Q67. Which ONE of the following pathways oxidizes 1 mole of glucose to 2 moles
of pyruvic acid along with one mole each of ATP, NADH and NADPH, in
Pseudomonas spp., but not in Bacillus spp.?
(A) Gluconeogenesis
(B) Embden–Meyerhoff Pathway (EMP)
(C) Entner–Doudoroff (ED) Pathway
(D) Pentose Phosphate Pathway (PPP)
The Entner-Doudoroff (ED) Pathway is the correct answer: (C). This pathway oxidizes 1 mole of glucose to 2 moles of pyruvic acid, producing 1 mole each of ATP, NADH, and NADPH in Pseudomonas spp., while Bacillus spp. primarily use the Embden-Meyerhoff Pathway (EMP).
Option Analysis
Gluconeogenesis (A): This anabolic pathway synthesizes glucose from non-carbohydrate precursors like pyruvate or lactate, consuming ATP rather than producing it. It does not oxidize glucose to pyruvic acid and operates in both Pseudomonas and Bacillus under specific conditions.
Embden-Meyerhoff Pathway (EMP) (B): Known as glycolysis, EMP converts 1 glucose to 2 pyruvates with a net yield of 2 ATP and 2 NADH (no NADPH). Both Pseudomonas and Bacillus spp. possess EMP, but Pseudomonas favor ED for glucose catabolism.
Entner-Doudoroff (ED) Pathway (C): ED pathway starts with glucose-6-phosphate oxidation to 6-phosphogluconate, dehydration to KDPG, and cleavage to pyruvate and glyceraldehyde-3-phosphate (yielding second pyruvate via lower glycolysis). Net products: 1 ATP, 1 NADH (from GAP dehydrogenase), 1 NADPH (from G6P dehydrogenase). Characteristic of Pseudomonas spp. (Gram-negative); absent in Bacillus spp. (Gram-positive, rely on EMP).
Pentose Phosphate Pathway (PPP) (D): Oxidative PPP generates NADPH and ribose-5-phosphate from glucose-6-phosphate but does not fully oxidize to 2 pyruvates. Non-oxidative PPP recycles intermediates; incomplete for the specified products and active in both genera.
The Entner-Doudoroff pathway represents a unique bacterial glucose catabolism route, oxidizing 1 mole of glucose to 2 moles of pyruvic acid alongside 1 mole each of ATP, NADH, and NADPH specifically in Pseudomonas spp. but absent in Bacillus spp. This distinction is crucial for CSIR NET Life Sciences aspirants tackling microbial metabolism questions.
Pathway Mechanisms
ED pathway bypasses EMP’s phosphofructokinase step: glucose → glucose-6-P (hexokinase, ATP used) → 6-phosphogluconolactone (G6P dehydrogenase, NADPH) → 6-phosphogluconate → KDPG (dehydratase) → pyruvate + GAP (aldolase); GAP yields second pyruvate, NADH, and ATP via EMP lower half. Pseudomonas relies on ED (up to 95% glucose flux), often via periplasmic oxidation.
Bacterial Specificity
Pseudomonas (e.g., P. putida, P. aeruginosa) uses ED due to lacking key EMP enzymes, channeling glucose via gluconate/6PG. Bacillus (Gram-positive, e.g., B. subtilis) employs EMP (60-70% glucose flux) + minor PPP, lacking ED enzymes like KDPG aldolase.
Comparison Table
| Pathway | Glucose to Pyruvate | ATP | NADH | NADPH | Pseudomonas | Bacillus |
|---|---|---|---|---|---|---|
| EMP (B) | 1 → 2 | 2 | 2 | 0 | Present (minor) | Primary |
| ED (C) | 1 → 2 | 1 | 1 | 1 | Primary | Absent |
| PPP (D) | Partial | 0 | 0 | Multiple | Minor | Minor |
| Gluconeogenesis (A) | Reverse | Consumes | Varies | Varies | Anabolic | Anabolic |
ED’s lower ATP yield suits rapid growth in nutrient-rich environments, providing NADPH for biosynthesis. Master this for CSIR NET success in molecular biology and microbiology sections.
