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Importance Of Chemistry : The Citric Acid Cycle

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Abstract
The citric acid cycle (CAC), also known as the Krebs cycle and the tricarboxylic acid cycle, is a stage of cellular respiration. The role of the cycle is to oxidise fuel molecules and to provide precursors in the form of carbon skeletons for other metabolic pathways (e.g. fatty acid synthesis, amino acid synthesis etc.). The CAC is described as amphibolic, as it has both catabolic and anabolic functions. Catabolism is the breaking down of substances to provide chemically available energy (e.g. ATP) and/or to generate intermediates used in anabolic reactions, whereas anabolism involves the formation of complex molecules from simpler ones and the storage of energy. The cycle can be regulated to either increase or decrease the rate of reaction, and regulation is often carried out via allosteric inhibition of CAC enzymes.
Step Reactants Enzyme Products Reaction type
1 Acetyl CoA (2C), Oxaloacetate (4C) and H2O Citrate synthase Citrate (6C), CoA and H+ Aldol condensation and hydrolysis

2 Citrate (6C) Aconitase Isocitrate (6C) Dehydrating and then rehydrating

3 Isocitrate (6C) and NAD+ Isocitrate dehydrogenase α-Ketoglutarate (5C), CO2 and NADH Oxidative decarboxylation

4 α-Ketoglutarate (5C), NAD+ and CoA α-Ketoglutarate dehydrogenase complex Succinyl CoA (4C), CO2 and NADH Oxidative decarboxylation

5 Succinyl CoA (4C), Pi, GDP and H2O Succinyl CoA synthase Succinate (4C), GTP and CoA Substrate-level phosphorylation

6 Succinate (4C) and FAD Succinnate

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