Q84. A mutant of E. coli grows normally on glucose or on glycerol but not on acetate. The most likely metabolic pathway that is defective in this mutant is

1. Glyoxylate cycle
2. Hexose monophosphate shunt
3. Krebs cycle
4. Entner–Doudoroff pathway
   

   E. coli Mutant Glyoxylate Cycle Defect

   E. coli mutants reveal key metabolic pathways by their growth patterns on specific carbon sources. This article explains why the glyoxylate cycle is defective in a mutant growing on glucose or glycerol but not acetate.

   Correct Answer

   The most likely defective pathway is the glyoxylate cycle. This cycle allows E. coli to use acetate as a carbon source by bypassing CO₂-releasing steps of the Krebs cycle, enabling net synthesis of gluconeogenic precursors like malate and succinate.

   Metabolic Context

   Glucose enters via glycolysis to pyruvate and acetyl-CoA, feeding the Krebs cycle for energy. Glycerol converts to dihydroxyacetone phosphate, entering gluconeogenesis or glycolysis, independent of glyoxylate enzymes. Acetate, however, activates acetyl-CoA via acetate kinase and phosphotransacetylase, requiring the glyoxylate shunt for growth since the full Krebs cycle loses carbon as CO₂.

   A glyoxylate cycle defect blocks acetate utilization while sparing glucose (glycolysis) and glycerol (gluconeogenesis) pathways.​

   Option Breakdown

   Option	Description	Why Not Defective Here
   Glyoxylate cycle	Bypasses isocitrate dehydrogenase in Krebs cycle; isocitrate lyase cleaves isocitrate to glyoxylate + succinate; malate synthase forms malate. Essential for C₂ compounds like acetate.	Matches phenotype: no growth on acetate; normal on glucose/glycerol .
   Hexose monophosphate shunt (Pentose phosphate pathway)	Oxidative/non-oxidative branches generate NADPH, ribose-5-P; alternative to glycolysis for glucose.	Glucose growth intact; not required for glycerol/acetate ​.
   Krebs cycle (TCA cycle)	Full oxidation of acetyl-CoA to CO₂; provides energy/intermediates.	Full block would impair all; partial use possible, but glyoxylate specifically enables acetate ​.
   Entner-Doudoroff pathway	Glucose to pyruvate via KDPG; minor in E. coli, absent natively for glucose.	E. coli relies on glycolysis for glucose; irrelevant for glycerol/acetate ​.

   Why Glyoxylate Cycle Fits Perfectly

   Without isocitrate lyase (key glyoxylate enzyme), acetyl-CoA from acetate cannot replenish oxaloacetate for gluconeogenesis, halting growth. Glucose and glycerol bypass this via glycolysis/gluconeogenesis convergence. This mutant highlights glyoxylate’s role in microbial C₂ assimilation, crucial for exams in microbiology and biochemistry.