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What is the source of the electrons generated in the Krebs cycle?

What is the source of the electrons generated in the Krebs cycle?



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In the Krebs cycle, where do the hydrogens and electrons that NAD+ and FAD accept come from? It seems that citric acid only loses two hydrogens because it starts out with eight hydrogens and then becomes oxaloacetic acid, which has four hydrogens.


This is a question that has been a source of disagreement between some quite senior biochemists, so, although I am no angel, I waited a little to find an elusive volume of a journal before rushing in. At the outset it seemed to me that:

  1. What was important in the oxidation of acetyl-CoA in the tricarboxylic acid (TCA) cycle was the source of electrons, not the source of hydrogen atoms. Oxidation is the removal of electrons.
  2. The source of the electrons must be the carbohydrate being oxidized. It could not come from anywhere else (such as water).

This point was made in a comment (not an answer) by TomD, who pointed to his answer to a different but related SE Biology question. In that answer he refers to two 'letters to the editor' on the subject in TIBS vol.6, p.6 (1981), which unfortunately are not available in the online version of the journal. I recalled them from my own university library's archives and have made them available here as a text pdf, although in the meantime TomD posted a scan himself. I recommend reading the letters carefully to consider their arguments, but quote extracts from each here to show why I personally tend towards their view.

D.E. Atkinson

Atkinson's key point, as I understand it, is that although the H may come from water, this is only as a result of ionization - the water is not oxidized, so cannot provide electrons:

“Electrons cannot be tagged and traced, but the stoichiometry is clear: in the oxidation of an acetyl group to 2CO2, eight electrons are lost. Of these, six are used in the reduction of NAD+ to NADH and two in the reduction of the flavin of succinate dehydrogenase. Water is not oxidized; thus no electrons are supplied from water to be: 'raised to the level of NADH2' or for any other role. When water ionizes, the electrons all remain associated with the OH- ion; whatever the proton may do, it cannot reduce anything.”

B. Herreros and J. Garcia-Sancho

These authors emphasize that it is the C-C and C-H bonds that are providing the electrons:

“As pointed out by Losada the carbon compounds themselves are the only source of reducing power in the TCA cycle, not water. The difficulty in realizing it arises from the contabilization of the reducing equivalents as pairs of H rather than as electron pairs. It is true that one molecule of glucose can not supply 12 pairs of H, but it can certainly supply 12 pairs of electrons…

… In the TCA cycle, the electrons shared in the C-C and the C-H bonds are the source of reducing power; they are transfered first to pyridine nucleotide (and flavin nucleotide) and then to the vicinity of oxygen through the respiratory chain.”

Towards the end of his letter, Atkinson writes “I have found it a useful pedagogical device to ask students to work through respiration (glycolysis plus the citrate cycle), keeping careful account of water and proton so that their equations will sum to the proper balanced overall equation.” I am afraid I haven't provided that in this answer, limiting myself to surveying the wood rather than examining each tree.


The two electrons and a proton of NADH comes from a H$^-$ ion of a Krebs cycle intermediate during Isocitrate dehydrogenase reaction and Malate dehydrogenase reaction.

In the following reaction, Isocitrate is oxidised to Oxalosuccinate and in the process two Hydrogens are lost one as H$^-$ and the other as H+. This H$^-$ is accepted by NAD+ to form NADH.

Note: Oxalosuccinate is an intermediate formed during oxidative decarboxylation of Isocitrate.

Now coming to FADH$_2$ $-$

During oxidation of Succinate to Fumarate, Succinate looses a H$^-$ and a H+ ion which constitute the two protons and electrons of FADH$_2$ . Here's an image that illustrate the mechanism:

The alpha-ketoglutarate dehydrogenase complex reaction is similar to pyruvate dehydrogenase complex reaction(PDC). PDC reaction is an oxidative decarboxylation reaction of Pyruvate in which the 2e$^-$ of NADH are derived from the electron shared between COO$^2-$ and C=O groups of Pyruvate.

The Hygrogen (H+) of NADH is either derived from the H+ of CoA-SH or from the medium. There's a series of complex reactions which need to be studied to fully understand the source of H of this NADH.

So you can understand that the 2 e$^-$ of NADH produced in Alpha-ketoglutarate dehydrogenase complex reaction are the electrons between COO$^2-$ and C=O groups of Alpha-ketoglutarate and the H is either from the medium or the H+ of CoA-SH.

Reference: Biochemistry by Berg


There is a theoretical maximum of 38 ATP produced from a single glucose molecule: 2 NADH produced in glycolysis (3 ATP each) + 8 NADH produced in Krebs cycle (3 ATP each) + 2 FADH2 produced I don’t know where (2 ATP each) + 2 ATP produced in the Krebs cycle + 2 ATP produced in glycolysis = 6 + 24 + 4 + 2 + 2 = 38 ATP.

