Citric Acid Cycle Practice 2 - Online Tutor, Practice Problems & Exam Prep

The citric acid cycle converts 1 mole of Acetyl CoA into 2 moles of CO2, producing 1 FADH₂, 3 NADH, and 1 GTP, which can be converted to ATP. Key reactions include the conversion of alpha-ketoglutarate to succinyl CoA by alpha-ketoglutarate dehydrogenase, which shares similarities with pyruvate dehydrogenase. Citrate is prochiral, interacting with aconitase as if chiral. The glyoxylate cycle, utilized by plants and bacteria, allows the conversion of Acetyl CoA into sugars, highlighting differences in metabolic pathways between organisms.

Citric Acid Cycle Practice 2

If you haven't completed the questions, pause the video now and give them a try. The conversion of 1 mole of Acetyl CoA to 2 moles of CO₂ results in the production of 1 mole of FADH. Now, in each turn of the citric acid cycle, you'll use 1 acetyl CoA and you'll produce 2 CO₂ as the problem said but you're also going to make 1 FADH₂, 3 NADH, and one GTP which can be converted to ATP but won't necessarily be. So that is why the rest of the answer choices are wrong.

This should be 3. This should be GTP. It should be 1. And citrate is, or rather it's only 1 mole of citrate that's going to be produced. But it will, of course, be used up too. That's why I was crossing it out. It's not really like a product, you know. You will generate citrate in the process but it's just part of the cycle. Anyway, question 7, the reaction in the citric acid cycle most similar to pyruvate dehydrogenase converts alpha-ketoglutarate to succinyl CoA and that is carried out by alpha-ketoglutarate dehydrogenase. Now, this reaction is also carried out by dehydrogenase.

The reaction in question does not use the same cofactors and substrates as pyruvate dehydrogenase. However, there are some similarities; it does generate NADH but the similarities end there. With alpha-ketoglutarate dehydrogenase, you're using TPP, lipoate, and FAD. You are also using NAD⁺ and CoA. So there are a lot of similarities between pyruvate dehydrogenase and alpha-ketoglutarate dehydrogenase. The reaction in the citric acid cycle that produces GTP is the one that converts succinyl CoA into succinate because, remember, it's breaking that thioester bond that provides the energy for the substrate-level phosphorylation to convert GDP and inorganic phosphate into GTP. Of course, as we've already said, that GTP can be used to make ATP, but it won't necessarily be used for that purpose.

Now, which citric acid cycle intermediate is considered prochiral? Well, what is prochiral? Prochiral means that the molecule is not actually chiral but it's going to behave like it’s chiral. The answer is citrate. You might remember that citrate is a symmetric molecule but aconitase will interact with citrate as if citrate were chiral. Just looking at these choices, I also want to point out that succinate is also a symmetric molecule but it is not prochiral, right? It can be randomized in its orientation in succinate dehydrogenase.

Now, the conversion of 1 mole of pyruvate to 3 moles of carbon dioxide by pyruvate dehydrogenase and the citric acid cycle will produce 4 moles of NADH; one from pyruvate oxidation (Pyox), three from the citric acid cycle. It will produce 1 mole of FADH also from the citric acid cycle and it will produce 1 mole of ATP again from the citric acid cycle. The answer is 'd.' Now, the glyoxylate cycle is not something that we talked about, but it is something you need to know for the exam. And this is something that is carried out not by humans but by plants, fungi, and bacteria and what this does is it uses Acetyl CoA for energy and the synthesis of biosynthetic precursors. So, if you recall, we cannot convert Acetyl CoA back into sugar, right? Well, plants can. That's basically the story here, you know. They can use the acetyl CoA for other things too but but they can, through the glyoxylate cycle.

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Here’s what students ask on this topic:

What are the main products of the citric acid cycle per mole of Acetyl CoA?

The citric acid cycle, also known as the Krebs cycle, converts 1 mole of Acetyl CoA into 2 moles of CO₂. Additionally, it produces 3 moles of NADH, 1 mole of FADH₂, and 1 mole of GTP, which can be converted to ATP. The overall reaction can be summarized as:

$1 \mathrm{Acetyl-CoA} + 3 \mathrm{NAD⁺} + 1 \mathrm{FAD} + 1 \mathrm{GDP} + 1 \mathrm{P_i} \to 2 \mathrm{CO₂} + 3 \mathrm{NADH} + 1 \mathrm{FADH₂} + 1 \mathrm{GTP}$

Which enzyme in the citric acid cycle is most similar to pyruvate dehydrogenase?

The enzyme in the citric acid cycle most similar to pyruvate dehydrogenase is alpha-ketoglutarate dehydrogenase. Both enzymes catalyze oxidative decarboxylation reactions and share several cofactors, including thiamine pyrophosphate (TPP), lipoate, FAD, NAD⁺, and CoA. The reaction catalyzed by alpha-ketoglutarate dehydrogenase converts alpha-ketoglutarate to succinyl-CoA, producing NADH and CO₂ in the process.

What is the role of citrate in the citric acid cycle, and why is it considered prochiral?

Citrate plays a crucial role in the citric acid cycle as an intermediate that is formed from the condensation of Acetyl CoA and oxaloacetate. It is considered prochiral because, although it is a symmetric molecule, it interacts with the enzyme aconitase in a manner that makes it behave as if it were chiral. This interaction is essential for the proper progression of the cycle.

What is the glyoxylate cycle, and how does it differ from the citric acid cycle?

The glyoxylate cycle is a metabolic pathway found in plants, fungi, and bacteria, but not in humans. It allows these organisms to convert Acetyl CoA into sugars, which is not possible in the citric acid cycle. The glyoxylate cycle bypasses the decarboxylation steps of the citric acid cycle, enabling the conservation of carbon atoms for gluconeogenesis. Key enzymes unique to the glyoxylate cycle include isocitrate lyase and malate synthase.

Which reaction in the citric acid cycle produces GTP, and how is this energy utilized?

The reaction in the citric acid cycle that produces GTP is the conversion of succinyl-CoA to succinate, catalyzed by succinyl-CoA synthetase. This reaction involves substrate-level phosphorylation, where the energy from the thioester bond in succinyl-CoA is used to convert GDP and inorganic phosphate (P_i) into GTP. The GTP produced can be directly used for energy or converted to ATP by nucleoside diphosphate kinase.