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Krebs cycle: Its importance, Nomenclature, and reactions

Importance of Krebs cycle for cellular respiration

Krebs cycle is also commonly referred to as the citric acid cycle. It plays an integral role in the cellular respiration process and the term “Krebs” has been named after Hans Krebs. The whole cycle comprises of a series of dynamic chemical reactions that aerobic organisms make use of in order to generate energy. The biochemical pathway of Krebs cycle suggests that it might possibly be one of the first parts related to the metabolism of a cell that evolved. Before understanding Krebs cycle, let us briefly go through what a link reaction is. This is one of the most important parts of the evolution process of a cell and is also known as Pyruvate Decarboxylation or Swanson Conversion. As per this reaction, a linkage is formed between glycolysis and Krebs cycle or citric acid cycle through metabolic pathways.

Hence, the Krebs cycle comes right after linkage reaction and helps in providing electrons and hydrogen that are needed for the transport chain of electrons. This whole reaction occurs inside the Mitochondria which are considered as the powerhouse of a cell. There are many other names of this cycle such as the tricarboxylic acid (TCA) and involves oxidation of Acetyl that is derived from Fats, Carbohydrates and Proteins straight into carbon dioxide which helps in the formation of chemical energy taking the form of GTP, which stands for guanosine triphosphate. In addition to that, the cycle also tends to provide precursors related to many amino acids in addition to NADH which is a reducing agent used in biochemical reactions. The central importance of this cycle in biochemical pathways goes on to suggest that it might just be one of the oldest and earliest components to be recognized in cellular metabolism.

Nomenclature of Krebs cycle

As we have said before that the name of this cycle emerges from Hans Krebs who came up with the concept first. Citric acid has a vital role to play in the process as it helps in the regeneration process in order to complete the entire cycle. Furthermore, acetate is consumed by the cycle with water and reducing agent NADH is formed which gives carbon dioxide as the waste by-product. The NADH that is generated by TCA is eventually fed into oxidative phosphorylation which happens to be a pathway for transporting electrons. The net result of the two pathways closely linked to each other would be the oxidation process of nutrients that are used to produce chemical energy of usable form. This is also known as ATP.

Krebs cycle in Eukaryotic cells

The citric acid cycle takes place in Eukaryotic cells in the form of a matrix inside the Mitochondria. On the other hand, there are Prokaryotic cells that lack Mitochondria in them. An example of a prokaryotic cell would be bacteria and the TCA sequence of reaction occurs in the cytosol along with proton gradient that helps in the production of ATP. The plasma membrane of a cell is where ATP production occurs, which happens to be the inner membrane attached to a Mitochondrion.

Krebs cycle as enzymatic reactions

As we know that citric acid cycle or Krebs cycle as it is commonly known, happens to be a range of enzymatic reactions that catalyze the whole aerobic metabolism process of the fuel molecules to water and carbon dioxide, hence generating chemical energy which helps in the production of ATP i.e. Adenosine Triphosphate molecules. The British biochemist Hans Krebs was the one to introduce the concept during his years in University of Sheffield, UK. There are lots of varieties of fuel molecules that can easily be drawn into the cycle and utilized in an efficient manner, which includes acetyl coenzyme, also known as acetyl-CoA. This is derived from the process of fatty acid oxidation and glycolysis. It is a much-contemplated fact that there are various types of amino acids whose metabolic process occurs with the help of enzymatic reactions through Krebs cycle.

Oxidation-reduction reactions in citric acid cycle

Almost all of the Eukaryotic cells have Krebs cycle present in them similar to Mitochondria that is present in all cells that are eukaryotic in nature. This cycle only functions as an integral part of the aerobic metabolism, only when oxygen is available. The requirement of oxygen occurs to the bonding exists between the electron transport chain of mitochondria and Krebs cycle. There is a total of four oxidation-reduction reactions that take place in a citric acid cycle. In the whole process, there is a phosphate bond that is generated from high energy and takes the form of GTP. This phosphate bond emitting high amounts of energy at a later stage of the process is transferred towards Adenosine Diphosphate also known as ADP. This leads to the formation of Adenosine Triphosphate (ATP). As soon as the oxidation process of the enzymes related to the Krebs cycle takes place, the fuel molecules are oxidized to carbon dioxide. In this way, there are coenzymes such as FAD, NAD+ and Q, also known as ubiquinone that are reduced eventually.

It has to be ensured that the cycle keeps continuing and the coenzymes that are reduced must be re-oxidized. This can occur only if electrons are transferred to oxygen which leads to the production of water. Henceforth, the electrons are finally accepted by the fuel molecules that are oxidized during the Krebs cycle oxidation process. But understand that the inhibition of this cycle will take place only if oxygen is absent.

Amphibole pathway of citric acid cycle

The whole citric acid cycle is nothing but a pathway for amphibole nature. This means that we can expect both the degradation of biomolecules and synthesis process to take place at the same time. Hence, since it finds usage in both the process, this cycle is absolutely vital to the mechanism of a cell and justifies the reason why this cycle was one of the first findings related to a cell. Apart from Acetyl CoA which as we know, is generated through fatty acids, glucose and ketogenic amino acids, is metabolized with the help of other biomolecules by the cycle. Many amino acids are usually degraded and go on to become intermediates of a cycle as we know. In a similar way, fatty acids that belong to odd chains are gradually metabolized which a lead to the formation of Succinyl-Coenzyme A that happens to be another important intermediate of Krebs cycle. There are many organisms that make use of these intermediates in order to perform synthesis process of biomolecules that are important to them.

There are some organisms that make use of Krebs cycle intermediates such as oxaloacetate and ketoglutarate for the synthesis process of amino acids of different nature and types. The Succinyl-Coenzyme A is used for the synthesis process of porphyrin rings that are used for the manufacturing process of heme and biosynthesis of chlorophyll. Malate and oxaloacetate are utilized towards the synthesis of glucose and the whole process is commonly referred to as gluconeogenesis.

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