During citric acid cycle, 36 ATP molecules are produced. So, all together there are 38 molecules of ATP produced in aerobic respiration and 2 ATP are formed outside the mitochondria. Thus, option A is correct.


Pyruvate oxidation

Pyruvate oxidation is much shorter than the other steps of cellular respiration, it is key in linking glycolysis and the Kreb’s cycle.

Pyruvate (a 3 carbon molecule) is converted to acetyl CoA, a two-carbon molecule attached to coenzyme A. This reaction releases a molecule of carbon dioxide and reduces a NAD+ to NADH. In eukaryotes, pyruvate oxidation takes place in the matrix, the central compartment of mitochondria. Acetyl-CoA, acts as fuel for the Kreb’s cycle (also called the citric acid cycle). Before the reactions in this process can begin, pyruvate must enter the mitochondrion, passing through its inner membrane and to the matrix. In the matrix, pyruvate is modified in a series of steps:

Firstly, a carboxyl group is removed from pyruvate and released as carbon dioxide. The resulting two carbon molecule is oxidized, and NAD+ acts as the electron acceptor for the lost electrons, forming NADH. The oxidized two-carbon molecule is attached to Coenzyme A to form acetyl CoA. Acetyl CoA carries the acetyl group to the Kreb’s cycle.

These steps are carried out by a large enzyme complex called the pyruvate dehydrogenase complex , which consists of three component enzymes and includes over 60 subunits. The pyruvate dehydrogenase complex is a key target for regulation, as it controls the amount of acetyl-CoA that can enter the Kreb’s cycle. For each glucose molecule, 2 molecules of pyruvate are converted into 2 molecules of acetyl-CoA during pyruvate oxidation, releasing 2 carbons as carbon dioxide and generating 2 NADH from NAD+ . Acetyl-CoA serves as fuel for the Kreb’s cycle in the next stage of cellular respiration.


How are glycolysis Krebs cycle and etc linked?

The citric acid cycle is a series of chemical reactions that removes high-energy electrons and uses them in the electron transport chain to generate ATP. As the electrons are passed from NADH or FADH2 down the electron transport chain, they lose energy. The products of the electron transport chain are water and ATP.

One may also ask, how many ATP are produced in glycolysis Krebs cycle and electron transport? Glycolysis produces 2 ATP molecules, and the Krebs cycle produces 2 more. Electron transport begins with several molecules of NADH and FADH2 from the Krebs cycle and transfers their energy into as many as 34 more ATP molecules.

People also ask, what is the function of the Krebs cycle and electron transport chain?

The citric acid cycle, also known as the Krebs cycle, is involved in cell respiration and produces NADH and FADH2 for the electron transport chain. The Krebs cycle also produces two ATP, but much more ATP is produced later, in the electron transport chain, so that is not its main purpose.

What is the purpose of the Krebs cycle in cellular respiration?

The Krebs cycle is the second of three stages of cellular respiration, in which glucose, fatty acids and certain amino acids, the so-called fuel molecules, are oxidized (see Figure). The oxidation of these molecules is primarily used to transform the energy contained in these molecules into ATP.


How much ATP is produced in the Krebs cycle?

In eukaryotes, the Krebs cycle uses a molecule of acetyl CoA to generate 1 ATP, 3 NADH, 1 FADH2, 2 CO2, and 3 H+. Both the NADH and FADH2 molecules made in the Krebs cycle are sent to the electron transport chain, the last stage of cellular respiration.

Also Know, how much ATP is produced in each step of cellular respiration? Depending on how many NADH molecules are available, the electron transport chain makes a total of 32 or 34 ATP. These 32-34 ATP combined with 2 ATP from glycolysis and 2 ATP from the Krebs cycle means that one molecule of glucose (sugar) can make a total of 36-38 ATP.

Correspondingly, how is 36 ATP produced?

Cellular respiration produces 36 total ATP per molecule of glucose across three stages. Breaking the bonds between carbons in the glucose molecule releases energy. There are also high energy electrons captured in the form of 2 NADH (electron carriers) which will be utilized later in the electron transport chain.

How many ATP are used in electron transport chain?

This accounts for about two ATP molecules. A total of 32 ATP molecules are generated in electron transport and oxidative phosphorylation.


Which is not produced during the Krebs cycle?

Acetyl-CoA is not produced during Krebs cycle. It is produced from the decarboxylation of a pyruvate molecule, which occurs before the Krebs cycle can begin. Each turn of Krebs cycle is initiated by one acetyl-CoA molecule.

