Question

Citrate is isomerized to isocitrate by the action of enzyme

• A. Triose isomerase
• B. Aconitase
• C. Isocitrate dehydrogenase
• D. Pyruvate dehydrogenase

Hint:

TCA Cycle Citrate $\xrightarrow{\text{Aconitase}}$ cis-Aconitate $\xrightarrow{\text{Aconitase}}$ Isocitrate. (Contains a $[4\text{Fe}-4\text{S}]$ cluster).

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
The second step of the Krebs tricarboxylic acid (TCA) cycle is the stereospecific isomerization of symmetric citrate to asymmetric D-isocitrate.
Key Formula or Approach:
\[ \text{Citrate} \xrightleftharpoons[-\text{H}_2\text{O}]{\text{Aconitase}} \text{cis-Aconitate} \xrightleftharpoons[+\text{H}_2\text{O}]{\text{Aconitase}} \text{D-Isocitrate} \]

Step 2: Detailed Explanation:
In the TCA cycle:
- Aconitase (Aconitate hydratase) contains a catalytic $[4\text{Fe}-4\text{S}]$ iron-sulfur cluster.
- It catalyzes a reversible two-step isomerization: first dehydrating tertiary alcohol citrate to enzyme-bound cis-aconitate intermediate, followed by stereospecific rehydration to form the secondary alcohol D-isocitrate, setting up the substrate for oxidative decarboxylation.

Step 3: Final Answer:
Thus, citrate is isomerized to isocitrate by Aconitase, corresponding to option (B).