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Overview of the citric acid cycle. The citric acid cycle—also known as the Krebs cycle, Szent–Györgyi–Krebs cycle, or 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, proteins, and alcohol.
The rather complex two-step mechanism by which this occurs is important, as it increases the efficiency of proton transfer. If, instead of the Q cycle, one molecule of QH 2 were used to directly reduce two molecules of cytochrome c, the efficiency would be halved, with only one proton transferred per cytochrome c reduced. [5]
To fully oxidize the equivalent of one glucose molecule, two acetyl-CoA must be metabolized by the Krebs cycle. Two low-energy waste products, H 2 O and CO 2, are created during this cycle. [12] [13] The citric acid cycle is an 8-step process involving 18 different enzymes and co-enzymes.
The reverse Krebs cycle, also known as the reverse TCA cycle (rTCA) or reductive citric acid cycle, is an alternative to the standard Calvin-Benson cycle for carbon fixation. It has been found in strict anaerobic or microaerobic bacteria (as Aquificales ) and anaerobic archea .
The former, often eponymously known as the "Krebs cycle", is the sequence of metabolic reactions that allows cells of oxygen-respiring organisms to obtain far more ATP from the food they consume than anaerobic processes such as glycolysis can supply; and its discovery earned Krebs a Nobel Prize in Physiology or Medicine in 1953.
It functions as a pace-making enzyme in the first step of the citric acid cycle (or Krebs cycle). [5] Citrate synthase is located within eukaryotic cells in the mitochondrial matrix, but is encoded by nuclear DNA rather than mitochondrial. It is synthesized using cytoplasmic ribosomes, then transported into the mitochondrial matrix.
The Krebs cycle – This is the second stage, and the products of this stage of the aerobic system are a net production of one ATP, one carbon dioxide molecule, three reduced NAD + molecules, and one reduced flavin adenine dinucleotide (FAD) molecule.
[5]: 572 To the right is an illustration of the amphibolic properties of the TCA cycle. The glyoxylate shunt pathway is an alternative to the tricarboxylic acid (TCA) cycle, for it redirects the pathway of TCA to prevent full oxidation of carbon compounds, and to preserve high energy carbon sources as future energy sources. This pathway occurs ...