Glyoxylate cycle identity check Which enzyme set is specific to the glyoxylate cycle (the bypass that enables growth on C2 compounds by conserving carbon)?

Introduction / Context:
The glyoxylate cycle is a metabolic shunt used by many bacteria, fungi, and plants to convert acetyl-CoA into four-carbon dicarboxylic acids without loss of carbon as CO2. It is essential for growth on acetate, fatty acids, or other C2 substrates and is distinguished by two hallmark enzymes.

Given Data / Assumptions:

• The cycle bypasses the CO2-producing steps of the TCA cycle.
• Two unique enzymes define this bypass.
• Objective: identify those specific enzymes.

Concept / Approach:

Isocitrate lyase cleaves isocitrate into succinate and glyoxylate. Malate synthase then condenses glyoxylate with acetyl-CoA to form malate. Together they enable net assimilation of acetyl units into cell material, avoiding the decarboxylating steps of TCA.

Step-by-Step Solution:

Verification / Alternative check:

Mutants lacking either enzyme cannot grow on acetate as sole carbon source, underscoring their specificity to the glyoxylate pathway.

Why Other Options Are Wrong:

• Isocitrate lyase or malate synthase alone: Incomplete pathway.
• Anaplerotic: Describes replenishing reactions (for example, PEP carboxylase), not the glyoxylate-specific enzymes.
• Pyruvate dehydrogenase: Part of central metabolism but not the glyoxylate-specific step.

Common Pitfalls:

• Confusing “anaplerotic” with “glyoxylate”; while related to TCA balance, they are distinct concepts.

Final Answer:

both (a) and (b)