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What Happens to Acetyl-CoA during the Citric Acid Cycle? - Open Education Online

What Happens to Acetyl-CoA during the Citric Acid Cycle?

Biochemistry is a broad discipline incorporating various chemical concepts that are related to living organisms. Several major topics are essential for a complete understanding of Biochemistry, no matter which sub-discipline one chooses. One of such issues is considered to be metabolism.

The idea of metabolism should be appealing not only to the scientists but also to people with different educational backgrounds. The fact is that metabolism is a crucial process through which we generate and use energy to carry functions essential for life. None of the living organisms would have managed to live without metabolic processes occurring in their bodies.

A metabolic pathway is generally divided into several steps. The energy is generated and released during these steps through various chemical reactions. One of the most important parts of the metabolic pathway is the Citric Acid Cycle which involves the reactions with Acetyl Coenzyme-A (or simply Acetyl-CoA) acting as a precursor.

To clearly explain all major concepts related to the involvement of Acetyl-CoA in the Citric Acid Cycle, the article will be divided into 4 sections. The first 3 sections provide general information about metabolism, Acetyl-CoA, and the Citric Acid Cycle, while the last part of the article examines what happens to Acetyl-CoA during the Citric Acid Cycle.

If you are struggling to understand what really happens at this stage of metabolism, or you are just curious how the Citric Acid Cycle works, this article is for you!

Metabolism

Metabolism is the process by which any living organism generates and uses energy to maintain functions necessary for life. To describe the term in a simpler manner, metabolism is the process of conversion of every nutrient that you intake into free energy that is essential for the proper functioning of your body. Therefore, it can be concluded that any reaction that occurs in a living organism is part of the metabolism.

One of the essential terms that you should know when talking about metabolism is a metabolic pathway, which is the combination of various enzymatically catalyzed chemical reactions occurring within a living organism.

Generally, metabolism is divided into two parts which are the following:

  • Catabolism – nutrients are broken down and the energy is released.
  • Anabolism – various biomolecules are synthesized and the energy is absorbed.

There are several common metabolic pathways, including Glucose metabolism, Glycogen metabolism, gluconeogenesis, and the Citric Acid Cycle. Each pathway consists of several chemical reactions involving different reaction mechanisms and specific enzymes to successfully transform nutrients into the energy for the body to use.

The free energy that is released as a result of catabolism is captured as ATP or other high-energy compounds. ATP can transfer its phosphoryl group to another compound so that the energy released can be used to drive different reactions.

Along with that, metabolic pathways also comprise various redox (oxidation-reduction) reactions involving common electron carriers: NAD+ (Nicotinamide Adenine Dinucleotide) and FAD (Flavin Adenine Dinucleotide). The role of these compounds is to receive electrons from reduced metabolites and donate them to other compounds.

Consequently, all the compounds that we intake through drinking and eating must go through the metabolic pathway for the free energy to be released. The compounds that enter the metabolic pathway are referred to as metabolic intermediates (precursors, metabolites). Therefore, any compound that is introduced to a living organism is converted into a proper precursor of the metabolic pathway.

Acetyl Coenzyme-A

As it was mentioned earlier, metabolic pathways involve the degradation of nutrients and biosynthesis of various biomolecules. One of such nutrients is Glycogen, which represents a branched polymer of Glucose. Glycogen metabolism incorporates a sequence of reactions involving particular enzymes.

Glycogen is first converted to Glucose-1-Phosphate by Glycogen Phosphorylase, which catalyzes glycogen breakdown. Then, Glycogen Debranching Enzyme, called Glucosyltransferase, transfers terminal glucose residues from one end to another. The resulting compound is further converted to Glucose-6-Phosphate by Phosphoglucomutase. G6P is converted to Fructose-6-Phosphate by Phosphoglucose Isomerase. The product requires the conversion to Fructose-1,6-bisphosphate, so Phosphofructokinase uses the ATP molecule to produce a bisphosphate. At this stage, the 6-carbon compound must be converted into two 3-carbon compounds; Therefore, Aldolase is now introduced to the system resulting in Dihydroxyacetone Phosphate (DHAP) and Glyceraldehyde-3-Phosphate (GAP). The next 4 steps involve the further conversion of GAP that can be shown below:

GAP à 1,3-Bisphosphoglycerate à 3-Phosphoglycerate à  2-Phosphoglycerate à Phosphoenolpyruvate

Finally, Phosphoenolpyruvate is converted to Pyruvate by Pyruvate kinase. Pyruvate is a precursor for the synthesis of Acetyl Coenzyme-A resulting from 5 sequential reactions involving oxidative decarboxylation.

Acetyl Coenzyme-A is a critical biomolecule participating in a wide range of biochemical reactions. It acts as an intermediate in various pathways, including cellular respiration, fatty acid metabolism, synthesis of biomolecules, allosteric regulation, and the Citric Acid Cycle.

Regarding the structure of the compound, acetic acid is linked to the Coenzyme A with a thioester bond. The acetic acid part of the Acetyl-CoA is the one that is transferred to specific molecules in various reactions occurring in a living organism.

Citric Acid Cycle

Every living organism requires metabolic fuels resulting from various energy-yielding pathways. Along with the catabolic reactions (e.g., Glucose and Glycogen catabolism), cells still need energy, since most of the chemical potential energy of carbohydrates is wasted during the Glycolysis. The fact is that further oxidation of the end product of Glycolysis leads to the recovery of even more energy.

