Glycolysis

Summary of Glycolysis

• Converts glucose to pyruvate in ten enzymatic steps.
• Occurs in the cytoplasm and is anaerobic.
• Uses 2 ATP in the preparatory phase and produces 4 ATP in the payoff phase.
• Net gain: 2 ATP and 2 NADH per glucose molecule.
• Key enzymes: Hexokinase, Phosphofructokinase-1 (PFK-1), Pyruvate kinase.
• Pyruvate enters fermentation (anaerobic) or the Krebs cycle (aerobic) depending on oxygen availability.

Keywords

Glycolysis, Glucose, Pyruvate, ATP, NADH, Cytoplasm, Anaerobic, Enzymes, Energy Production, Cellular Respiration, Preparatory Phase, Payoff Phase, Metabolic Pathway.

Glycolysis

Introduction

Glycolysis is a key metabolic pathway that breaks down glucose (a six-carbon sugar) into two molecules of pyruvate (a three-carbon compound). This process takes place in the cytoplasm of all living cells and does not require oxygen, making it an anaerobic process.

It serves as the first step in cellular respiration, supplying energy to both aerobic and anaerobic organisms.

Overview of Glycolysis

• Location: Cytoplasm
• Oxygen Requirement: None (Anaerobic)
• Starting Molecule: Glucose (C₆H₁₂O₆)
• End Products: 2 Pyruvate, 2 NADH, and 2 ATP (net gain)
• Total Steps: 10 enzyme-catalyzed reactions

Phases:

1.     Preparatory (Energy Investment) Phase

2.     Payoff (Energy Yielding) Phase

A. Preparatory Phase (Energy Investment Phase)

This phase consumes two ATP molecules to activate glucose.

1.     Phosphorylation of Glucose

o    Enzyme: Hexokinase

o    Reaction: Glucose → Glucose-6-phosphate (G6P)

o    ATP Used: 1

o    Purpose: Traps glucose inside the cell.

2.     Isomerization

o    Enzyme: Phosphoglucose isomerase

o    Reaction: G6P → Fructose-6-phosphate (F6P)

3.     Second Phosphorylation (Key Regulatory Step)

o    Enzyme: Phosphofructokinase-1 (PFK-1)

o    Reaction: F6P → Fructose-1,6-bisphosphate (F1,6BP)

o    ATP Used: 1

o    Purpose: Controls the rate of glycolysis.

4.     Cleavage of F1,6BP

o    Enzyme: Aldolase

o    Reaction: F1,6BP → Dihydroxyacetone phosphate (DHAP) + Glyceraldehyde-3-phosphate (G3P)

5.     Isomerization of DHAP

o    Enzyme: Triose phosphate isomerase

o    Reaction: DHAP → G3P

o    Purpose: Converts all 3-carbon intermediates to G3P.

B. Payoff Phase (Energy Harvesting Phase)

This phase generates ATP and NADH.

6.     Oxidation and Phosphorylation

o    Enzyme: Glyceraldehyde-3-phosphate dehydrogenase

o    Reaction: G3P → 1,3-Bisphosphoglycerate (1,3-BPG)

o    NADH Produced: 1 per G3P (Total = 2)

7.     ATP Formation (Substrate-Level Phosphorylation)

o    Enzyme: Phosphoglycerate kinase

o    Reaction: 1,3-BPG → 3-Phosphoglycerate (3PG)

o    ATP Produced: 2 (one per G3P)

8.     Mutase Reaction

o    Enzyme: Phosphoglycerate mutase

o    Reaction: 3PG → 2-Phosphoglycerate (2PG)

9.     Dehydration Reaction

o    Enzyme: Enolase

o    Reaction: 2PG → Phosphoenolpyruvate (PEP)

o    Byproduct: H₂O

10. Formation of Pyruvate (Final Step)

o    Enzyme: Pyruvate kinase

o    Reaction: PEP → Pyruvate

o    ATP Produced: 2 (one per G3P)

Key Enzymes to Remember

• Hexokinase: Catalyzes the first phosphorylation step.
• Phosphofructokinase-1 (PFK-1): Main regulatory enzyme.
• Pyruvate kinase: Catalyzes the final ATP-producing reaction.

Significance of Glycolysis

• Universal Pathway: Found in nearly all living organisms.
• Quick Energy Source: Essential for tissues like red blood cells and muscles.
• Central Role: Provides intermediates for other metabolic pathways.
• Connection to Respiration:
• Under aerobic conditions, pyruvate enters the Krebs cycle.
• Under anaerobic conditions, it undergoes fermentation to produce lactic acid or ethanol.