P
        • powered by 1 ATP
        • Irreversible rate-limiting step (main regulatory site)
          • PFK reaction is the first irreversible step in this pathway that is unique to glycolysis
            • Glucose-6-phosphate can be converted into glycogen or fed into the pentose phosphate pathway
      • Fructose-1,6-bisphosphate → [Aldolase] dihydroxyacetone phosphate (DHAP) + glyceraldehyde-3-phosphate (G3P)
        • Triosephosphate isomerase (TPI) catalyzes the interconversion between DHAP and G3P
          • Only G3P proceeds to the payoff phase
            • As G3P gets converted to the next intermediate, it decreases in concentration
            • This ‘pulls’ more DHAP to convert to G3P based on Le Chatlier’s principle
            • Eventually all DHAP will either be converted to G3P to enter the payoff phase
          • For every 1 molecule of glucose, 2 molecules of G3P enter payoff phase
    • Regulation
      • Regulation is based on energy need and blood glucose levels
        • Glycolysis speeds up when cells need to produce more energy and slows down when they don’t
        • Glycolysis speeds up when blood glucose levels are high and slows down when they are low
      • Regulatory steps
        • PFK reaction is the primary regulatory step
          • Regulation at other reversible steps as well
      • Glycolysis speeds up (activation)
        • High energy need
          • AMP
        • High blood glucose levels
          • Insulin
            • Insulin activates PFK-2, increasing Fructose-2,6-bisphosphate (F26BP) levels
              • Fructose-6-phosphate → [phosphofructokinase-2 (PFK-2)] Fructose-2,6-bisphosphate
              • F26BP activates PFK
                • Allows liver cells to override ATP inhibition
            • Insulin activates Glucokinase
              • Glucokinase is an alternative catalyst to hexokinase for the first reaction of glycolysis
              • Glucokinase is only found in the liver and in the pancreas.
      • Glycolysis slows down (inhibition)
        • Low energy need
          • ATP
          • Citrate
            • Citrate is a TCA intermediate, so high Citrate levels mean higher levels of energy production
          • Glucose-6-phosphate
            • Feedback inhibition on hexokinase
              • Glucose-6-phosphate builds up when the pathway is regulated at a later step
        • Low blood glucose levels
          • Glucagon
            • Inhibits PFK-2, decreasing Fructose-2,6-bisphosphate (F26BP) levels