Glyoxylate cycle and carbon sources: Some microorganisms employ the glyoxylate shunt when which compound is the sole source of carbon for growth?

Introduction / Context:
The glyoxylate cycle is a metabolic bypass that enables cells to grow on C2 compounds by conserving carbon skeletons that would otherwise be lost as CO2 in the tricarboxylic acid (TCA) cycle. Understanding when microbes switch on this pathway explains growth on acetate, fatty acids, or ethanol.

Given Data / Assumptions:

• Organism faces a sole carbon source with two carbons per molecule.
• Objective is net synthesis of C4 and C6 intermediates for biosynthesis.
• Key enzymes: isocitrate lyase and malate synthase.

Concept / Approach:

When acetate is the only carbon source, acetyl-CoA is the entry currency. The conventional TCA cycle decarboxylates carbon at isocitrate dehydrogenase and α-ketoglutarate dehydrogenase steps, preventing net carbon gain. The glyoxylate shunt bypasses these CO2-losing steps: isocitrate → succinate + glyoxylate (via isocitrate lyase), and glyoxylate + acetyl-CoA → malate (via malate synthase). This preserves carbon and allows gluconeogenesis and biosynthetic precursor formation.

Step-by-Step Solution:

Verification / Alternative check:

Mutants lacking isocitrate lyase cannot grow on acetate but grow normally on glucose, confirming the pathway’s role in C2 metabolism.

Why Other Options Are Wrong:

Nitrate: a nitrogen source/electron acceptor, not a carbon source.

Carbon dioxide: chemoautotrophs fix CO2 via Calvin cycle or other CO2-fixing routes, not glyoxylate shunt.

“all of these”: incorrect because only acetate fits the context.

Common Pitfalls:

Confusing glyoxylate cycle with Calvin cycle; assuming any poor carbon source requires glyoxylate shunt.

Final Answer:

acetate