Can someone help me understand this entire situatuion!!!! Question : The CO 2 pr

Question

Can someone help me understand this entire situatuion!!!!

Question: The CO2 produced in one round of the citric acid cycle does not originate in the acetyl carbons that entered that round. If acetylCoA is labeled with 14C at its carbonyl carbon, how many rounds of the cycle are required before 14CO2 is released?

Answser: The labeled carbon becomes C4 of the succinyl moiety of succinylCoA. Because succinate is symmetrical, the label appears at C1 and C4 of succinate. When the resulting oxaloacetate begins the second round, the labeled carbons appear as 14CO2 in the isocitrate dehydrogenase and the alpha-ketoglutarate dehydrogenase reactions

The text book states that this figure helps

Explanation / Answer

It is better to give numbers to all the carbons. The 2nd carbon of Acetyl CoA is labelled.

In the first step of Krebs cycle, when Acetyl CoA binds to OAA, citrate is formed. The labelled carbon comes on 5th position of citrate-----> from here it comes to 5th position of isocitrate in the second step-----> in third step, one CO2 is removed from the 3rd position of isocitrate and alpha-ketoglutarate is formed. The labelled carbon is still at the 5th position of 5C alpha ketoglutarate. -------> In the fourth step, one more CO2 is removed from the first position of alpha-ketoglutarate. This is the C-1 of oxaloacetate. So, the labelled carbon of Acetyl CoA becomes C-4 of succinate ----> through a series of steps clearly shown in the diagram this labelled carbon becomes C-1 of oxaloacetate.

So, in the next round, the labelled carbon will be removed when it reaches the fourth step (given in bold in the above flowchart).

So, 2 rounds of Krebs cycle are required to remove the labelled carbon at C-2 of Acetyl CoA.

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