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The citric acid cycle

Overview and steps of the citric acid cycle, also known as the Krebs cycle or tricarboxylic acid (TCA) cycle.

Introduction

How important is the citric acid cycle? So important that it has not one, not two, but three different names in common usage today!
The name we'll primarily use here, the citric acid cycle, refers to the first molecule that forms during the cycle's reactions—citrate, or, in its protonated form, citric acid. However, you may also hear this series of reactions called the tricarboxylic acid (TCA) cycle, for the three carboxyl groups on its first two intermediates, or the Krebs cycle, after its discoverer, Hans Krebs.
Whatever you prefer to call it, the citric cycle is a central driver of cellular respiration. It takes acetyl start text, C, o, A, end text—produced by the oxidation of pyruvate and originally derived from glucose—as its starting material and, in a series of redox reactions, harvests much of its bond energy in the form of start text, N, A, D, H, end text, start text, F, A, D, H, end text, start subscript, 2, end subscript, and start text, A, T, P, end text molecules. The reduced electron carriers—start text, N, A, D, H, end text and start text, F, A, D, H, end text, start subscript, 2, end subscript—generated in the TCA cycle will pass their electrons into the electron transport chain and, through oxidative phosphorylation, will generate most of the ATP produced in cellular respiration.
Below, we’ll look in more detail at how this remarkable cycle works.

Overview of the citric acid cycle

In eukaryotes, the citric acid cycle takes place in the matrix of the mitochondria, just like the conversion of pyruvate to acetyl start text, C, o, A, end text. In prokaryotes, these steps both take place in the cytoplasm. The citric acid cycle is a closed loop; the last part of the pathway reforms the molecule used in the first step. The cycle includes eight major steps.
Simplified diagram of the citric acid cycle. First, acetyl CoA combines with oxaloacetate, a four-carbon molecule, losing the CoA group and forming the six-carbon molecule citrate. After citrate undergoes a rearrangement step, it undergoes an oxidation reaction, transferring electrons to NAD+ to form NADH and releasing a molecule of carbon dioxide. The five-carbon molecule left behind then undergoes a second, similar reaction, transferring electrons to NAD+ to form NADH and releasing a carbon dioxide molecule. The four-carbon molecule remaining then undergoes a series of transformations, in the course of which GDP and inorganic phosphate are converted into GTP—or, in some organisms, ADP and inorganic phosphate are converted into ATP—an FAD molecule is reduced to FADH2, and another NAD+ is reduced to NADH. At the end of this series of reactions, the four-carbon starting molecule, oxaloacetate, is regenerated, allowing the cycle to begin again.
In the first step of the cycle, acetyl start text, C, o, A, end text combines with a four-carbon acceptor molecule, oxaloacetate, to form a six-carbon molecule called citrate. After a quick rearrangement, this six-carbon molecule releases two of its carbons as carbon dioxide molecules in a pair of similar reactions, producing a molecule of start text, N, A, D, H, end text each timestart superscript, 1, end superscript. The enzymes that catalyze these reactions are key regulators of the citric acid cycle, speeding it up or slowing it down based on the cell’s energy needssquared.
The remaining four-carbon molecule undergoes a series of additional reactions, first making an start text, A, T, P, end text molecule—or, in some cells, a similar molecule called start text, G, T, P, end text—then reducing the electron carrier start text, F, A, D, end text to start text, F, A, D, H, end text, start subscript, 2, end subscript, and finally generating another start text, N, A, D, H, end text. This set of reactions regenerates the starting molecule, oxaloacetate, so the cycle can repeat.
Overall, one turn of the citric acid cycle releases two carbon dioxide molecules and produces three start text, N, A, D, H, end text, one start text, F, A, D, H, end text, start subscript, 2, end subscript, and one start text, A, T, P, end text or start text, G, T, P, end text. The citric acid cycle goes around twice for each molecule of glucose that enters cellular respiration because there are two pyruvates—and thus, two acetyl start text, C, o, A, end texts—made per glucose.

