Q.78 Which one of the following enzyme combinations allows some bacteria to utilize acetate through
glyoxylate pathway?
(P) Isocitrate lyase
(Q) Isocitrate dehydrogenase
(R) Succinyl CoA synthetase
(S) Malate synthase
(A) P & S
(B) P & R
(C) Q & S
(D) Q & R
The glyoxylate pathway enables certain bacteria to metabolize acetate as a carbon source by bypassing CO2-losing steps of the TCA cycle. The correct enzyme combination is (A) P & S: Isocitrate lyase and Malate synthase.
Pathway Overview
Bacteria like E. coli activate the glyoxylate shunt during growth on acetate, converting two acetyl-CoA molecules into one succinate and one malate for gluconeogenesis. Isocitrate lyase (P) cleaves isocitrate into glyoxylate and succinate, while malate synthase (S) condenses glyoxylate with acetyl-CoA to form malate. These unique enzymes distinguish the pathway from the standard TCA cycle.
Option Analysis
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(P) Isocitrate lyase: Essential; splits isocitrate, avoiding decarboxylation to enable net carbon gain from acetate. Mutants lacking it fail to grow on acetate.
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(Q) Isocitrate dehydrogenase: TCA cycle enzyme; oxidatively decarboxylates isocitrate to alpha-ketoglutarate, losing CO2—bypassed in glyoxylate pathway, so not required for acetate use.
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(R) Succinyl CoA synthetase: Common to both TCA and glyoxylate cycles; converts succinyl-CoA to succinate but not unique or sufficient alone for the shunt.
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(S) Malate synthase: Critical; forms malate from glyoxylate and acetyl-CoA, completing the bypass. Mutants show impaired acetate assimilation.
Why P & S?
Only isocitrate lyase (P) and malate synthase (S) are the dedicated glyoxylate shunt enzymes, allowing bacteria to generate four-carbon intermediates from acetate without carbon loss. Options with Q or R fail because Q defeats the bypass purpose and R is non-specific.
