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: Understanding the Concept:
The citric acid cycle (or Krebs cycle) is a series of eight enzyme-catalyzed chemical reactions that form a key part of cellular respiration. The question asks to arrange a given subset of these enzymes in the order they appear in the cycle. Note: Option B lists the substrate "Fumarate," but it represents the enzyme "Fumarase."

Step 2: Detailed Explanation of the Sequence:
Let's place the given enzymes in the context of the full cycle:
- (Step 1: Citrate Synthase)
-
Step 2: A. Aconitase - converts citrate to its isomer, isocitrate.
-
Step 3: D. Isocitrate dehydrogenase - oxidizes isocitrate to \(\alpha\)-ketoglutarate, producing 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 cycle's starting substrate.

Step 3: Final Answer:
The correct sequence of the listed enzymes as they function in the citric acid cycle is Aconitase \(\rightarrow\) Isocitrate dehydrogenase \(\rightarrow\) Fumarase \(\rightarrow\) Malate dehydrogenase. This corresponds to the order A, D, B, C. Therefore, option (D) is correct.