Definition of the Citric Acid Cycle
The citric acid cycle, also known as the tricarboxylic acid (TCA) cycle or the Krebs cycle, represents one part of the three- part aerobic cellular respiration pathway. In cellular respiration, a molecule of glucose enters the pathway and adenosine triphosphate (ATP) ultimately gets produced for use as a source of energy to run cellular processes.
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Background
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Any organism which respires aerobically will undergo the citric acid cycle as a part of the cellular respiration pathway. Organisms that respire anaerobically have a different pathway to get energy.
Cellular Respiration
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Cellular respiration occurs in three stages. The first stage, glycolysis, converts a molecule of the monosaccharide glucose into pyruvate. As a byproduct, ATP and NADH gets released from the reaction.
The pyruvate, after being converted into acetyl-Coenzyme A (CoA), enters the citric acid cycle in the mitochondrial matrix. While a small amount of ATP is created in the citric acid cycle, the main byproducts are NADH and FADH2. These molecules enter the next step, oxidative phosphorylation, to create the bulk of the ATP that occurs from the cellular respiration process.
The final stage, oxidative phosphorylation, occurs in the mitochondrial membrane. The NADH and FADH2 that were produced in the citric acid cycle get oxidized to FAD+ and NAD+. Through chemiosmosis, the change in proton gradient and subsequent pumping of protons across the mitochondrial membrane allow adenosine diphosphate (ADP) to undergo phosphorylation to ATP.
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Products
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The products from one acetyl CoA entering the citric acid cycle include: two molecules of carbon dioxide, three molecules of NADH, two molecules of FADH2, one molecule of guanine triphosphate (GTP) or ATP, and CoASH.
Convergence Of Pathways
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The citric acid cycle has greater importance than just an intermediary step in aerobic cellular respiration. Other cellular pathways include the citric acid cycle as a major step during catabolism. Carbohydrate catabolism, fat catabolism and protein catabolism all use the citric acid cycle at different steps in the breakdown of products in those pathways.
Disease
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Mutations in the genes responsible for enzymes that function in the citric acid cycle can cause decreased function in the cycle and other diseases stemming from the mutation. For example, a mutation in the gene for the succinyl-CoA synthetase enzyme leads to autosomal recessive encephalopathy and Leigh syndrome. In the citric acid cycle, succinyl-CoA synthetase converts succinyl CoA to succinate.
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