Citric Acid Cycle Practice 1 - Online Tutor, Practice Problems & Exam Prep

The pyruvate dehydrogenase complex converts pyruvate to acetyl CoA, utilizing cofactors like TPP, lipoate, and FAD, while biotin is not involved. In glycolysis, labeled glucose loses its label during decarboxylation, resulting in unlabeled acetyl CoA. Malonate inhibits succinate dehydrogenase, increasing succinate and decreasing fumarate. Acetyl CoA is not an intermediate of the citric acid cycle; it combines with oxaloacetate to form citrate. After one cycle, 50% of oxaloacetate retains its radioactivity due to the loss of two carbons during decarboxylation.

Citric Acid Cycle Practice 1

Before watching the answer to each question, you should attempt those questions on your own. I'll pause briefly between each question so you can pause the video, but you can also just try all the questions and then watch the video explanation. Now, question 1, which of the following is not true of the pyruvate dehydrogenase complex? And the answer is that Biotin participates. Biotin doesn't have anything to do with pyruvate dehydrogenase complex. If you recall the cofactors, it uses are TPP, lipoate, and FAD. All of the rest of the statements are true. We have 2 different thiol groups with TPP and with the CoA. Both NAD and FAD participate. We're going to generate NADH from NAD⁺ and we're going to actually use FAD to do that. And these reactions indeed occur in the mitochondria, in the matrix to be precise.

Alright, question 2. Excuse me, glucose labeled in the 3^rd and 4^th carbons, where C-14 is converted to pyruvate by glycolysis and to acetyl CoA with the pyruvate dehydrogenase complex. Where is the labeled acetyl CoA? Or where is the label in the Acetyl CoA? The answer is there's none there at all. If you look at pyruvate, here is pyruvate, and you probably remember this is carbon 1 or 6, 2 or 5, and 3 or 4, and when this is converted into Acetyl CoA, this gets decarboxylated as CO2. So all that remains are carbons 1 or 6 or 2 or 5 from the original glucose.

Now, malonate is a competitive inhibitor of succinate dehydrogenase. If malonate is added to the mitochondria, we would expect the product of succinate dehydrogenase to decrease in concentration, right? So fumarate, we would expect that to decrease in concentration. We would actually expect succinate to increase in concentration. If succinate dehydrogenase is being inhibited, then succinate will probably build up a little bit. And these of course, well alpha-ketoglutarate and isocitrate are precursors to succinate, and pyruvate is not even part of the citric acid cycle. It comes in before the citric acid cycle.

Now, which of the following is not an intermediate of the citric acid cycle? Tricky question, right? It's actually Acetyl CoA. And this might surprise you. Acetyl CoA is not an intermediate. It's what enters the cycle, but it's not considered an intermediate, whereas both citrate, what it enters the cycle to become, and oxaloacetate, the thing it combines with within the cycle, are intermediates of the citric acid cycle. Of course, maleate and isocitrate are also intermediates of the cycle.

Oxaloacetate uniformly labeled with carbon-14 in all carbons, meaning that all carbons have the same radiolactivity or the same radiolabel, is added to the mitochondria with an adequate supply of acetyl CoA that is unlabeled. In the first cycle, how much radiolactivity will remain in oxaloacetate? The answer is 50%. So, if you recall, oxaloacetate is made of 4 carbons. I'm not going to draw the whole molecule. This is like my mini oxaloacetate and it combines with 2 carbon acetyl CoA, right, and then you form this molecule citrate. Okay. So let me scroll down a little so you can see my drawing. So this carbon and this carbon are going to decarboxylate in that in this turn of the cycle. So these 2 are going to disappear. Now, we said that all of these oxaloacetates were labeled, right? But these 2 carbons are going to decarboxylate. Those are these 2 carbons right here. So, only these 2 carbons are going to remain in the next turn of the cycle. These acetyl CoA carbons were unlabeled, right? So these are carbon-14 and these are regular carbon-12, right? So then in our next oxaloacetate let me draw it up here. We are going to have 2 carbon-14s and you can't really say which two it's going to be. We just don't know. So right. Because there is the randomization of citrate's orientation. So we can't say which 2 it'll be. Right? It could be the 2. I've just drawn in as 2 here, but the point is we're only going to have 50% of that radiolabeled carbon remaining in the next turn because we're going to lose 2 to decarboxylation in that first turn.

Here’s what students ask on this topic:

What are the main cofactors involved in the pyruvate dehydrogenase complex?

The main cofactors involved in the pyruvate dehydrogenase complex are Thiamine pyrophosphate (TPP), lipoate, and Flavin adenine dinucleotide (FAD). These cofactors play crucial roles in the conversion of pyruvate to acetyl CoA. TPP assists in the decarboxylation of pyruvate, lipoate acts as a carrier of acyl groups, and FAD is involved in the oxidation-reduction reactions. Notably, biotin is not a cofactor in this complex.

How does malonate affect the citric acid cycle?

Malonate is a competitive inhibitor of succinate dehydrogenase, an enzyme in the citric acid cycle. When malonate is present, it competes with succinate for the active site of succinate dehydrogenase, leading to a decrease in the production of fumarate. Consequently, succinate accumulates because it cannot be converted to fumarate. This inhibition disrupts the normal flow of the citric acid cycle.

Why is acetyl CoA not considered an intermediate of the citric acid cycle?

Acetyl CoA is not considered an intermediate of the citric acid cycle because it is the molecule that enters the cycle rather than one that is formed and transformed within the cycle. Acetyl CoA combines with oxaloacetate to form citrate, which then undergoes a series of transformations. Intermediates are those molecules that are formed and transformed within the cycle, such as citrate, isocitrate, and oxaloacetate.

What happens to the radioactivity of uniformly labeled oxaloacetate after one cycle of the citric acid cycle?

When oxaloacetate uniformly labeled with carbon-14 is added to the citric acid cycle, 50% of the radioactivity remains in oxaloacetate after one cycle. This is because oxaloacetate combines with unlabeled acetyl CoA to form citrate, which then undergoes decarboxylation, losing two carbons as CO₂. Since all four carbons of oxaloacetate were initially labeled, the loss of two carbons results in 50% of the original radioactivity remaining in the oxaloacetate after one cycle.

What is the fate of labeled glucose in glycolysis and the citric acid cycle?

When glucose labeled at the 3rd and 4th carbons with carbon-14 undergoes glycolysis, it is converted to pyruvate. During the conversion of pyruvate to acetyl CoA by the pyruvate dehydrogenase complex, the labeled carbons are lost as CO₂. Therefore, the resulting acetyl CoA is unlabeled. This means that the label from the original glucose does not enter the citric acid cycle.