Question

The only 5C intermediate formed during reactions of TCA cycle is ________

• A. Oxalosuccinic acid
• B. $\alpha$-ketoglutarate
• C. Succinyl CoA
• D. Cis - Aconitate

Hint:

Carbon flow: 6C $\rightarrow$ 5C ($\alpha$-ketoglutarate) $\rightarrow$ 4C

Answer 1

The Correct Option is B

Concept: The TCA (Krebs) cycle involves a series of reactions converting a 2-carbon acetyl group into CO$_2$ with formation of intermediates of different carbon lengths.
• 6C compounds: Citrate, Isocitrate
• 5C compound: $\alpha$-ketoglutarate
• 4C compounds: Succinyl CoA, Succinate, Fumarate, Malate, Oxaloacetate

Step 1: Track carbon numbers.
After decarboxylation of isocitrate (6C), a 5C compound is formed.

Step 2: Identify the 5C intermediate.
This intermediate is $\alpha$-ketoglutarate.

Step 3: Analyze options.
• (A) Oxalosuccinic acid: Intermediate but unstable and not counted as main 5C intermediate
• (B) $\alpha$-ketoglutarate: Stable 5C compound (correct)
• (C) Succinyl CoA: 4C
• (D) Cis-aconitate: 6C

Step 4: Conclusion.
Thus, the only 5-carbon intermediate is $\alpha$-ketoglutarate. Final Answer: Option (B)