Question

Suppose that in the citric acid cycle, oxalacetate is labeled with 14C in the carboxyl carbon farthest from the keto group. Where would you expect to find the 14C label when alpha-ketoglutarate is converted to succinate?

Answer 1

Answer:

All of alpha-ketoglutarate formed in the citric acid cycle would contain the radioactive $^{14}C$ while none of succinate would contain $^{14}C$, and all of carbon dioxide released would contain $^{14}C$.

Explanation:

When oxaloacetate in the citric acid cycle is labeled with $^{14}C$ in carboxyl carbon atom which is farthest from keto group, alpha ketoglutarate formed from this oxaloactetate has the full radioactive label. In the next step, the carboxylic group (that contains the $^{14}C$) is eliminated in the form of the release of the carbon dioxide and succinate is formed. Succinate thus will not have radioactivity. $CO_2$ released had all the radioactivity.