Glycolytic Pathway - Stage Three

May 21, 2024

Glycolytic Pathway: Stage Three

Overview

  • Focus on steps in stage three of glycolysis
  • Harvest energy from glyceraldehyde-3-phosphates (GAP)
  • Formation of ATP, NADH, and pyruvate
  • NADH and pyruvate can be further processed under aerobic conditions for more ATP

Step 1: Conversion of GAP to 1,3-Bisphosphoglycerate (1,3-BPG)

  • Enzyme: Glyceraldehyde-3-Phosphate Dehydrogenase (GAPDH)
  • Reaction:
    • GAP is converted to 1,3-BPG
    • NAD+ is reduced to NADH
    • Addition of orthophosphate (Pi) to GAP

Mechanism

  • Hydride transfer from GAP to NAD+
  • Formation of a thioester intermediate
  • NADH leaves the active site, replaced by NAD+
  • NAD+ helps depolarize and weaken thioester bond
  • Orthophosphate enters and forms 1,3-BPG
  • Enzyme catalytic residues: Cys 149 and His 176
  • Enzyme active site restores catalytic residues

Uncatalyzed Reaction

  • Divides into two reactions: Oxidation-reduction and dehydration
  • Energy release and consumption balance out
  • Without enzyme, carboxylic acid intermediate is too stable to proceed

Catalyzed Reaction

  • Formation of a less stable thioester intermediate instead of carboxylic acid
  • Thioester intermediate is higher in energy
  • Enzyme couples favorable oxidation-reduction with the unfavorable dehydration

Step 2: Phosphate Transfer from 1,3-BPG to ADP

  • Enzyme: Phosphoglycerate Kinase
  • Reaction: Substrate-Level Phosphorylation
    • 1,3-BPG transfers a phosphate group to ADP, forming ATP
  • Energy Consideration:
    • 1,3-BPG has a higher phosphoryl transfer potential than ATP
    • Formation of 3-phosphoglycerate
  • Net Result: Formation of 2 ATP molecules
    • 2 molecules of 1,3-BPG form 2 ATP and 2 3-phosphoglycerates

Summary

  • Step 1 forms 2 NADH and 2 H+ and 2 1,3-BPG
  • Step 2 forms 2 ATP from ADP and 2 3-phosphoglycerate
  • Net ATP at this stage is zero (2 used in stage one and 2 produced in stage three)

Key Terminology:

  • Glyceraldehyde-3-Phosphate (GAP): Substrate molecule
  • 1,3-Bisphosphoglycerate (1,3-BPG): Product of GAP conversion
  • NAD+: Nicotinamide adenine dinucleotide, oxidized form
  • NADH: Reduced form of NAD+
  • Orthophosphate (Pi): Inorganic phosphate
  • Substrate-Level Phosphorylation: Direct transfer of a phosphate group to ADP to form ATP