- last actualisation
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Learning objectives
- Explain how a circular pathway such as the citric acid cycle is significantly different from a linear pathway such as glycolysis
- Describe how pyruvate, a product of glycolysis, is prepared to enter the citric acid cycle
If oxygen is available, aerobic respiration will continue. In eukaryotic cells, the pyruvate molecules produced at the end of glycolysis are transported to the mitochondria, which are the sites of cellular respiration. There, the pyruvate is converted to an acetyl group, which is taken up and activated by a carrier called coenzyme A (CoA). The resulting compound is calledAcetylo-CoA. CoA is made from vitamin B5, pantothenic acid. Acetyl-CoA can be used by the cell in various ways, but its primary function is to transfer the acetyl group derived from pyruvate to the next step in the glucose catabolism pathway.
pyruvate breakdown
In order for pyruvate, a product of glycolysis, to enter the next metabolic pathway, it must undergo several transformations. The conversion takes place in a three-step process (Figure \(\PageIndex{1}\)).
Step 1. The carboxyl group is removed from the pyruvate, releasing a carbon dioxide molecule into the surrounding environment. The result of this step is a hydroxyethyl group with two carbon atoms attached to the enzyme (pyruvate dehydrogenase). This is the first of the six carbon atoms from the original glucose molecule to be removed. This step is performed twice (remember: there aretwopyruvate molecules, formed at the end of glycolysis) for each glucose molecule metabolized; Thus, at the end of both steps, two of the six carbon atoms have been removed.
Step 2. The hydroxyethyl group is oxidized to an acetyl group and electrons are accepted by NAD+formation of NADH. The high-energy electrons from NADH are later used to make ATP.
Step 3. The acetyl group attached to the enzyme is transferred to CoA, forming an acetyl-CoA molecule.
It should be noted that in the second phase of glucose metabolism, each time a carbon atom is removed, it binds to two oxygen atoms, producing carbon dioxide, one of the main end products of cellular respiration.
Acetyl-CoAs CO2
In the presence of oxygen, acetyl-CoA donates its acetyl group to a four-carbon molecule, oxaloacetate, to form citrate, a six-carbon molecule with three carboxyl groups. This time it will get the rest of the energy that can be extracted from the original glucose molecule. This unique path is referred to by various names:The citric acid cycle(for the first intermediate formed - citric acid or citrate - when acetate combines with oxaloacetate),TCA-Zyklus(Because citric acid or citrate and isocitrate are tricarboxylic acids)Krebs cycle, according to Hans Krebs, who in the 1930s first identified the stages in pigeon flight muscles.
The citric acid cycle
Like the conversion of pyruvate to acetyl-CoA, the citric acid cycle takes place in the mitochondrial matrix. Almost all citric acid cycle enzymes are soluble, with the exception of the enzyme succinate dehydrogenase, which is embedded in the inner mitochondrial membrane. Unlike glycolysis, the citric acid cycle is a closed loop: the final part of the pathway regenerates the compound used in the first step. The eight stages of the cycle are a series of redox reactions, dehydration, hydration and decarboxylation, resulting in the formation of two molecules of carbon dioxide, GTP/ATP and reduced forms of NADH and FADH2(Image \(\PageIndex{2}\)). This is considered an aerobic pathway due to the presence of NADH and FADH2The electrons created must transfer their electrons to the next path in the oxygen-using system. If this transfer does not occur, the oxidation steps of the citric acid cycle will not occur. It should be noted that the citric acid cycle directly produces very little ATP and does not use oxygen directly.
Stages of the citric acid cycle
Step 1. Before the start of the first step, there is a transition phase in which pyruvic acid is converted to acetyl-CoA. Then the first stage of the cycle begins: this is the condensation stage where a two-carbon acetyl group combines with a four-carbon oxaloacetate molecule to form a six-carbon citrate molecule. CoA is attached to the sulfhydryl group (-SH) and diffuses to eventually connect to another acetyl group. This step is irreversible because it is highly exergonic. The rate of this response is controlled by negative feedback and the amount of ATP available. As the level of ATP increases, the rate of this reaction decreases. When ATP is low, the speed increases.
