glycolysis step

BNO TeamApril 26, 2025

Answered

Glycolysis is a fundamental metabolic pathway that converts one molecule of glucose (a six-carbon sugar) into two molecules of pyruvate (a three-carbon compound). This process occurs in the cytoplasm of cells and is the first step in both aerobic and anaerobic respiration. Glycolysis consists of 10 enzyme-catalyzed steps, which can be divided into two phases: the energy investment phase and the energy payoff phase.

Energy Investment Phase (Steps 1–5):

• Step 1: Phosphorylation of Glucose

  • Enzyme: Hexokinase (or glucokinase in liver cells)

  • Reaction: Glucose + ATP → Glucose-6-phosphate (G6P) + ADP

  • This step traps glucose within the cell and destabilizes it for further reactions.​

• Step 2: Isomerization of G6P to F6P

  • Enzyme: Phosphoglucose isomerase

  • Reaction: Glucose-6-phosphate ↔ Fructose-6-phosphate (F6P)

  • This rearrangement prepares the molecule for subsequent phosphorylation.​

• Step 3: Phosphorylation of F6P to F1,6BP

  • Enzyme: Phosphofructokinase-1 (PFK-1)

  • Reaction: Fructose-6-phosphate + ATP → Fructose-1,6-bisphosphate (F1,6BP) + ADP

  • This is a key regulatory and irreversible step in glycolysis.​

• Step 4: Cleavage of F1,6BP

  • Enzyme: Aldolase

  • Reaction: Fructose-1,6-bisphosphate ↔ Glyceraldehyde-3-phosphate (G3P) + Dihydroxyacetone phosphate (DHAP)

  • This step splits the six-carbon sugar into two three-carbon sugars.

• Step 5: Isomerization of DHAP to G3P

  • Enzyme: Triose phosphate isomerase

  • Reaction: Dihydroxyacetone phosphate ↔ Glyceraldehyde-3-phosphate

  • This ensures that both molecules proceed through the same subsequent steps.​

Energy Payoff Phase (Steps 6–10):

• Step 6: Oxidation and Phosphorylation of G3P

  • Enzyme: Glyceraldehyde-3-phosphate dehydrogenase

  • Reaction: Glyceraldehyde-3-phosphate + NAD⁺ + Pi ↔ 1,3-Bisphosphoglycerate (1,3-BPG) + NADH + H⁺

  • This step produces NADH and adds a high-energy phosphate group

• Step 7: Substrate-Level Phosphorylation

  • Enzyme: Phosphoglycerate kinase

  • Reaction: 1,3-Bisphosphoglycerate + ADP ↔ 3-Phosphoglycerate + ATP

  • This is the first ATP-generating step in glycolysis.​

• Step 8: Conversion of 3-Phosphoglycerate to 2-Phosphoglycerate

  • Enzyme: Phosphoglycerate mutase

  • Reaction: 3-Phosphoglycerate ↔ 2-Phosphoglycerate

  • This rearrangement prepares the molecule for dehydration.​

• Step 9: Dehydration of 2-Phosphoglycerate

  • Enzyme: Enolase

  • Reaction: 2-Phosphoglycerate ↔ Phosphoenolpyruvate (PEP) + H₂O

  • This step creates a high-energy enol phosphate.​

• Step 10: Second Substrate-Level Phosphorylation

  • Enzyme: Pyruvate kinase

  • Reaction: Phosphoenolpyruvate + ADP → Pyruvate + ATP

  • This final step yields ATP and forms pyruvate, the end product of glycolysis.​

Net Energy Yield from Glycolysis:

• ATP Consumed: 2 (Steps 1 and 3)

• ATP Produced: 4 (Steps 7 and 10, two ATP per step as each occurs twice per glucose molecule)

• Net ATP Gain: 2

• NADH Produced: 2 (Step 6)