Question

The first three steps of the $\beta$-oxidation pathway of fatty acyl-CoA, chemically resemble the citric acid cycle reactions carrying out which of the following biochemical conversion?

• A. Oxaloacetate to $\alpha$-ketoglutarate
• B. Citrate to Succinyl-CoA
• C. Succinate to Oxaloacetate
• D. Isocitrate to Succinate

Hint:

This three-step chemical sequence (oxidation-hydration-oxidation) is a conserved metabolic motif used by cells whenever a stable, unreactive single carbon-carbon bond needs to be activated for cleavage.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Metabolic pathways often share common chemical mechanisms to convert stable hydrocarbon chains into reactive carbonyl groups.
Both the mitochondrial $\beta$-oxidation of fatty acids and the Citric Acid (Krebs) Cycle utilize a conserved sequence of oxidation, hydration, and oxidation reactions.

Step 2: Detailed Explanation:
Let us compare the chemical steps of the two pathways:
1. In the $\beta$-oxidation pathway, a fatty acyl-CoA (which has a saturated, alkane-like hydrocarbon chain) is converted into a $\beta$-ketoacyl-CoA through three sequential reactions:
- First Step (Oxidation): Acyl-CoA dehydrogenase introduces a trans double bond between C-2 and C-3, reducing FAD to FADH${}_2$ and forming trans-2-enoyl-CoA.
- Second Step (Hydration): Enoyl-CoA hydratase adds water across the double bond, converting the alkene into a secondary alcohol, L-3-hydroxyacyl-CoA.
- Third Step (Oxidation): 3-hydroxyacyl-CoA dehydrogenase oxidizes the hydroxyl group to a ketone, reducing NAD${}^+$ to NADH and forming 3-ketoacyl-CoA.
2. In the Citric Acid Cycle, the conversion of Succinate to Oxaloacetate proceeds through the exact same sequence of chemical transformations:
- First Step (Oxidation): Succinate dehydrogenase oxidizes the saturated C-C bond of succinate (alkane) to form fumarate (alkene with a double bond), reducing FAD to FADH${}_2$.
- Second Step (Hydration): Fumarase adds water across the double bond of fumarate to form L-malate (secondary alcohol).
- Third Step (Oxidation): Malate dehydrogenase oxidizes the secondary alcohol group of L-malate to a ketone, forming oxaloacetate and reducing NAD${}^+$ to NADH.
Therefore, the first three steps of $\beta$-oxidation chemically resemble the conversion of succinate to oxaloacetate.

Step 3: Final Answer:
The biochemical conversion resembling the first three steps of $\beta$-oxidation is the conversion of Succinate to Oxaloacetate, which corresponds to option (C).