Question

The correct sequence of metabolic intermediates in the Krebs cycle is:

• A. \(\alpha\)-ketoglutarate \(⇒\) fumarate \(⇒\) succinate \(⇒\) malate
• B. fumarate \(⇒\) malate \(⇒\) succinate \(⇒ \alpha\)-ketoglutarate
• C. \(\alpha\)-ketoglutarate \(⇒\) succinate \(⇒\) fumarate \(⇒\) malate
• D. succinate \(⇒ \alpha\)-ketoglutarate \(⇒\) malate \(⇒\) fumarate

Hint:

Mnemonic for Krebs cycle intermediates: “{Citrate Is Krebs’ Starting Substrate For Making Oxaloacetate}” → Citrate, Isocitrate, \(\alpha\)-ketoglutarate, Succinyl-CoA, Succinate, Fumarate, Malate, Oxaloacetate.

Answer 1

The Correct Option is C

The correct order of intermediates in the Krebs cycle (TCA cycle) after citrate and isocitrate is:
\[ \alpha\text{-ketoglutarate} \;⇒\; succinyl-CoA \;⇒\; succinate \;⇒\; fumarate \;⇒\; malate \;⇒\; oxaloacetate \] - Option (A) is incorrect because fumarate cannot precede succinate.
- Option (B) is incorrect as the order is mixed up.
- Option (D) is incorrect because succinate does not precede \(\alpha\)-ketoglutarate.
- Option (C) is correct as it preserves the right order.
\fbox{Therefore, the correct sequence is (C).}