Question

The TCA cycle starts with the condensation of acetyl group with

• A. $\alpha$-Ketoglutaric acid
• B. Succinic acid
• C. Oxaloacetic acid
• D. Citric acid

Hint:

Oxaloacetic acid accepts the acetyl group to start the TCA cycle, forming citric acid.

Answer 1

The Correct Option is C

• TCA Cycle Overview: The tricarboxylic acid (TCA) cycle, also known as the Krebs cycle, begins with the condensation of an acetyl group (from acetyl-CoA) with a four-carbon compound.
• Reaction Analysis: The first step involves the combination of acetyl-CoA (2 carbons) with oxaloacetic acid (4 carbons) to form citric acid (6 carbons), catalyzed by citrate synthase.
• Option Analysis:
  • (1) $\alpha$-Ketoglutaric acid: An intermediate later in the cycle, not the starting compound. Incorrect.
  • (2) Succinic acid: Another intermediate, not involved in the initial condensation. Incorrect.
  • (3) Oxaloacetic acid: The correct four-carbon acceptor that condenses with acetyl-CoA. Correct.
  • (4) Citric acid: The product of the initial condensation, not the reactant. Incorrect.
• Conclusion: The correct answer is (3) Oxaloacetic acid, as it is the compound that initiates the TCA cycle.