Step 2. In the second step, citrate loses one water molecule and gains another as citrate is converted to its isomer, isocitrate.
Step 3. In the third step, isocitrate is oxidized to form a five-carbon molecule, α-ketoglutarate, along with a CO molecule2and two NAD reducing electrons+to NADH. This step is also regulated by the negative feedback of ATP and NADH and the positive influence of ADP.
Steps 3 and 4. Steps three and four are both oxidation and decarboxylation steps that release NAD reducing electrons+into NADH and release carboxyl groups that form CO2Particles. The α-ketoglutarate is the product of the third step and the succinyl group is the product of the fourth step. CoA binds the succinyl group to form succinyl-CoA. The enzyme that catalyzes the fourth step is regulated by the inhibition feedback of ATP, succinyl-CoA and NADH.
Step 5. In the fifth step, Coenzyme A is replaced with a phosphate group and a high-energy bond is formed. This energy is used in substrate-level phosphorylation (when converting the succinyl group to succinate) to form guanine triphosphate (GTP) or ATP. There are two forms of enzymes at this stage, called isoenzymes, depending on the type of animal tissue in which they are found. One form is found in tissues that use large amounts of ATP, such as the heart and skeletal muscles. This form produces ATP. Another form of the enzyme is found in tissues that have a large number of anabolic signaling pathways, such as the liver. This form is generated by GTP. GTP is energetically equivalent to ATP; However, its use is more limited. GTP is especially used in protein synthesis.
Step 6. Step six is the dehydration process that converts succinate to fumarate. Two hydrogen atoms are transferred to FAD to form FADH2. The energy contained in the electrons of these atoms is insufficient to reduce NAD+but enough to reduce FAD. Unlike NADH, this carrier remains bound to the enzyme and transfers electrons directly to the electron transport chain. This process is made possible by the localization of the enzyme that catalyzes this step in the inner mitochondrial membrane.
Step 7. In the seventh step, water is added to the fumarate to form malate. The last step of the citric acid cycle regenerates oxaloacetate by oxidizing malate. This creates another NADH molecule.
Interactive link
Take a look at itVideo showing the steps of the citric acid cycle.
Products of the citric acid cycle
Two carbon atoms enter the citric acid cycle from each acetyl group, representing four of the six carbon atoms in the glucose molecule. With each turn of the cycle, two molecules of carbon dioxide are released; However, they do not necessarily contain recently added carbon atoms. The two acetyl carbons are finally released in later rounds of the cycle; Therefore, in the end, all six carbon atoms of the original glucose molecule are incorporated into carbon dioxide. In each round of the cycle, three NADH and one FADH are produced2Molecule. These carriers combine with the final part of aerobic respiration to form ATP molecules. GTP or ATP is also produced in each cycle. Several intermediates in the citric acid cycle can be used for the synthesis of non-essential amino acids; Therefore, the cycle is amphibolic (both catabolic and anabolic).
Abstract
In the presence of oxygen, pyruvate is converted to an acetyl group attached to the coenzyme A carrier molecule. The resulting acetyl-CoA can enter via several routes, but most commonly the acetyl group is donated to the citric acid cycle for further catabolism. During the conversion of pyruvate to an acetyl group, one molecule of carbon dioxide and two high-energy electrons are removed. The carbon dioxide forms two (converting two pyruvate molecules) from the six carbon atoms of the original glucose molecule. Electrons are taken by NAD+and NADH transfers electrons to the next ATP production pathway. At this point, the glucose molecule that originally entered cellular respiration is fully oxidized. The chemical potential energy stored in the glucose molecule is transferred to electron carriers or used to synthesize part of ATP.
The citric acid cycle consists of a series of redox and decarboxylation reactions that remove high-energy electrons and carbon dioxide. Electrons are temporarily stored in NADH and FADH molecules2are used to generate ATP in the next pathway. In each cycle, one molecule of GTP or ATP is created by phosphorylation at the substrate level. There is no comparison between a cyclic path and a linear path.
review questions
What is removed from pyruvate when it is converted to an acetyl group?
