Question

In experiments carried out to investigate the citric acid cycle, oxaloacetate labeled with C¹⁴ in the carboxyl carbon closest to the carbonyl atom was introduced to an active preparation of mitochondria. From your knowledge of the citric acid cycle when would you expect the labeled carbon atom to show up as released C¹⁴O₂? Please show all necessary intermediate structures.

Answer 1

In the citric acid cycle experiment with 14C-labeled oxaloacetate, the labeled carbon atom would be expected to be released as CO2 first when isocitrate is converted to α-ketoglutarate and again when α-ketoglutarate is converted to succinyl-CoA.

Step-by-step explanation:

The citric acid cycle, also known as the Krebs cycle or TCA cycle, involves a series of chemical reactions used by all aerobic organisms to generate energy through the oxidation of acetyl-CoA derived from carbohydrates, fats, and proteins into carbon dioxide and chemical energy.

In the experiment mentioned, oxaloacetate labeled with 14C in the carboxyl carbon closest to the carbonyl atom is introduced into mitochondria. The labeled carbon atom would first be released as CO2 in the step where isocitrate is converted to α-ketoglutarate, catalyzed by isocitrate dehydrogenase.

This is because isocitrate, which is formed after citrate has been isomerized, loses a carbon as CO2 before converting into α-ketoglutarate. With continued cycling, a second labeled CO2 will be released when α-ketoglutarate is converted to succinyl-CoA, catalyzed by α-ketoglutarate dehydrogenase. Hence, indeed, the labeled carbon atom will show up as released 14CO2 twice, first after the isocitrate step and again after the α-ketoglutarate step in the cycle.