Question

Given below are two statements related to Krebs cycle.
Statement (I): There are four steps of oxidation in Krebs cycle, catalyzed by dehydrogenases using \(\text{NAD}^{+}\) or \(\text{FAD}\) as the coenzyme.
Statement (II): Two molecules of GTP are produced for every molecule of citric acid oxidized.
In light of the above statements, choose the most appropriate answer from the options given below.

• A. Both Statement (I) and Statement (II) are correct.
• B. Both Statement (I) and Statement (II) are incorrect.
• C. Statement (I) is correct but Statement (II) is incorrect.
• D. Statement (I) is incorrect but Statement (II) is correct.

Hint:

The Krebs cycle has four oxidation steps but gives only one GTP per turn.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept
The Krebs cycle oxidises acetyl CoA, and some steps give reduced coenzymes while one step gives GTP or ATP.

Step 2: Statement (I)
There are four oxidation steps: isocitrate to oxalosuccinate and alpha-ketoglutarate to succinyl CoA (using \(\text{NAD}^{+}\)), succinate to fumarate (using FAD) and malate to oxaloacetate (using \(\text{NAD}^{+}\)). This is correct.

Step 3: Statement (II)
Only one substrate-level phosphorylation occurs per turn, when succinyl CoA is converted to succinate. It gives one GTP, not two. So this is incorrect.

Final Answer:
Statement (I) is correct and Statement (II) is incorrect. \[ \boxed{\text{(C) }\text{Statement (I) is correct but Statement (II) is incorrect.}} \]