Steps of the citric acid cycle

You've already gotten a preview of the molecules produced during the citric acid cycle. But how, exactly, are those molecules made? We’ll walk through the cycle step by step, seeing how start text, N, A, D, H, end text, start text, F, A, D, H, end text, start subscript, 2, end subscript, and start text, A, T, P, end text/start text, G, T, P, end text are produced and where carbon dioxide molecules are released.
Step 1. In the first step of the citric acid cycle, acetyl start text, C, o, A, end text joins with a four-carbon molecule, oxaloacetate, releasing the start text, C, o, A, end text group and forming a six-carbon molecule called citrate.
Step 2. In the second step, citrate is converted into its isomer, isocitrate. This is actually a two-step process, involving first the removal and then the addition of a water molecule, which is why the citric acid cycle is sometimes described as having nine steps—rather than the eight listed herecubed.
Step 3. In the third step, isocitrate is oxidized and releases a molecule of carbon dioxide, leaving behind a five-carbon molecule—α-ketoglutarate. During this step, start text, N, A, D, end text, start superscript, plus, end superscript is reduced to form start text, N, A, D, H, end text. The enzyme catalyzing this step, isocitrate dehydrogenase, is important in regulating the speed of the citric acid cycle.
Step 4. The fourth step is similar to the third. In this case, it’s α-ketoglutarate that’s oxidized, reducing start text, N, A, D, end text, start superscript, plus, end superscript to start text, N, A, D, H, end text and releasing a molecule of carbon dioxide in the process. The remaining four-carbon molecule picks up Coenzyme A, forming the unstable compound succinyl start text, C, o, A, end text. The enzyme catalyzing this step, α-ketoglutarate dehydrogenase, is also important in regulation of the citric acid cycle.
Detailed diagram of the citric acid cycle, showing the structures of the various cycle intermediates and the enzymes catalyzing each step.
Step 1. Acetyl CoA combines with oxaloacetate in a reaction catalyzed by citrate synthase. This reaction also takes a water molecule as a reactant, and it releases a SH-CoA molecule as a product.
Step 2. Citrate is converted into isocitrate in a reaction catalyzed by aconitase.
Step 3. Isocitrate is converted into α-ketoglutarate in a reaction catalyzed by isocitrate dehydrogenase. An NAD+ molecule is reduced to NADH + H+ in this reaction, and a carbon dioxide molecule is released as a product.
Step 4. α-ketoglutarate is converted to succinyl CoA in a reaction catalyzed by α-ketoglutarate dehydrogenase. An NAD+ molecule is reduced to NADH + H+ in this reaction, which also takes a SH-CoA molecule as reactant. A carbon dioxide molecule is released as a product.
Step 5. Succinyl CoA is converted to succinate in a reaction catalyzed by the enzyme succinyl-CoA synthetase. This reaction converts inorganic phosphate, Pi, and GDP to GTP and also releases a SH-CoA group.
Step 6. Succinate is converted to fumarate in a reaction catalyzed by succinate dehydrogenase. FAD is reduced to FADH2 in this reaction.
Step 7. Fumarate is converted to malate in a reaction catalyzed by the enzyme fumarase. This reaction requires a water molecule as a reactant.
Step 8. Malate is converted to oxaloacetate in a reaction catalyzed by malate dehydrogenase. This reaction reduces an NAD+ molecule to NADH + H+.
image credit: modified from "Oxidation of pyruvate and citric acid cycle: Figure 2" by OpenStax College, Biology, CC BY 3.0
Step 5. In step five, the start text, C, o, A, end text of succinyl start text, C, o, A, end text is replaced by a phosphate group, which is then transferred to start text, A, D, P, end text to make start text, A, T, P, end text. In some cells, start text, G, D, P, end text—guanosine diphosphate—is used instead of start text, A, D, P, end text, forming start text, G, T, P, end text—guanosine triphosphate—as a product. The four-carbon molecule produced in this step is called succinate.
Step 6. In step six, succinate is oxidized, forming another four-carbon molecule called fumarate. In this reaction, two hydrogen atoms—with their electrons—are transferred to start text, F, A, D, end text, producing start text, F, A, D, H, end text, start subscript, 2, end subscript. The enzyme that carries out this step is embedded in the inner membrane of the mitochondrion, so start text, F, A, D, H, end text, start subscript, 2, end subscript can transfer its electrons directly into the electron transport chain.
Step 7. In step seven, water is added to the four-carbon molecule fumarate, converting it into another four-carbon molecule called malate.
Step 8. In the last step of the citric acid cycle, oxaloacetate—the starting four-carbon compound—is regenerated by oxidation of malate. Another molecule of start text, N, A, D, end text, start superscript, plus, end superscript is reduced to start text, N, A, D, H, end text in the process.

Products of the citric acid cycle

Let’s take a step back and do some accounting, tracing the fate of the carbons that enter the citric acid cycle and counting the reduced electron carriers—start text, N, A, D, H, end text and start text, F, A, D, H, end text, start subscript, 2, end subscript—and start text, A, T, P, end text produced.
In a single turn of the cycle,
  • two carbons enter from acetyl start text, C, o, A, end text, and two molecules of carbon dioxide are released;
  • three molecules of start text, N, A, D, H, end text and one molecule of start text, F, A, D, H, end text, start subscript, 2, end subscript are generated; and
  • one molecule of start text, A, T, P, end text or start text, G, T, P, end text is produced.
These figures are for one turn of the cycle, corresponding to one molecule of acetyl start text, C, o, A, end text. Each glucose produces two acetyl start text, C, o, A, end text molecules, so we need to multiply these numbers by 2 if we want the per-glucose yield.
Two carbons—from acetyl start text, C, o, A, end text—enter the citric acid cycle in each turn, and two carbon dioxide molecules are released. However, the carbon dioxide molecules don’t actually contain carbon atoms from the acetyl start text, C, o, A, end text that just entered the cycle. Instead, the carbons from acetyl start text, C, o, A, end text are initially incorporated into the intermediates of the cycle and are released as carbon dioxide only during later turns. After enough turns, all the carbon atoms from the acetyl group of acetyl start text, C, o, A, end text will be released as carbon dioxide.

Where’s all the start text, A, T, P, end text?