Additionally, how many ATP molecules are not produced in the Krebs cycle? These energy carriers join the 2 ATP and 2 NADH produced in glycolysis and the 2 NADH produced in the conversion of 2 pyruvates to 2 Acetyl-CoA molecules. At the conclusion of the Krebs Cycle, glucose is completely broken down, yet only four ATP have been produced.

Then, which are products of the Krebs cycle?

Each acetyl coenzyme A proceeded once through the citric acid cycle. Therefore, in total, it created 6 NADH + H+ molecules, two FADH2 molecules, four carbon dioxide molecules, and two ATP molecules. That's a lot of products!

How many electron carriers are produced in the Krebs cycle?

The citric acid cycle, which makes six NADH and two FADH2 . These carriers bring their electrons to the electron transport chain, which creates a hydrogen ion gradient in intermembrane of the mitochondria. The mitochondria is the powerhouse of the cell.


Referencing this Article

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Transition Reaction, Coenzyme A, and the Krebs Cycle

Glycolysis produces pyruvate, which can be further oxidized to capture more energy. For pyruvate to enter the next oxidative pathway, it must first be decarboxylated by the enzyme complex pyruvate dehydrogenase to a two-carbon acetyl group in the transition reaction, also called the bridge reaction. In the transition reaction, electrons are also transferred to NAD + to form NADH. To proceed to the next phase of this metabolic process, the comparatively tiny two-carbon acetyl must be attached to a very large carrier compound called coenzyme A (CoA). The transition reaction occurs in the mitochondrial matrix of eukaryotes in prokaryotes, it occurs in the cytoplasm because prokaryotes lack membrane-enclosed organelles.

The Krebs cycle transfers remaining electrons from the acetyl group produced during the transition reaction to electron carrier molecules, thus reducing them. The Krebs cycle also occurs in the cytoplasm of prokaryotes along with glycolysis and the transition reaction, but it takes place in the mitochondrial matrix of eukaryotic cells where the transition reaction also occurs. The Krebs cycle is named after its discoverer, British scientist Hans Adolf Krebs (1900–1981) and is also called the citric acid cycle, or the tricarboxylic acid cycle (TCA) because citric acid has three carboxyl groups in its structure. Unlike glycolysis, the Krebs cycle is a closed loop: The last part of the pathway regenerates the compound used in the first step. The eight steps of the cycle are a series of chemical reactions that capture the two-carbon acetyl group (the CoA carrier does not enter the Krebs cycle) from the transition reaction, which is added to a four-carbon intermediate in the Krebs cycle, producing the six-carbon intermediate citric acid (giving the alternate name for this cycle). As one turn of the cycle returns to the starting point of the fourcarbon intermediate, the cycle produces two CO2 molecules, one ATP molecule (or an equivalent, such as guanosine triphosphate [GTP]) produced by substrate-level phosphorylation, and three molecules of NADH and one of FADH 2 .

Although many organisms use the Krebs cycle as described as part of glucose metabolism, several of the intermediate compounds in the Krebs cycle can be used in synthesizing a wide variety of important cellular molecules, including amino acids, chlorophylls, fatty acids, and nucleotides therefore, the cycle is both anabolic and catabolic.


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October 24, 2007

So my cell biology class is taking a very modular approach to Fatty Acid Oxidation, the Krebs Cycle and Glycolysis. Im sure that my biochemistry class will not be nearly so informal, however, for completeness in my discussion of the mitochondria and because I said I would, here is a quick overview of these three important cycles that lead to the cell getting energy from food.

There are two standard food inputs that the cell uses to make ATP–Fatty Acids and Glucose. Ultimately, Glucose is reduced to two molecules of pyruvate in the cytosol, which are then further reduced to two molecules of Acetyl-CoA in the matrix space. In the fatty acid cycle, fatty acids are reduced in length to make units of Acetyl-CoA. This supply of Acetyl-CoA is what drives the Citric acid cycle in the cell.

Let us first tackle glycolysis. Using one ATP, Glycolysis is phosphorylated by hexokinase (remember that kinases are enzymes involved in phosphorylation) and changed into fructose 6-P by phosphoglucose isomerase. Using 1 ATP, phosphofructokinase phosphorylates the fructose on its first carbon, making fructose 1,6-P. This product is then cut by aldolase into hydroxyacetone phosphate and glyceraldehyde 3-phosphate. The hydroxyacetone P is transformed into another glyceraldehyde by tirose phosphate isomerase, giving us two glyceraldehyde 3-P. By gaining a phosphate, each glyceraldehyde 3-P reduces a NAD + , yielding twp 1,3-biphosphoglycerate and two NADH. Each biophosphoglycerate interacts with a phosphoglycerate kinase to phosphorylate an ADP, yielding 2 ATP and a 3-phosphoglycerate. Phosphoglycerate mutase then moves the phosphate from the third carbon to the second, producing 2-phosphoglycerate, each of which is used to produce phosphoenolpyruvate through interaction with enolase. Each enolpyruvate is then used to phosphorylate an ATP via catalysis by pyruvate kinase, yielding pyruvate and ATP.

