Glycolysis: Reactions and Regulation

Glycolysis Overview - Sugar Split Kickoff

• Definition: Breakdown of glucose (6C) into two pyruvate (3C) molecules.
• Location: Cytoplasm of all cells.
• Types: Aerobic (pyruvate to Acetyl-CoA) & Anaerobic (pyruvate to lactate/ethanol).
• Overall Reaction: $C_6H_{12}O_6 + 2NAD^+ + 2ADP + 2Pi \rightarrow 2 Pyruvate + 2NADH + 2H^+ + 2ATP + 2H_2O$.
• Significance:
  • Sole energy source for RBCs.
  • Primary energy for brain (fed state).
  • Rapid ATP for muscles during intense exercise.

⭐ Glycolysis occurs in the cytoplasm of all cells and can proceed aerobically or anaerobically.

Glycolytic Pathway & Energetics - Ten Step Energy Dance

Glycolysis, a 10-step anaerobic cytoplasmic pathway, converts one molecule of glucose ($C_6H_{12}O_6$) into two molecules of pyruvate ($CH_3COCOO^−$). It has two phases: Preparatory (energy investment) and Payoff (energy generation).

• Preparatory (Investment) Phase (Steps 1-5):
  • Glucose is phosphorylated (Step 1: Hexokinase/Glucokinase, irreversible), isomerized, then phosphorylated again (Step 3: PFK-1, irreversible, rate-limiting).
  • Fructose-1,6-bisphosphate is cleaved by Aldolase (Step 4) into Dihydroxyacetone Phosphate (DHAP) and Glyceraldehyde-3-Phosphate (G3P). DHAP is isomerized to G3P (Step 5).
  • Consumes 2 ATP.
• Payoff Phase (Steps 6-10):
  • Two G3P molecules are oxidized and phosphorylated, yielding ATP and NADH.
  • Step 6: G3P Dehydrogenase produces 2 NADH.
  • Step 7 (Phosphoglycerate Kinase) & Step 10 (Pyruvate Kinase, irreversible) generate ATP via substrate-level phosphorylation.
  • Produces 4 ATP and 2 NADH.

📌 Enzyme Mnemonic: "Hungry Peter Pan And The Giant Pigeon Picked Enormous Pumpkins" (Hexokinase, PGI, PFK-1, Aldolase, TPI, G3PDH, PGK, PGM, Enolase, PK).

Net Energetics per Glucose:

• ATP: -2 ATP (investment) + 4 ATP (payoff) = Net 2 ATP.
• NADH: 2 NADH (each yields ~2.5 ATP via oxidative phosphorylation in aerobic conditions).
• Products: 2 Pyruvate, 2 $H_2O$, 2 $H^+$.

⭐ The three irreversible enzymes of glycolysis are Hexokinase/Glucokinase, Phosphofructokinase-1 (PFK-1), and Pyruvate Kinase. These are key regulatory points.

Glycolysis Regulation - Pathway Traffic Control

Control at 3 irreversible steps manages glucose flux:

• Key Regulatory Enzymes & Effectors:

- Most potent PFK-1 allosteric activator.
- Levels set by bifunctional PFK-2/FBPase-2 enzyme:
    - Insulin (dephosphorylates enzyme): ↑PFK-2 activity → ↑F-2,6-BP → ↑Glycolysis.
    - Glucagon (phosphorylates enzyme via PKA): ↑FBPase-2 activity → ↓F-2,6-BP → ↓Glycolysis.

⭐ Fructose-2,6-bisphosphate is the most potent allosteric activator of PFK-1, and its levels are controlled by the bifunctional enzyme PFK-2/FBPase-2.

Pyruvate's Fate & Clinicals - Post-Glycolysis Junction

• Fates of Pyruvate:
  • Aerobic: Pyruvate $\rightarrow$ Acetyl-CoA (by PDH complex) $\rightarrow$ TCA cycle.
  • Anaerobic (regenerates $NAD^+$):
    • Humans (RBCs, exercising muscle): Pyruvate $\rightarrow$ Lactate (by LDH). Cori cycle (lactate to liver).
    • Yeast: Pyruvate $\rightarrow$ Ethanol.
• Clinical Correlations:
  • Pyruvate Kinase (PK) Deficiency: Hemolytic anemia (due to $\downarrow$ATP in RBCs).
  • Lactic Acidosis: Lactate accumulation.
  • Warburg Effect: Aerobic glycolysis in cancer cells.
  • Poisons: Arsenate (inhibits G3PDH, uncouples ATP), Fluoride (inhibits Enolase).

⭐ Pyruvate Kinase deficiency is an autosomal recessive disorder causing chronic non-spherocytic hemolytic anemia due to insufficient ATP for RBC membrane integrity.

High‑Yield Points - ⚡ Biggest Takeaways

• Glycolysis occurs in the cytosol and is an anaerobic pathway.
• Net products per glucose: 2 ATP (substrate-level phosphorylation), 2 NADH, and 2 Pyruvate.
• Key irreversible regulatory enzymes: Hexokinase/Glucokinase, Phosphofructokinase-1 (PFK-1), and Pyruvate Kinase.
• PFK-1 is the rate-limiting step; activated by AMP, Fructose-2,6-bisphosphate; inhibited by ATP, Citrate.
• Arsenate inhibits Glyceraldehyde-3-phosphate dehydrogenase, uncoupling ATP synthesis.
• Fluoride is an inhibitor of Enolase.
• Pyruvate Kinase deficiency causes hemolytic anemia due to decreased ATP in RBCs.