You may be thinking that the start text, A, T, P, end text output of the citric acid cycle seems pretty unimpressive. All that work for just one start text, A, T, P, end text or start text, G, T, P, end text?
It’s true that the citric acid cycle doesn’t produce much start text, A, T, P, end text directly. However, it can make a lot of start text, A, T, P, end text indirectly, by way of the start text, N, A, D, H, end text and start text, F, A, D, H, end text, start subscript, 2, end subscript it generates. These electron carriers will connect with the last portion of cellular respiration, depositing their electrons into the electron transport chain to drive synthesis of ATP molecules through oxidative phosphorylation.

Want to join the conversation?

  • aqualine seed style avatar for user PrincessDeen101
    Is there a difference between ATP and GTP?
    (13 votes)
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  • piceratops seed style avatar for user VINI
    Explain why citric acid cycle can't operate in the absence of oxygen?
    (16 votes)
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    • blobby green style avatar for user Ali Sasani
      Cooper is right...

      Once the ETC stops oxidizing NADH to NAD+ there is no longer any NAD+ available for the Krebs cycle to reduce back to NADH and the cycle comes to a halt. Therefore, the Krebs cycle is actually regulated by the availability of NAD+

      It is true that the Fermentation process an contribute to NAD+ regeneration but remember that under anaerobic condition most of the cell's pyruvate is being sent to the Fermentation pathway... and even when NAD+ is regenerated during Fermentation, there will be much less Pyruvate (I'm trying to avoid words like "none") entering the Kreb's Cycle.

      All this being said, yes technically it can but it would not be contributing to the overall goal (ETC) so the ATP production will be significantly less.
      (20 votes)
  • blobby green style avatar for user Mohammad mahdy yousefi
    How kerebs found this cycle?
    (5 votes)
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    • piceratops ultimate style avatar for user Darmon
      Krebs was working on the problem of finding the chemicals that act as intermediaries in cellular respiration. He discovered that when he added certain chemicals to pigeon breast muscle cells, their oxygen consumption would increase, thus indicating that more respiration reactions were taking place. These chemicals are the same ones we now identify as those making up the Kreb's Cycle. :)
      (18 votes)
  • leafers ultimate style avatar for user Devon Dryer
    Which provides more energy output, 1 ATP molecule or 1 GTP molecule?
    (6 votes)
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  • blobby green style avatar for user fiky95
    I was wondering whether it's necessary to remember the formula of each compound? Thank you! :)
    (3 votes)
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    • leafers ultimate style avatar for user William H
      Most basic biology classes, even AP bio don't require you to know the exact structure, although you might want to know their basic structure such as oh this is glucose with a phosphate group attached, this is a molecule with an extra proton, since most questions in that topic will revolve around that.
      (11 votes)
  • leaf green style avatar for user BSnailer
    Going from Malate to Oxaloacetic Acid 2 hydrogen ions are hydrolyzed but only one NADH is formed. Where did the other Hydrogen Ion go?
    (5 votes)
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    • male robot donald style avatar for user Maxime
      NAD+ needs 2 electrons en 1 proton to make NADH.
      The oxidation of malate transfers these products to NAD+. There is indeed a remaining H+ ion that is released in the matrix as a proton. That way the charge is kept on both ends of the reaction.
      This also happens with the other times that NADH is formed (releasing a proton) but there the proton was released beforehand when the carboxyl group was created.
      (4 votes)
  • orange juice squid orange style avatar for user Martin
    Can GTP serve the same functions as ATP?
    (3 votes)
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  • blobby green style avatar for user caominh11122000
    In the picture "Oxidation of pyruvate and citric acid cycle", in step 3 and 4, I saw there are 2 H+ ions produced but I'm not sure where they came from. I think there should be no spare H+ ion in step 3 and 4.
    (4 votes)
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  • hopper cool style avatar for user Jose Gelves Cabrera
    Why doesn't it produce hydrolisis when water added in the step 7?
    (2 votes)
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    • piceratops ultimate style avatar for user RowanH
      One thing that determines which reactions happen in a cell is which enzymes there are, since they are the catalysts that increase the rate of feasible reactions so that they actually happen at a useful rate. So enzymes are often responsible for deciding at exactly which position in a large molecule a reaction will happen, where in a test tube, the chemicals might slightly prefer to react at a different position.

      That being said, from a chemical point of view, fumarate and water reacting the way they do makes sense, whereas I don't see how a hydrolysis reaction could take place. In hydrolysis reactions, the negatively charged oxygen of water 'attacks' some partially positive charged atom on another molecule and shares its electrons with it in a new bond, and a 'leaving group', which is happier to take its electrons with it, is displaced and leaves. It then picks up the spare H+ from the original water. In this way an OH- has been added to one part of the original molecule, and a H+ to the other, and the molecule has been split (lysed) into two by water(i.e hydrolysed).

      But in step 7, fumarate has a double bond across which the water is added (in a 'hydration' reaction). So the molecule is not split in two, because a bond remains linking the two atoms that got either the H+ or OH- added to them.

      Does this help?
      (4 votes)
  • piceratops ultimate style avatar for user Jonah Lee
    How is GTP turned into ATP?
    (2 votes)
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