Question

Using pyruvate, labeled with \(^{14} \mathrm{C}\) in its keto group, via the pyruvate dehydrogenase reaction and the TCA cycle, where would the carbon label be at the end of one turn of the TCA cycle? Where would the carbon label be at the end of the second turn of the cycle?

Short answer

After two turns of the TCA cycle, where is the carbon label (^14C)? Answer: After one turn of the TCA cycle, the ^14C label is at C1 of oxaloacetate. After two turns of the TCA cycle, the ^14C label is released as CO2 and is no longer in the cycle.

Step-by-step solution

Understand the pyruvate dehydrogenase reaction and enter the TCA cycle

Pyruvate first undergoes decarboxylation by the pyruvate dehydrogenase complex to yield Acetyl-CoA, which enters the TCA cycle. In this reaction, the carboxyl carbon (C1) of pyruvate is released as CO2, and the remaining two carbons (C2 and C3) form acetyl-CoA. The \(^{14}\mathrm{C}\) label is in the keto group of pyruvate, which is C2.

First turn of the TCA cycle

As Acetyl-CoA enters the TCA cycle, it combines with oxaloacetate (4 carbons) to form citrate (6 carbons). In this reaction, labeled C2 becomes C2 of the resulting citrate molecule. Through the following reactions of the TCA cycle, C2 of citrate maintains its position as C2: 1. Citrate → Isocitrate (via citrate synthase, aconitase enzymes) 2. Isocitrate → α-Ketoglutarate (via isocitrate dehydrogenase enzyme) In the reaction of α-Ketoglutarate → Succinyl-CoA (via α-ketoglutarate dehydrogenase enzyme), carbon C1 of α-Ketoglutarate is released as CO2. Therefore, our labeled carbon (originally C2) is now at position C1 of Succinyl-CoA. During the remaining steps of the TCA cycle, the labeled C1 carbon holds its position as follows: 1. Succinyl-CoA → Succinate 2. Succinate → Fumarate 3. Fumarate → Malate 4. Malate → Oxaloacetate At the end of the first turn of the TCA cycle, our labeled carbon (\(^{14}\mathrm{C}\)) is at C1 in oxaloacetate.

Second turn of the TCA cycle

In the second TCA cycle turn, the labeled oxaloacetate (C1 position) combines with another Acetyl-CoA molecule to form citrate. As before, the labeled carbon keeps its position in citrate: 1. Citrate → Isocitrate 2. Isocitrate → α-Ketoglutarate In the α-Ketoglutarate → Succinyl-CoA reaction, the labeled carbon (C1 of α-Ketoglutarate) will be released as CO2. As a result, after the second turn of the TCA cycle, the \(^{14}\mathrm{C}\) label is no longer present in the cycle. In conclusion, at the end of one turn of the TCA cycle, the \(^{14}\mathrm{C}\) label is at C1 of oxaloacetate. At the end of the second turn of the cycle, the \(^{14}\mathrm{C}\) label is released as CO2 and is no longer in the cycle.