Question

Select the correct sequence of steps involved in TCA cycle:
A. $\alpha$-Ketoglutarate
B. Succinyl CoA
C. Fumarate
D. Citrate
E. Oxaloacetate

• A. A, B, C, D, E
• B. D, A, B, C, E
• C. D, C, E, A, B
• D. D, E, B, C, A

Hint:

Mnemonic for TCA Cycle Intermediates:
Citrate $\to$ Isocitrate $\to$ $\alpha$-Ketoglutarate $\to$ Succinyl-CoA $\to$ Succite $\to$ Fumarate $\to$ Malate $\to$ Oxaloacetate.
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Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks to arrange five key intermediate metabolites of the Tricarboxylic Acid (TCA Krebs) cycle in their correct chronological order of generation.

Step 2: Key Formula or Approach:
Trace the sequential pathway of the TCA cycle starting from condensation of Acetyl-CoA ($2\text{C}$) with Oxaloacetate ($4\text{C}$): Citrate (6C) $\rightarrow{}$ Isocitrate (6C) $\rightarrow{}$ \alpha-Ketoglutarate (5C) $\rightarrow{}$ Succinyl-CoA (4C) $\rightarrow{}$ Succite (4C) $\rightarrow{}$ Fumarate (4C) $\rightarrow{}$ Malate (4C) $\rightarrow{}$ Oxaloacetate (4C)

Step 3: Detailed Explation:

• 1. Citrate (D): Formed first by Citrate Synthase condensing Oxaloacetate and Acetyl-CoA ($6\text{C}$ compound).

• 2. $\alpha$-Ketoglutarate (A): Produced via oxidative decarboxylation of Isocitrate by Isocitrate Dehydrogese ($5\text{C}$ compound, releasing $\text{CO}_2$ and $\text{DH}$).

• 3. Succinyl CoA (B): Produced via second oxidative decarboxylation by $\alpha$-Ketoglutarate Dehydrogese complex ($4\text{C}$ compound, releasing $\text{CO}_2$ and $\text{DH}$).

• 4. Fumarate (C): Produced downstream when Succite is oxidized by Succite Dehydrogese (Complex II, generating $\text{FADH}_2$).

• 5. Oxaloacetate (E): Regenerated at the fil step when Malate is oxidized by Malate Dehydrogese (generating $\text{DH}$).

Step 4: Fil Answer:
The correct sequential order of intermediate generation in the TCA cycle is D, A, B, C, E.