Question

Arrange the following enzymes of the citric acid cycle in the correct sequence.
A. Aconitase
B. Fumarase
C. Malate dehydrogenase
D. Isocitrate dehydrogenase
Choose the most appropriate answer from the options given below:

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

Hint:

A helpful mnemonic for the substrates of the Krebs cycle is: "Citrate Is Krebs' Starting Substrate For Making Oxaloacetate."

\textbf{A}conitase acts on \textbf{C}itrate.
\textbf{I}socitrate \textbf{d}ehydrogenase acts on \textbf{I}socitrate.
\textbf{F}umarase acts on \textbf{F}umarate.
\textbf{M}alate \textbf{d}ehydrogenase acts on \textbf{M}alate. \end{itemize} This helps confirm the sequence A, D, B, C.

Answer 1

The Correct Option is D

Step 1: Concept Overview:
The citric acid cycle (Krebs cycle) involves eight enzyme-catalyzed reactions crucial for cellular respiration. The task is to arrange a subset of these enzymes in their cycle order. Note that Option B, listing "Fumarate," actually refers to the enzyme "Fumarase."

Step 2: Sequence Elaboration:
Placing the given enzymes within the complete cycle:
- (Step 1: Citrate Synthase)
-
Step 2: A. Aconitase - converts citrate to isocitrate.
-
Step 3: D. Isocitrate dehydrogenase - oxidizes isocitrate to \(\alpha\)-ketoglutarate, generating NADH.
- (Step 4: \(\alpha\)-Ketoglutarate dehydrogenase)
- (Step 5: Succinyl-CoA synthetase)
- (Step 6: Succinate dehydrogenase)
- Step 7: B. Fumarase - hydrates fumarate to form malate.
- Step 8: C. Malate dehydrogenase - oxidizes malate to oxaloacetate, producing NADH and regenerating the starting substrate.

Step 3: Conclusion:
The correct enzyme sequence in the citric acid cycle is Aconitase \(\rightarrow\) Isocitrate dehydrogenase \(\rightarrow\) Fumarase \(\rightarrow\) Malate dehydrogenase, corresponding to A, D, B, C. Therefore, option (D) is correct.