Q.81 An E.coli mutant defective for an enzyme is unable to grow on acetate but grows on glycerol as the
sole carbon source. Which ONE of the following enzymes is likely to be defective in this mutant?
(A) Isocitrate dehydrogenase (B) Glyceraldehyde 3-phospahte dehydrogenase
(C) Pyruvate dehydrogenase (D) Isocitrate lyase
The correct answer is (D) Isocitrate lyase. This enzyme is essential for E. coli to utilize acetate as a sole carbon source via the glyoxylate cycle, while growth on glycerol relies on glycolysis and gluconeogenesis, which bypass this defect.
Question Breakdown
E. coli mutants defective in certain enzymes reveal metabolic pathways for carbon utilization. Acetate requires the glyoxylate shunt to replenish TCA cycle intermediates for biosynthesis, whereas glycerol feeds directly into glycolysis.
Option Analysis
(A) Isocitrate Dehydrogenase
This enzyme converts isocitrate to α-ketoglutarate in the TCA cycle. Mutants can still grow on acetate because the glyoxylate cycle bypasses this step by diverting isocitrate via isocitrate lyase; its activity is even regulated (inhibited) during acetate growth.
(B) Glyceraldehyde 3-Phosphate Dehydrogenase
A core glycolytic enzyme converting glyceraldehyde 3-phosphate to 1,3-bisphosphoglycerate. Glycerol metabolism produces this intermediate directly, and a defect would impair glycerol growth, not selectively acetate.
(C) Pyruvate Dehydrogenase
Converts pyruvate to acetyl-CoA for TCA entry. For acetate (already acetyl-CoA), it’s unnecessary; glycerol generates pyruvate via gluconeogenesis without needing this enzyme.
(D) Isocitrate Lyase
Cleaves isocitrate to succinate and glyoxylate in the glyoxylate cycle, unique for C2 compounds like acetate. Mutants fail to grow on acetate (long lag, slow/no growth) but thrive on glycerol via standard pathways.
Introduction
In microbiology and biochemistry, understanding how an E.coli mutant defective enzyme impacts growth on acetate versus glycerol is key for competitive exams like GATE Life Sciences. This E.coli mutant unable to grow on acetate but grows on glycerol highlights the glyoxylate cycle’s role, where isocitrate lyase is crucial for acetate metabolism.
Glyoxylate Cycle in E.coli
E.coli uses the glyoxylate cycle to grow on acetate as the sole carbon source, bypassing CO2-releasing TCA steps. Acetyl-CoA from acetate condenses with oxaloacetate to citrate, then aconitate to isocitrate. Isocitrate lyase splits isocitrate into succinate (TCA refill) and glyoxylate, which malate synthase converts to malate.
Without it, no net carbon gain for biosynthesis occurs on acetate.
Why Growth Succeeds on Glycerol
Glycerol enters as glycerol-3-phosphate, then dihydroxyacetone phosphate, feeding glycolysis to pyruvate and gluconeogenesis. No glyoxylate cycle needed; TCA runs fully for energy.
Enzyme Options Explained
| Enzyme | Role | Impact on Acetate Mutant | Impact on Glycerol Mutant |
|---|---|---|---|
| Isocitrate dehydrogenase | TCA: isocitrate → α-KG | Bypassed by glyoxylate; growth ok | Glycolysis unaffected; growth ok |
| Glyceraldehyde 3-P dehydrogenase | Glycolysis core | Acetate independent; growth ok | Blocks glycolysis; no growth |
| Pyruvate dehydrogenase | Pyruvate → acetyl-CoA | Acetate provides acetyl-CoA; growth ok | Gluconeogenesis bypasses; growth ok |
| Isocitrate lyase | Glyoxylate: isocitrate → succinate + glyoxylate | Essential; no growth | Not required; growth ok |
Exam Relevance for GATE Life Sciences
This PYQ tests metabolic adaptation in bacteria. Remember: C2 substrates (acetate) need glyoxylate shunt; C3 (glycerol) use EMP pathway.
