Preparatory phase reactions

Investment Phase - Priming the Pump

The preparatory phase of glycolysis converts one glucose molecule ($C_6H_{12}O_6$) into two glyceraldehyde-3-phosphate (G3P) molecules. This requires an investment of 2 ATP.

• Goal: Phosphorylate glucose and convert it to a high-energy, cleavable intermediate.
• ATP Investment: 2 ATP consumed.
• Net Products: 2 molecules of Glyceraldehyde-3-P.
• Irreversible Steps: Catalyzed by Hexokinase/Glucokinase and PFK-1.

⭐ The two irreversible enzymatic steps in the preparatory phase are catalyzed by Hexokinase/Glucokinase and Phosphofructokinase-1 (PFK-1).

The Five Steps - Enzyme & Reaction Lineup

📌 Mnemonic: Hungry Peter Pan And The Pirates (Hexokinase, Phosphoglucose Isomerase, PFK-1, Aldolase, Triose Phosphate Isomerase)

• Step 1: Hexokinase / Glucokinase
  • Reaction: $Glucose + ATP -> Glucose-6-Phosphate (G6P) + ADP$
  • Cofactor: $Mg^{2+}$
  • Irreversible trap; phosphorylates glucose to keep it inside the cell.

⭐ Hexokinase vs. Glucokinase

• Hexokinase: Found in most tissues. Low $K_m$ (high affinity), low $V_{max}$. Inhibited by its product, G6P.
• Glucokinase: Found in liver & pancreatic β-cells. High $K_m$ (low affinity), high $V_{max}$. Induced by insulin; not inhibited by G6P.

• Step 2: Phosphoglucose Isomerase

  • Reaction: $G6P <-> Fructose-6-Phosphate (F6P)$
  • Reversible isomerization.

• Step 3: Phosphofructokinase-1 (PFK-1)

  • Reaction: $F6P + ATP -> Fructose-1,6-bisphosphate (F-1,6-BP) + ADP$
  • Cofactor: $Mg^{2+}$
  • Irreversible; the rate-limiting step of glycolysis.

• Step 4: Aldolase

  • Reaction: $F-1,6-BP <-> Dihydroxyacetone phosphate (DHAP) + Glyceraldehyde-3-phosphate (G3P)$
  • Reversible cleavage.

• Step 5: Triose Phosphate Isomerase

  • Reaction: $DHAP <-> G3P$
  • Reversible. Ensures all glucose carbons proceed through glycolysis as G3P.

PFK-1 Regulation - The Master Switch

Phosphofructokinase-1 (PFK-1) catalyzes the primary committed, rate-limiting step of glycolysis. Its activity is tightly controlled by allosteric effectors, reflecting the cell's energy status.

• Allosteric Activators (Low Energy State):

  • ↑ AMP, ADP
  • ↑ Fructose-2,6-bisphosphate

• Allosteric Inhibitors (High Energy State):

  • ↑ ATP (at allosteric site)
  • ↑ Citrate (TCA cycle intermediate)

⭐ Fructose-2,6-bisphosphate is the most potent allosteric activator of PFK-1, overriding inhibition by ATP.

High‑Yield Points - ⚡ Biggest Takeaways

• This is the energy investment phase of glycolysis, consuming 2 ATP per glucose molecule.
• It features two key irreversible, ATP-dependent kinase reactions: Hexokinase/Glucokinase and PFK-1.
• Phosphofructokinase-1 (PFK-1) is the rate-limiting enzyme and the most critical regulatory point.
• Hexokinase has a low Km (high affinity), while liver-specific Glucokinase has a high Km.
• The phase concludes by cleaving a 6-carbon sugar into two 3-carbon molecules (G3P).