Q.37 The enzyme which converts citrate to isocitrate in TCA cycle is:
1. Aconitase
2. Isocitrate dehydrogenase
3. Fumarase
4. Trans ketolase
Aconitase is the enzyme that converts citrate to isocitrate in the TCA cycle, making option 1 the correct answer for this biochemistry question.
Correct Answer
1. Aconitase
Aconitase catalyzes the reversible isomerization of citrate to isocitrate via cis-aconitate intermediate in the mitochondrial matrix during the Krebs cycle (TCA cycle). It uses a [4Fe-4S] cluster to dehydrate/rehydrate the substrate, enabling the cycle’s progression.
Option Breakdown
1. Aconitase
This iron-sulfur enzyme performs the second step of TCA cycle, dehydrating citrate to cis-aconitate then hydrating it to isocitrate. Essential for cycle flux and regulated by iron availability.
2. Isocitrate Dehydrogenase
Follows aconitase; oxidatively decarboxylates isocitrate to α-ketoglutarate, producing NADH/NADPH and CO₂. Rate-limiting step, allosterically controlled by ADP/ATP and Ca²⁺.
3. Fumarase
Later TCA enzyme converting fumarate to malate via hydration. Stereospecific and reversible, located in mitochondria; aids final steps before oxaloacetate regeneration.
4. Transketolase
Not a TCA enzyme; pentose phosphate pathway thiamine-dependent enzyme transferring C₂ units in non-oxidative phase for NADPH/ribose production, unrelated to citrate metabolism.
