Citrate cycle (TCA cycle) - Homo sapiens (human)

Curator(s): Joe Strothman

The citrate cycle (TCA cycle, Krebs cycle) is an important aerobic pathway for the final steps of the oxidation of carbohydrates and fatty acids. The cycle starts with acetyl-CoA, the activated form of acetate, derived from glycolysis and pyruvate oxidation for carbohydrates and from beta oxidation of fatty acids. The two-carbon acetyl group in acetyl-CoA is transferred to the four-carbon compound of oxaloacetate to form the six-carbon compound of citrate. In a series of reactions two carbons in citrate are oxidized to CO2 and the reaction pathway supplies NADH for use in the oxidative phosphorylation and other metabolic processes. The pathway also supplies important precursor metabolites including 2-oxoglutarate. At the end of the cycle the remaining four-carbon part is transformed back to oxaloacetate. According to the genome sequence data, many organisms seem to lack genes for the full cycle, but contain genes for specific segments.    
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Citrate cycle (TCA cycle) - Homo sapiens (human) Fatty acid biosynthesis Fatty acid metabolism node32 Acetyl-CoA Fatty acid metabolism->node32 Fatty acid elongation in mitochondria node1 Valine, leucine, and isoleucine degradation node1->node32 node38 Succinyl-CoA node1->node38 node2 Alanine, aspartate and glutamate metabolism Oxaloacetate node2->Oxaloacetate node37 2-Oxoglutarate node2->node37 Glyoxylate and dicarboxylate metabolism Glyoxylate and dicarboxylate metabolism->Oxaloacetate node3 node3->Oxaloacetate node35 (S)-Malate node3->node35 node4 node34 Phosphoenol-pyruvate node4->node34 node5 node5->node34 node6 node6->node32 node6->Oxaloacetate Citrate node6->Citrate node7 node7->node32 node7->Oxaloacetate node7->Citrate node8 node8->Oxaloacetate Tyrosine metabolism Fumarate Tyrosine metabolism->Fumarate node9 node9->node35 node9->Fumarate Arginine and proline metabolism Arginine and proline metabolism->Fumarate node10 node10->Fumarate Succinate node10->Succinate node11 node11->Fumarate node11->Succinate node12 node12->Succinate node12->node38 node13 node13->Succinate node13->node38 node14 node24 Dihydrolipoamide-E node14->node24 node14->node38 node15 Glycolysis / Gluconeogenesis Pyruvate node15->Pyruvate node16 node16->node32 node16->Pyruvate node17 node17->Citrate node36 cis-Aconitate node17->node36 Isocitrate node17->Isocitrate node18 1 node19 node19->node18 node22 2-Hydroxyethyl -ThPP node19->node22 node23 S-Acetyldihydrolipoamide node19->node23 ThPP node19->ThPP node20 node20->node23 node20->node24 node20->node32 node21 node21->node24 node25 Lipoamide-E node21->node25 node22->node19 node25->node19 node29 node25->node29 node26 node26->Oxaloacetate node27 node27->Isocitrate node27->node37 Oxalosuccinate node27->Oxalosuccinate node28 node28->Isocitrate node28->node37 node29->ThPP node39 3-Carboxy -1-hydroxypropyl -ThPP node29->node39 node30 node30->node37 node30->node38 Ascorbate and aldarate metabolism Ascorbate and aldarate metabolism->node37 node31 D-Gln and D -Glu Metabolism node31->node37 ThPP->node19 ThPP->node29 node32->Fatty acid biosynthesis node32->Fatty acid elongation in mitochondria node33 2 Pyruvate->node19 Pyruvate->node26 Pyruvate->node33 Pyruvate node34->node15 Oxaloacetate->node2 Oxaloacetate->Glyoxylate and dicarboxylate metabolism Oxaloacetate->node4 Oxaloacetate->node5 Citrate->node8 node37->node2 node37->node29 node37->Ascorbate and aldarate metabolism node37->node31 node39->node29 print "