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Entner–Doudoroff pathway

The Entner–Doudoroff (ED) pathway is a glucose-catabolizing route found predominantly in aerobic Gram-negative bacteria (notably Pseudomonas, Azotobacter, Rhizobium, Agrobacterium, and some Zymomonas strains under aerobic conditions). Unlike the ubiquitous Embden–Meyerhof–Parnas (EMP) glycolytic pathway, the ED pathway yields only 1 net ATP per glucose (versus 2 in EMP) but produces higher amounts of NADPH and is energetically favorable under oxygen-rich, nutrient-limited conditions. 

1. Distribution

  • Dominant in aerobic Gram-negative α-, β-, and γ-proteobacteria (Pseudomonas, Gluconobacter, Azotobacter, Rhizobium, Agrobacterium, etc.).
  • Present in some archaea (e.g., Halobacterium, Sulfolobus, Thermoplasma).
  • Rare in Gram-positive bacteria and virtually absent in Enterobacteriaceae (which use EMP).
  • Zymomonas mobilis uses a modified ED pathway anaerobically for ethanol production (high yield: ~1.9 mol ethanol/mol glucose).

2. Two Major Variants

  • Classical (non-phosphorylative) ED pathway – found in most Pseudomonas species.
  • Phosphorylative (semi-phosphorylative) ED pathway – found in some organisms (e.g., Gluconobacter, Zymomonas) that incorporate a phosphorylation step at glyceraldehyde-3-phosphate.

3. Detailed Enzymatic Steps of the Classical Entner–Doudoroff Pathway

  1. Phosphorylation of Glucose: Glucose is first phosphorylated to glucose-6-phosphate (G6P) by hexokinase, consuming one ATP molecule. This phosphorylation helps retain glucose inside the cell and keeps intracellular glucose concentrations low.

  2. Oxidation to 6-Phosphogluconolactone: Glucose-6-phosphate dehydrogenase oxidizes G6P to 6-phosphogluconolactone, reducing NADP+ to NADPH, an important cellular reducing agent.

  3. Hydrolysis to 6-Phosphogluconate: A hydrolase enzyme converts 6-phosphogluconolactone to 6-phosphogluconate by opening the lactone ring.

  4. Dehydration to KDPG: 6-Phosphogluconate undergoes a dehydration reaction catalyzed by 6-phosphogluconate dehydratase to form 2-keto-3-deoxy-6-phosphogluconate (KDPG).

  5. Cleavage of KDPG: KDPG aldolase cleaves KDPG into pyruvate and glyceraldehyde-3-phosphate (GAP).

  6. Metabolism of Glyceraldehyde-3-Phosphate: GAP enters the EMP glycolytic pathway, where it is converted into pyruvate, generating ATP and NADH.

4. Regulation

  • Strongly induced by gluconate or glucose under aerobic conditions.
  • Catabolite repression by organic acids (succinate, malate) via Crc/CrcZ/Hfq system in Pseudomonas.
  • KDPG aldolase is often a key regulatory point; the enzyme is induced only when ED pathway intermediates accumulate.

5. Physiological Advantages

  • High NADPH yield → supports biosynthetic reactions and oxidative stress resistance (important in soil pseudomonads exposed to peroxides).
  • Lower ATP yield but higher carbon flux to pyruvate when ATP demand is low.
  • Allows growth on gluconate without prior phosphorylation (energy-saving under nutrient limitation).
  • In Zymomonas: anaerobic ED + pyruvate decarboxylase/alcohol dehydrogenase → near-theoretical ethanol yield.

6. Evolutionary and Ecological Significance

The ED pathway is ancestral in many proteobacteria and archaea. Its persistence despite lower ATP yield reflects selection for NADPH generation and survival under oxidative, oligotrophic environments rather than rapid fermentative growth.