5 - Oxidation of Odd Chain Length FA and Unsat. FA

What is the final product of β-oxidation of an odd-chain fatty acid?

Propionyl-CoA (3-carbon compound).

What is propionyl-CoA converted into for entry into metabolism?

Succinyl-CoA (a TCA cycle intermediate).

What is the intermediate product formed from propionyl-CoA before succinyl-CoA?

D-Methylmalonyl-CoA, then L-Methylmalonyl-CoA.

What enzyme converts L-Methylmalonyl-CoA to Succinyl-CoA?

Methylmalonyl-CoA mutase.

What vitamin-derived coenzyme is required by methylmalonyl-CoA mutase?

Vitamin B₁₂ (cobalamin).

What coenzyme is required by propionyl-CoA carboxylase in the first step of the conversion?

Biotin (Vitamin B₇), which helps in the carboxylation of propionyl-CoA.

What is the full pathway for converting propionyl-CoA to succinyl-CoA?

1. Propionyl-CoA → D-Methylmalonyl-CoA (via propionyl-CoA carboxylase, requires biotin)

2. D → L-Methylmalonyl-CoA (via racemase)

3. L-Methylmalonyl-CoA → Succinyl-CoA (via methylmalonyl-CoA mutase, requires Vitamin B₁₂)

What structural feature of unsaturated fatty acids complicates β-oxidation?

Presence of cis-double bonds, which are not compatible with standard β-oxidation enzymes.

What two additional enzymes are required for unsaturated fatty acid oxidation?

• Enoyl-CoA isomerase

• 2,4-Dienoyl-CoA reductase

When is Enoyl-CoA isomerase used during β-oxidation of unsaturated fatty acids?

When a single cis-Δ³ double bond is encountered.

When are both Enoyl-CoA isomerase and 2,4-Dienoyl-CoA reductase required?

When there is a cis-Δ⁴ or multiple double bonds such that the intermediate is conjugated (2,4-dienoyl).

What is the position of the cis double bond that only requires isomerase?

A cis-Δ³ double bond (3rd carbon from the carboxyl end).

How does the isomerase help with a cis-Δ³ double bond?

Converts it to a trans-Δ² double bond suitable for standard β-oxidation.

What is the location of the cis double bonds that require both isomerase and reductase?

Cis-Δ⁴, or when two double bonds are present in conjugated positions (Δ² and Δ⁴).

What is the function of 2,4-Dienoyl-CoA reductase?

Reduces the 2,4-dienoyl-CoA intermediate to a trans-Δ³ intermediate, which is then handled by isomerase.

How does the energy yield from unsaturated fatty acids compare to saturated fatty acids?

Lower energy yield, because fewer FADH₂ molecules are produced.

Why is less FADH₂ produced during oxidation of unsaturated fatty acids?

Because acyl-CoA dehydrogenase is bypassed in the step where a cis-double bond is present, and that step normally generates FADH₂.