Question

Metabolism of polyunsaturated fatty acids through $\beta$-oxidation require enzymes in addition to enzymes of $\beta$-oxidation are
(A) $\Delta^3$-$\Delta^2$-enoyl CoA isomerase
(B) 2,4-dienoyl CoA reductase
(C) Enoyl CoA isomerase
(D) Methyl malonyl CoA mutase
Choose the correct answer from the options given below:

• A. (A), (B) and (D) only.
• B. (A), (B) and (C) only.
• C. (A), (B), (C) and (D).
• D. (B), (C) and (D) only.

Hint:

To remember auxiliary enzymes for fatty acid oxidation:
- Odd-numbered double bonds require only Isomerase.
- Even-numbered double bonds require both Reductase and Isomerase.
- Odd-chain fatty acids require Mutase at the end of the pathway.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Saturated fatty acids are degraded via the standard four-step spiral of \(\beta\)-oxidation.
However, polyunsaturated fatty acids contain cis-double bonds at various positions.
These double bonds present steric obstacles that the standard \(\beta\)-oxidation enzymes cannot process directly, requiring auxiliary enzymes.

Step 2: Detailed Explanation:
During the \(\beta\)-oxidation of polyunsaturated fatty acids, two major auxiliary enzymes are required:
- Isomerases (A and C): Standard \(\beta\)-oxidation requires a trans-\(\Delta^2\)-enoyl-CoA substrate.
When a cis-\(\Delta^3\)-double bond is encountered, cis-\(\Delta^3\)-enoyl-CoA isomerase (also referred to as \(\Delta^3\)-\(\Delta^2\)-enoyl-CoA isomerase) converts it to the trans-\(\Delta^2\) isomer, which is a normal intermediate of \(\beta\)-oxidation.
- Reductase (B): When a \(\Delta^4\) double bond is present, the action of acyl-CoA dehydrogenase yields a 2,4-dienoyl-CoA intermediate.
This intermediate cannot be processed by the next enzyme in the pathway.
2,4-dienoyl-CoA reductase uses NADPH to reduce this diene intermediate to a single trans-\(\Delta^3\)-enoyl-CoA, which is then isomerized by the isomerase.
- Methyl malonyl CoA mutase (D) is a cobalamin (vitamin B12)-dependent enzyme required for the metabolism of odd-chain fatty acids (converting propionyl-CoA to succinyl-CoA), not specifically polyunsaturated fatty acids.
Thus, (A), (B), and (C) are the necessary auxiliary enzymes.

Step 3: Final Answer:
The additional enzymes required for the metabolism of polyunsaturated fatty acids are (A), (B), and (C) only.