- oxygen
- ATP
- vitamin b
- carbon dioxide
- Answer
-
Answer: D
What do electrons add to NAD?+Also?
- They become part of the fermentation pathway.
- They take a different route to produce ATP.
- They promote the entry of the acetyl group into the citric acid cycle.
- They are converted to NADP.
- Answer
-
Answer: B
GTP or ATP is made when ________ is converted.
- Isocitrynate to α-ketoglutarate
- Succinyl-CoA is a succinate
- Fumarat z Malat
- Convert malate to oxaloacetate
- Answer
-
Answer: B
(Video) Krebs Cylcle Trick How to remember krebs cycle FOREVER!!
How many NADH molecules are produced in each round of the citric acid cycle?
- How
- two
- tri
- four
- Answer
-
Answer: C
Free reply
What is the main difference between a circular track and a linear track?
- Answer
-
Answer: In a cyclic path, the final reaction product is also the initial reactant. The path is self-contained as long as the intermediate products of the path exist. Circular courses can accommodate multiple entry and exit points, making them particularly suitable for amphibian courses. In the case of a linear path, the path trip completes the path, and the second trip would be an independent event.
glossary
- Acetylo-CoA
- A combination of an acetyl group derived from pyruvic acid and coenzyme A produced from pantothenic acid (a group of B vitamins).
- The citric acid cycle
- (also Krebs cycle) A series of chemical reactions catalyzed by enzymes, central to all living cells
- Krebs cycle
- (also citric acid cycle) an alternative name for the citric acid cycle, named after Hans Krebs, who first identified the steps of the metabolic pathway in the muscles of flying pigeons in the 1930s; see citric acid cycle
- TCA-Zyklus
- (also citric acid cycle) an alternative name for the citric acid cycle, named after the citric acid group, tricarboxylic acid (TCA); see citric acid cycle
Contributors and Attributions
Connie Rye (East Mississippi Community College), Robert Wise (University of Wisconsin, Oshkosh), Vladimir Jurukovski (Suffolk County Community College), Jean DeSaix (University of North Carolina at Chapel Hill), Jung Choi (Georgia Institute of Technology), Yael Avissar (Rhode Island College) among other contributing authors. Original OpenStax content (CC BY 4.0; free download athttp://cnx.org/contents/185cbf87-c72...f21b5eabd@9.87).
FAQs
Figure 7.4: Pyruvate Oxidation and the Citric Acid Cycle? ›
In Summary: Pyruvate Oxidation
In the presence of oxygen, pyruvate is transformed into an acetyl group attached to a carrier molecule of coenzyme A. The resulting acetyl CoA can enter several pathways, but most often, the acetyl group is delivered to the citric acid cycle for further catabolism.
In Summary: Pyruvate Oxidation
In the presence of oxygen, pyruvate is transformed into an acetyl group attached to a carrier molecule of coenzyme A. The resulting acetyl CoA can enter several pathways, but most often, the acetyl group is delivered to the citric acid cycle for further catabolism.
As a result of glycolysis, pyruvate oxidation and the citric acid cycle, only a small portion of the energy of glucose has been converted to ATP.
Is pyruvate oxidation the citric acid cycle? ›Breakdown of Pyruvate
In order for pyruvate, the product of glycolysis, to enter the next pathway, it must undergo several changes to become acetyl Coenzyme A (acetyl CoA). Acetyl CoA is a molecule that is further converted to oxaloacetate, which enters the citric acid cycle (Krebs cycle).
In aerobic conditions, pyruvate enters the citric acid cycle and undergoes oxidative phosphorylation leading to the net production of 32 ATP molecules.
What happens in the citric acid cycle step by step? ›Step 1: Acetyl CoA (two-carbon molecule) joins with oxaloacetate (four-carbon molecule) to form citrate (six-carbon molecule). Step 2: Citrate is converted to isocitrate (an isomer of citrate) Step 3: Isocitrate is oxidised to alpha-ketoglutarate (a five-carbon molecule) which results in the release of carbon dioxide.
Where does pyruvate oxidation and the citric acid cycle take place? ›Like the conversion of pyruvate to acetyl CoA, the citric acid cycle takes place in the matrix of mitochondria.