Q.38 The enzyme NOT involved in oxidation of the molecule shown below is
(A)Δ3, Δ2- Enoyl –CoA isomerase
(B) Propionyl-CoA caboxylase
(C) Acyl CoA dehydrogenase
(D)Enoyl CoA hydratase

Introduction

Oxidation of fatty acids is a crucial metabolic process that occurs primarily through β-oxidation in mitochondria. When a fatty acid contains double bonds, additional enzymes are required to rearrange these bonds so that normal β-oxidation can proceed.

In this question, an unsaturated fatty acid is shown, and we are asked to identify the enzyme NOT involved in its oxidation.

Correct Answer

✅ (B) Propionyl-CoA carboxylase

Understanding Oxidation of Unsaturated Fatty Acids

Unsaturated fatty acids undergo β-oxidation similarly to saturated fatty acids, but extra enzymes are required to handle cis double bonds. These include:

• Acyl-CoA dehydrogenase

• Enoyl-CoA hydratase

• Enoyl-CoA isomerase

However, propionyl-CoA carboxylase plays a role only in the final metabolism of odd-chain fatty acids, not in the oxidation steps of unsaturated fatty acids themselves.

Explanation of Each Option

Option (A): Δ³, Δ²-Enoyl-CoA Isomerase

🔹 Involved

This enzyme is essential during the β-oxidation of unsaturated fatty acids.
It converts cis-Δ³ double bonds into trans-Δ² double bonds, which are required for normal β-oxidation to continue.

✅ Correctly involved in oxidation.

Option (B): Propionyl-CoA Carboxylase

🔹 NOT involved ✅ Correct Answer

This enzyme functions after β-oxidation, specifically when odd-chain fatty acids are completely broken down and propionyl-CoA is formed.

Its role:

• Converts propionyl-CoA → methylmalonyl-CoA

• Helps enter the TCA cycle

❌ It does not participate in the oxidation steps of the fatty acid shown.

Option (C): Acyl-CoA Dehydrogenase

🔹 Involved

This enzyme catalyzes the first step of each β-oxidation cycle, introducing a double bond between the α and β carbons.

It is required for:

• Saturated fatty acids

• Unsaturated fatty acids

✅ Essential enzyme in oxidation.

Option (D): Enoyl-CoA Hydratase

🔹 Involved

This enzyme hydrates the trans-Δ²-enoyl-CoA intermediate to form β-hydroxyacyl-CoA.

It acts in:

• Normal β-oxidation

• Unsaturated fatty acid oxidation (after isomerization)

✅ Clearly involved in oxidation.

Why Option (B) Is Correct

• The molecule shown is an unsaturated fatty acid

• Its oxidation requires isomerase, dehydrogenase, and hydratase

• Propionyl-CoA carboxylase works only after oxidation, during metabolism of odd-chain end products

👉 Therefore, it is NOT involved in the oxidation of the given molecule.