Each of the steps of glycolysis is reversible, except for those catalyzed by pyruvate kinase, phosphofructokinase and hexokinase, thus once you get a molecule of pyruvate, it cannot wend its way back along the path–it is stuck as a pyruvate (unless it finds another metabolic path to follow).

In this procedure, two molecules of ATP have been used to catalyze phosphorylation of the sugar, and four ATP’s were yielded, along with two molecules of NADH. This makes the net yield of glycolysis 2 ATP, 2 NADH and 2 Pyruvate for each molecule of Glucose. A schematic summary of this process is given in Alberts’ Molecular Biology and the Cell, availible here.

The products of glycolysis (pyruvate) are imported into the mitochondria, where they are decarboxylated and attached to the CoA enzyme. In the process of loosing CO2, pyruvate reduces a NAD + . The resulting Acetyl CoA then enters the Krebs cycle proper. Acetyl CoA is catalyzed by Citrate synthase to donate its Acetyl group to oxaloacetate (also involved in the malate-aspartate shuttle) to produce citrate. Citrate is transformed into isocitrate by aconitase, which is then used by isocitrate dehydrogenase to reduce an NAD + producing α-keytoglutarate (also involved in malate aspartate shuttle). The keytoglutarate then interacts with the α-keytoglutarate dehydrogenase complex and the CoA enzyme to reduce a NAD + , and decarboxylate, yielding succinyl-CoA. It then is phosphorylated by succinyl CoA synthase to make succinate, phosphorylating a GDP in the process. Succinate then reacts with the membrane bound succinate dehydrogenase (Complex II in the electron transport chain), reducing FAD and producing Fumarate. Fumarate then reacts with fumerase and water to make malate which reacts with malate dehydrogenase to yield one final NADH and to restore oxaloacetate for use in another cycle.

The net products of this reaction chain are 4 molecules of NADH, one molecule of GTP and one molecule of FADH2, which means for each molecule of Glucose, this has yielded 4 NADH, 2 GTP and 2 FADH2. The carbons added by pyruvate to the oxaloacetate are lost in steps 3 and 4 of the chain (the reactions from isocitrate to succinyl CoA, though it should be noted that the carbons lost are not the ones actually added on by the pyruvate, but rather those present from another cycle in the citric acid cycle. The energy released from pyruvate comes from the successive oxidation and decarboxylation of the molecule. A diagram of the reaction, from Alberts’ text, is available here

Fatty Acid oxidation also provides a source of Acetyl CoA. Fatty Acids are imported into the Mitochondria after they have been activated for oxidation in the cytosol. Once in the mitochondria fatty acid goes through a cycle of four reactions mediated by four enzymes. The first reaction is an oxidation mediated by acyl-CoA dehydrogenase (AD). AD has a prosthetic FAD group that transfers the electrons that it gains from the oxidation of the fatty acid to ETF, which passes them on to Coenzyme Q via ubiquinone oxioreductase. The oxidized fatty acid, a trans-Δ 2 -Enoyl-CoA, is then hydrated by enoyl-CoA Hydratase. Once hydrated, the compound is reduced once more, and the electrons lost in this oxidation are passed on to a NADH via 3-L-hydroxyacyl-CoA dehydrogenase. The resulting compound is then undergoes thiolysis. The thiolysis is acomplished via β-Keytoacyl-CoA thiolase. The thiolysis breaks the bond betwen the β carbonyl carbon and the α carbon, yielding Acetyl CoA and another Fatty acyl-CoA that is two atoms shorter than when the sequence of reactions started. A schematic of this from Stryers’ text can be found here.

Thus Fatty Acid Oxidation provides a FADH2, a NADH and a Acetyl-CoA per cycle, until the fatty acid chain has been fully oxidized.

On a per molecule basis, it becomes clear that Fats are a much better source of energy to make ATP than are glucose molecules. Fat molecules are arranged in triglycerides–fully reduced molecule with three fatty acid chains, each of which may contain 18 carbons. Thus a single triglyceride has the potential to drive (18/2)*3=27 rounds of the citric acid cycle—as compared to two per molecule of glucose.

The information for this post came from Alberts’ Molecular Biology and the Cell, as well as Voet and Voet’s Biochemistry.


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