Question

Enzymes involved in the complete oxidation and utilization of odd-chain fatty acids:
A. Propionyl CoA carboxylase
B. Thiolase
C. Fatty acyl CoA synthetase
D. Methylmalonyl CoA mutase
E. Methylmalonyl CoA racemase
Choose the correct sequence in which aforementioned enzymes act for the purpose of oxidation and utilization of odd-chain fatty acids as a source of energy.

• A. C, B, A, E, D
• B. B, C, A, D, E
• C. A, D, E, C, B
• D. A, E, D, B, C

Hint:

Odd-chain fatty acid oxidation always starts with activation (Synthetase) and \(\beta\)-oxidation (Thiolase), which produces Propionyl-CoA. This intermediate is then processed by a Carboxylase, a Racemase, and finally a Mutase to yield Succinyl-CoA.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Odd-chain fatty acids undergo \(\beta\)-oxidation in a manner similar to even-chain fatty acids.
However, the final cycle of \(\beta\)-oxidation yields one molecule of acetyl-CoA and one molecule of propionyl-CoA (a three-carbon unit).
Propionyl-CoA must be converted via a specialized metabolic pathway into succinyl-CoA to enter the citric acid cycle.

Step 2: Detailed Explanation:
Let us trace the complete sequence of enzymatic steps involved in oxidizing odd-chain fatty acids:
1. Fatty acyl CoA synthetase (C): This enzyme activates the free fatty acid in the cytosol by coupling it with Coenzyme A to form fatty acyl-CoA, allowing it to enter the mitochondria via the carnitine shuttle.
2. Thiolase (B): This enzyme catalyzes the final cleavage step of \(\beta\)-oxidation, releasing one molecule of acetyl-CoA and leaving behind a three-carbon propionyl-CoA.
3. Propionyl CoA carboxylase (A): This biotin-dependent enzyme carboxylates propionyl-CoA to form D-methylmalonyl-CoA in an ATP-dependent reaction.
4. Methylmalonyl CoA racemase (E): This enzyme converts D-methylmalonyl-CoA into its stereoisomer, L-methylmalonyl-CoA.
5. Methylmalonyl CoA mutase (D): This vitamin \(\text{B}_{12}\)-dependent enzyme catalyzes the structural rearrangement of L-methylmalonyl-CoA into succinyl-CoA, which can directly enter the Krebs cycle.
This sequential pathway follows the order: C \(\to\) B \(\to\) A \(\to\) E \(\to\) D.

Step 3: Final Answer:
The correct metabolic sequence is C, B, A, E, D, which corresponds to option (A).