Interconnected biochemical reactions releasing energy
Overview of the citric acid cycle
The citric acid cycle—also known as the Krebs cycle, Szent–Györgyi–Krebs cycle or the TCA cycle (tricarboxylic acid cycle)[1][2]—is a series of biochemical reactions to release the energy stored in nutrients through the oxidation of acetyl-CoA derived from carbohydrates, fats, and proteins. The chemical energy released is available under the form of ATP. The Krebs cycle is used by organisms that respire (as opposed to organisms that ferment) to generate energy, either by anaerobic respiration or aerobic respiration. In addition, the cycle provides precursors of certain amino acids, as well as the reducing agent NADH, that are used in numerous other reactions. Its central importance to many biochemical pathways suggests that it was one of the earliest components of metabolism.[3][4] Even though it is branded as a "cycle", it is not necessary for metabolites to follow only one specific route; at least three alternative segments of the citric acid cycle have been recognized.[5]
The name of this metabolic pathway is derived from the citric acid (a tricarboxylic acid, often called citrate, as the ionized form predominates at biological pH[6]) that is consumed and then regenerated by this sequence of reactions to complete the cycle. The cycle consumes acetate (in the form of acetyl-CoA) and water, reduces NAD+ to NADH, releasing carbon dioxide. The NADH generated by the citric acid cycle is fed into the oxidative phosphorylation (electron transport) pathway. The net result of these two closely linked pathways is the oxidation of nutrients to produce usable chemical energy in the form of ATP.
In eukaryotic cells, the citric acid cycle occurs in the matrix of the mitochondrion. In prokaryotic cells, such as bacteria, which lack mitochondria, the citric acid cycle reaction sequence is performed in the cytosol with the proton gradient for ATP production being across the cell's surface (plasma membrane) rather than the inner membrane of the mitochondrion.
For each pyruvate molecule (from glycolysis), the overall yield of energy-containing compounds from the citric acid cycle is three NADH, one FADH2, and one GTP.[7]
^Lowenstein JM (1969). Methods in Enzymology, Volume 13: Citric Acid Cycle. Boston: Academic Press. ISBN 978-0-12-181870-8.
^Kay J, Weitzman PD (1987). Krebs' citric acid cycle: half a century and still turning. London: Biochemical Society. pp. 25. ISBN 978-0-904498-22-6.
^Wagner A (2014). Arrival of the Fittest (First ed.). PenguinYork. p. 100. ISBN 978-1-59184-646-8.
^Lane N (2009). Life Ascending: The Ten Great Inventions of Evolution. New York: W. W. Norton & Co. ISBN 978-0-393-06596-1.
^Chinopoulos C (August 2013). "Which way does the citric acid cycle turn during hypoxia? The critical role of α-ketoglutarate dehydrogenase complex" (PDF). Journal of Neuroscience Research. 91 (8): 1030–1043. doi:10.1002/jnr.23196. PMID 23378250.
^Cite error: The named reference Voet_2004 was invoked but never defined (see the help page).
^Lieberman M (2013). Marks' basic medical biochemistry : a clinical approach. Marks, Allan D., Peet, Alisa (Fourth ed.). Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins. ISBN 978-1-60831-572-7. OCLC 769803483.
intermediate in the citricacidcycle, which occurs in the metabolism of all aerobic organisms. More than two million tons of citricacid are manufactured...
gluconeogenesis, the urea cycle, the glyoxylate cycle, amino acid synthesis, fatty acid synthesis and the citricacidcycle. Oxaloacetic acid undergoes successive...
[1](p.734) The urea cycle and the citricacidcycle are independent cycles but are linked. One of the nitrogen atoms in the urea cycle is obtained from the...
acid alanine and can be converted into ethanol or lactic acid via fermentation. Pyruvic acid supplies energy to cells through the citricacidcycle (also...
pseudo-igniarius), lichen, and Iceland moss. Fumarate is an intermediate in the citricacidcycle used by cells to produce energy in the form of adenosine triphosphate...
two ATPs is formed in the glycolysis cycle. The glycolysis pathway is later associated with the CitricAcidCycle which produces additional equivalents...
the citricacidcycle where carbon is lost in the form of CO2. The two initial steps of the glyoxylate cycle are identical to those in the citricacid cycle:...
fixation process, malate is a source of CO2 in the Calvin cycle. In the citricacidcycle, (S)-malate is an intermediate, formed by the addition of an...
Isocitric acid is a structural isomer of citricacid. Since citricacid and isocitric acid are structural isomers, they share similar physical and chemical...
tricarboxylic acid is citricacid. Citricacid, a type of tricarboxylic acid, is used in the citricacidcycle – also known as tricarboxylic acid (TCA) cycle or...
CO2 is formed.[citation needed] The citricacidcycle is also called the Krebs cycle or the tricarboxylic acidcycle. When oxygen is present, acetyl-CoA...
O2. Every cycle give 3 NADH, 1 FADH2, CO2 and GTP. The second role is biosynthetic, as citricacidcycle regenerate oxaloacetate when cycle intermediates...
amino acid synthesis (synthesis of glutamate, glutamine, proline and arginine) begins with α-ketoglutarate, an intermediate in the CitricAcidCycle. The...
the citricacidcycle. It is acted upon by the enzyme aconitase. Aconitic acid can be synthesized by dehydration of citricacid using sulfuric acid: (HO2CCH2)2C(OH)CO2H...
post-glycolytic pathways: Fatty acid synthesis Cholesterol synthesis The citricacidcycle which in turn leads to: Amino acid synthesis Nucleotide synthesis...
complex is an enzyme complex, most commonly known for its role in the citricacidcycle. Much like pyruvate dehydrogenase complex (PDC), this enzyme forms...
amino acids facilitates entering of their carbon skeleton into the cycle directly (as pyruvate or oxaloacetate), or indirectly via the citricacidcycle. The...
mitochondria in eukaryotes to generate acetyl-CoA. Acetyl-CoA enters the citricacidcycle, generating NADH and FADH2, which are electron carriers used in the...
condenses with oxaloacetate, to enter the citricacidcycle. During each turn of the cycle, two carbon atoms leave the cycle as CO2 in the decarboxylation reactions...
metabolites, in the end generating acetyl-CoA, the entry molecule for the citricacidcycle, the main energy supply of living organisms, including bacteria and...
the cell cycle. Amino acids also contribute to cellular energy metabolism by providing a carbon source for entry into the citricacidcycle (tricarboxylic...
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