Citric Acid Cycle

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This illustration shows the eight steps of the citric acid cycle. In the first step, the acetyl group from acetyl uppercase C lower case o upper case A is transferred to a four-carbon oxaloacetate molecule to form a six-carbon citrate molecule. In the second step, citrate is rearranged to form isocitrate. In the third step, isocitrate is oxidized to alpha-ketoglutarate. In the process, one N A D H is formed from N A D superscript plus sign baseline; and one carbon dioxide is released. In the fourth step, alpha-ketoglutarate is oxidized and upper C lower o upper A is added, forming succinyl upper C lower o upper A. In the process, another N A D H is formed and another carbon dioxide is released. In the fifth step, upper C lower o upper A is released from succinyl upper C lower o upper A, forming succinate. In the process, one G T P is formed, which is later converted into A T P. In the sixth step, succinate is oxidized to fumarate, and one F A D is reduced to F A D H subscript 2 baseline. In the seventh step, fumarate is converted into malate. In the eighth step, malate is oxidized to oxaloacetate, and another N A D H is formed.
In the citric acid cycle, the acetyl group from acetyl CoA is attached to a four-carbon oxaloacetate molecule to form a six-carbon citrate molecule. Through a series of steps, citrate is oxidized, releasing two carbon dioxide molecules for each acetyl group fed into the cycle. In the process, three NAD+ molecules are reduced to NADH, one FAD molecule is reduced to FADH2, and one ATP or GTP (depending on the cell type) is produced (by substrate-level phosphorylation). Because the final product of the citric acid cycle is also the first reactant, the cycle runs continuously in the presence of sufficient reactants. (credit: modification of work by “Yikrazuul”/Wikimedia Commons)

OpenStax Biology 2e

Like the conversion of pyruvate to acetyl CoA, the citric acid cycle takes place in the matrix of mitochondria. Almost all of the enzymes of the citric acid cycle are soluble, with the single exception of the enzyme succinate dehydrogenase, which is embedded in the inner membrane of the mitochondrion. Unlike glycolysis, the citric acid 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 redox, dehydration, hydration, and decarboxylation reactions that produce two carbon dioxide molecules, one GTP/ATP, and the reduced carriers NADH and FADH2. This is considered an aerobic pathway because the NADH and FADH2 produced must transfer their electrons to the next pathway in the system, which will use oxygen. If this transfer does not occur, the oxidation steps of the citric acid cycle also do not occur. Note that the citric acid cycle produces very little ATP directly and does not directly consume oxygen.

– What do you call the idea that life arose from non-life more than 3.5 billion years ago on Earth?

Steps in the Citric Acid Cycle

Step 1. Prior to the first step, a transitional phase occurs during which pyruvic acid is converted to acetyl CoA. Then, the first step of the cycle begins: This condensation step combines the two-carbon acetyl group with a four-carbon oxaloacetate molecule to form a six-carbon molecule of citrate. CoA is bound to a sulfhydryl group (-SH) and diffuses away to eventually combine with another acetyl group. This step is irreversible because it is highly exergonic. The rate of this reaction is controlled by negative feedback and the amount of ATP available. If ATP levels increase, the rate of this reaction decreases. If ATP is in short supply, the rate increases.

Step 2. In step two, citrate loses one water molecule and gains another as citrate is converted into its isomer, isocitrate.

Step 3. In step three, isocitrate is oxidized, producing a five-carbon molecule, α-ketoglutarate, along with a molecule of CO2 and two electrons, which reduce NAD+ to NADH. This step is also regulated by negative feedback from ATP and NADH and a positive effect of ADP.

– What is a metabolic process in which energy is harnessed for the production of ATP?

Step 4. Steps three and four are both oxidation and decarboxylation steps, which as we have seen, release electrons that reduce NADto NADH and release carboxyl groups that form CO2 molecules. Alpha-ketoglutarate is the product of step three, and a succinyl group is the product of step four. CoA binds with the succinyl group to form succinyl CoA. The enzyme that catalyzes step four is regulated by feedback inhibition of ATP, succinyl CoA, and NADH.

Step 5. In step five, a phosphate group is substituted for coenzyme A, and a high-energy bond is formed. This energy is used in substrate-level phosphorylation (during the conversion of the succinyl group to succinate) to form either guanine triphosphate (GTP) or ATP. There are two forms of the enzyme, called isoenzymes, for this step, depending upon the type of animal tissue in which they are found. One form is found in tissues that use large amounts of ATP, such as heart and skeletal muscle. This form produces ATP. The second form of the enzyme is found in tissues that have a high number of anabolic pathways, such as liver. This form produces GTP. GTP is energetically equivalent to ATP; however, its use is more restricted. In particular, protein synthesis primarily uses GTP.

Step 6. Step six is a dehydration process that converts succinate into fumarate. Two hydrogen atoms are transferred to FAD, reducing it to FADH2. (Note: the energy contained in the electrons of these hydrogens is insufficient to reduce NAD+ but adequate to reduce FAD.) Unlike NADH, this carrier remains attached to the enzyme and transfers the electrons to the electron transport chain directly. This process is made possible by the localization of the enzyme catalyzing this step inside the inner membrane of the mitochondrion.

Step 7. Water is added by hydrolysis to fumarate during step seven, and malate is produced. The last step in the citric acid cycle regenerates oxaloacetate by oxidizing malate. Another molecule of NADH is then produced in the process.

A functional citric acid cycle is absent in plasmodium parasites, and their main energy source is considered to be the ATPs generated during anaerobic fermentation of glucose. To meet the high energy requirement of the parasites, the infected red blood cell consume 30–100 times more glucose than uninfected ones, and glycolysis is essential for parasite survival.

Source:

Clark, M., Douglas, M., Choi, J. Biology 2e. Houston, Texas: OpenStax. Access for free at: https://openstax.org/details/books/biology-2e

https://www.britannica.com/science/abiogenesis

https://employees.csbsju.edu/cschaller/Reactivity/oxphos/OPintro.htm

https://www.sciencedirect.com/referencework/9780080450445/comprehensive-medicinal-chemistry-ii


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