Citric Acid Cycle 2: Videos & Practice Problems

The citric acid cycle begins with citrate synthase catalyzing the formation of citrate from acetyl CoA and oxaloacetate, a reaction characterized by a negative delta G, indicating its driving force. Aconitase then converts citrate to isocitrate through a reversible process involving prochirality. Isocitrate dehydrogenase subsequently oxidizes isocitrate to alpha-ketoglutarate, generating NADH and releasing CO₂, marking it as an oxidative decarboxylase. This cycle illustrates the interconnectedness of metabolic pathways, showcasing how enzymes like aconitase also play regulatory roles in iron metabolism.

Citric Acid Cycle 2

Alright. Step 1 of the citric acid cycle is carried out by citrate synthase and notice that nice negative delta G. Remember, this is one of those driving steps of the citric acid cycle. In the process of this reaction, we have our acetyl CoA interacting with oxaloacetate. We add water in, we bump off that CoA. See you later. And we are left with citrate right here.

Now, the following reaction, reaction number 2, is carried out by aconitase. And you might notice that it is listed as having a positive delta. Notice though that that's the Δ′°. At actual cellular conditions, this ΔG is closer to 0. It's a readily reversible reaction. Here, citrate is turned into this enzymatic intermediate, cis-aconitate, which you can see right here in the middle, and ultimately it's turned into isocitrate. During this process, we remove water and then we actually add water back in. That is why it's sometimes called the Ferris wheel; it is kind of like doing a turn.

What is interesting to note, and also important to note about this reaction, is that citrate is prochiral. What that means is that, even though it's not actually a chiral molecule, it behaves as if it were chiral and that's because aconitase binds citrate in one orientation. It just happens this way; you don't need to know why this is, but just notice that citrate is a symmetric molecule. So this is not a chiral center. However, right here is always where this double bond is going to be formed. Citrate wouldn't go in in the other orientation and have it be formed there. It just happens that way and we call that property prochiral, meaning not actually chiral but behaving as if it were chiral.

Something about aconitase that's interesting to note is that it contains these iron-sulfur complexes, which are actually held into the enzyme by cysteine residues. This is not going to be the last time we talk about iron-sulfur complexes. Those are going to come up in the next review when we talk about the electron transport chain because there are a ton of them in there. What's cool about aconitase, in terms of these iron-sulfur complexes, is that it's actually an iron response regulatory molecule. When there is a lack of iron, it changes its shape due to this lack; it can actually bind RNA. If it binds RNA, it will turn on and off various genes. What's going to happen is when iron is high in the blood, ferritin, this molecule, is going to be produced. And when iron is low in the blood, this molecule, transferrin, is going to be produced. Aconitase has this auxiliary function; it's not just an enzyme that's part of the citric acid cycle, and again, this continues the theme that biology recycles things. It always finds new purposes for old creations. It's always innovating.

The third reaction of the citric acid cycle is carried out by isocitrate dehydrogenase. We are going to take isocitrate and form alpha-ketoglutarate (alpha KG). I'm just abbreviating it that way because I'm being lazy. A couple of interesting notes about this reaction: as we've already said, it generates NADH and releases a CO₂. Definitely important to note. Here is our CO₂ that's coming off. So that is our released CO₂, and that makes this enzyme an oxidative decarboxylase. Notice how we are oxidizing that alcohol. It uses NAD⁺ or NADP⁺ as the electron acceptor. However, for us, for humans, we're going to be using NAD⁺ to generate NADH. It also uses manganese as a cofactor. Although it's a dehydrogenase, it does not use other molecules such as TPP, lipoate, FAD, and CoA like pyruvate dehydrogenase does. However, alpha-ketoglutarate dehydrogenase, which we will talk about in just a moment, uses all this stuff. That's why I point out here that isocitrate doesn't use it. Pyruvate dehydrogenase and alpha-ketoglutarate dehydrogenase do use these molecules. So, don't go thinking all dehydrogenases use those same molecules.

With that, let's flip the page.

Here’s what students ask on this topic:

What is the role of citrate synthase in the citric acid cycle?

Citrate synthase catalyzes the first step of the citric acid cycle, where it facilitates the condensation of acetyl CoA and oxaloacetate to form citrate. This reaction is characterized by a negative ΔG, indicating it is energetically favorable and acts as a driving force for the cycle. The reaction also involves the addition of water and the release of CoA. Citrate synthase's role is crucial as it sets the stage for subsequent reactions in the cycle.

How does aconitase function in the citric acid cycle?

Aconitase catalyzes the reversible conversion of citrate to isocitrate via the intermediate cis-aconitate. This reaction involves the removal and subsequent addition of water. Aconitase binds citrate in a specific orientation, making citrate behave as if it were chiral, a property known as prochirality. The enzyme contains iron-sulfur complexes, which are essential for its function and also play a role in iron metabolism by regulating gene expression in response to iron levels.

What is the significance of isocitrate dehydrogenase in the citric acid cycle?

Isocitrate dehydrogenase catalyzes the oxidative decarboxylation of isocitrate to alpha-ketoglutarate, producing NADH and releasing CO2. This reaction is significant because it contributes to the cell's energy production by generating NADH, which is used in the electron transport chain to produce ATP. The enzyme uses NAD+ as an electron acceptor and requires manganese as a cofactor. This step is crucial for maintaining the flow of the citric acid cycle and energy metabolism.

What is prochirality, and how does it relate to citrate in the citric acid cycle?

Prochirality refers to a molecule that is not chiral but can be converted into a chiral molecule by a single alteration. In the citric acid cycle, citrate is prochiral because, although it is symmetric and not chiral, it behaves as if it were chiral when bound by aconitase. Aconitase binds citrate in a specific orientation, leading to the formation of isocitrate in a manner that mimics chirality. This property is important for the enzyme's specificity and the proper progression of the cycle.

How do iron-sulfur complexes in aconitase contribute to its function?

Iron-sulfur complexes in aconitase are crucial for its enzymatic activity. These complexes are held in place by cysteine residues and facilitate the reversible conversion of citrate to isocitrate. Additionally, aconitase acts as an iron-responsive regulatory protein. When iron levels are low, aconitase changes shape and can bind RNA, influencing the expression of genes involved in iron metabolism, such as those coding for ferritin and transferrin. This dual role highlights the enzyme's importance in both the citric acid cycle and iron homeostasis.