Therefore, additional sequential reactions are required to generate more energy through oxidative reactions. The Citric Acid Cycle (also referred to as a Krebs Cycle, or Tricarboxylic Acid (TCA) Cycle) comprises 8 sequential reactions that oxidize Acetyl-CoA forming two carbon dioxide molecules, three NADH, one FADH2, and one high-energy compound (either GTP or ATP).

The Citric Acid Cycle, abbreviated as CAC, originates in the 1930s when Hans Krebs proposed a scheme of chain reactions involving interconversion of particular compounds with 2-3 carboxylic acid groups. Since Krebs was the first scientist to identify the linkage between the metabolism of CAC intermediates and the oxidation of metabolic fuels, the cycle is also known as the Krebs Cycle. Moreover, since the cycle involves tricarboxylic acid, it is also referred to as a Tricarboxylic Acid, or TCA cycle.

Before moving to the last section that examines the particular role of Acetyl-CoA in the CAC, it is essential to note that the Krebs cycle oxidizes acetyl groups from not only pyruvate but also from carbohydrates, fatty acids, and amino acids. Due to its importance for the normal functioning of a living organism, the Citric Acid Cycle is usually considered as the core of the cellular metabolism.

Acetyl Coenzyme-A in the Citric Acid Cycle

Involved in various biochemical reactions, Acetyl-CoA is also essential for the Citric Acid Cycle. The primary role of Acetyl-CoA is to transfer its acetyl group, which is further oxidized and the energy is released. As mentioned earlier, the involvement of Acetyl-CoA in the Krebs Cycle is essential for the production of additional energy in the form of NADH and FADH2. This is achieved through 8 sequential reactions precursor of which is Acetyl-CoA.

Reaction 1 – Attachment of Acetyl Group to Oxaloacetate
In the first reaction of the Citric Acid Cycle, Acetyl-CoA and Oxaloacetate are condensed with the involvement of Citrate Synthase. As a result of the acid-base catalyzed step, Citryl-CoA is formed and hydrolyzed to produce CoA and Citrate.

Reaction 2 – Interconversion of Citrate and Isocitrate
Aconitase is used to catalyze the isomerization reaction of Citrate and Isocitrate which proceeds with the production of cis-Aconitate as an intermediate. As the first step of a reaction, Citrate is dehydrated and then rehydrated to form Isocitrate.

Reaction 3 – Generation of the First NADH Molecule
As a result of the third reaction of the Citric Acid Cycle, the first NADH molecule is released. The first stem of the reaction involves NAD+-dependent Isocitrate Dehydrogenase, which promotes the conversion of Isocitrate to Oxalosuccinate by deprotonation of Isocitrate and the release of NADH. Further protonation of Oxalosuccinate produces alfa-Ketoglutarate.

Reaction 4 – Generation of the Second NADH Molecule
The fourth reaction of the Krebs cycle involves oxidative decarboxylation of alfa-Ketoglutarate to Succinyl-CoA. The enzyme involved in the reaction is called alfa-Ketoglutarate Dehydrogenase. Similarly to Reaction 3 above, NAD+ is also essential for this reaction to deprotonate alfa-Ketoglutarate and generate a second NADH molecule.

Reaction 5 – Generation of GTP
At this stage, high-energy molecule Succinyl-CoA is cleaved to form Succinyl-Phosphate and CoA. The phosphate group from Succinyl-Phosphate is transferred to a Histidine residue on the enzyme called Succinyl-CoA Synthase. As a last step of the reaction, the phosphoryl group is transferred to GDP to generate GTP.

Reaction 6 – Generation of FADH2 Molecule
Succinate is dehydrogenated to form Fumarate. Succinate Dehydrogenase is the enzyme that catalyzes this reaction. Since the enzyme contains FAD as a prosthetic group, the dehydrogenation of Succinate leads to the formation of the FADH2 molecule.

Reaction 7 – Conversion of Fumarate to Malate
With the involvement of Fumarase, Fumarate is hydrated to produce Malate through a carbanion transition state.

Reaction 8 – Generation of the Third NADH Molecule
In the last reaction of the Citric Acid Cycle, the hydroxyl group of Malate is oxidized through the reaction catalyzed by Malate Dehydrogenase. The reaction is NAD+-dependent; therefore, the conversion of Malate to Oxaloacetate also releases the third and last molecule of NADH.

Considering all the information provided throughout the article, Acetyl-CoA plays a vital role in the Citric Acid Cycle. Along with the generation of high-energy compounds such as NADH and FADH2, Acetyl-CoA is also crucial for the regulation of the cycle. Accurate and precise control of the reactions of the Citric Acid Cycle is essential for the proper functioning of a living organism. Thus, the regulation of different enzymes is achieved by several methods, including product inhibition and covalent modification by phosphorylation or dephosphorylation.

In both cases, the involvement of Acetyl-CoA is essential. For instance, the regulation of Pyruvate Dehydrogenase is achieved via product inhibition by NADH and Acetyl-CoA. In such cases, NADH and Acetyl-CoA act as competitors for the binding sites on the enzymes. This process decreases the rate at which pyruvate decarboxylation occurs.

As already mentioned earlier, NADH and Acetyl-CoA are the products of the Pyruvate Dehydrogenase reaction. Therefore, in the case of covalent modification, NADH and Acetyl-CoA can activate the Pyruvate Dehydrogenase Kinase. As a consequence, phosphorylation of a particular dehydrogenase Serine residue inactivates Pyruvate Dehydrogenase.

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