ENTRY ko00020 Pathway NAME Citrate cycle (TCA cycle) DESCRIPTION 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 [MD:M00009], but contain genes for specific segments [MD:M00010 M00011]. CLASS Metabolism; Carbohydrate metabolism PATHWAY_MAP ko00020 Citrate cycle (TCA cycle) MODULE M00003 Gluconeogenesis, oxaloacetate => fructose-6P [PATH:ko00020] M00009 Citrate cycle (TCA cycle, Krebs cycle) [PATH:ko00020] M00010 Citrate cycle, first carbon oxidation, oxaloacetate => 2-oxoglutarate [PATH:ko00020] M00011 Citrate cycle, second carbon oxidation, 2-oxoglutarate => oxaloacetate [PATH:ko00020] M00307 Pyruvate oxidation, pyruvate => acetyl-CoA [PATH:ko00020] M00740 Methylaspartate cycle [PATH:ko00020] DBLINKS GO: 0006099 ORTHOLOGY K01647 citrate synthase [EC:2.3.3.1] K05942 citrate (Re)-synthase [EC:2.3.3.3] K01659 2-methylcitrate synthase [EC:2.3.3.5] K27797 2-methylcitrate synthase [EC:2.3.3.5] K01648 ATP citrate (pro-S)-lyase [EC:2.3.3.8] K15230 ATP-citrate lyase alpha-subunit [EC:2.3.3.8] K15231 ATP-citrate lyase beta-subunit [EC:2.3.3.8] K01681 aconitate hydratase [EC:4.2.1.3] K27802 aconitate hydratase A / 2-methylisocitrate dehydratase [EC:4.2.1.3 4.2.1.99] K01682 aconitate hydratase 2 / 2-methylisocitrate dehydratase [EC:4.2.1.3 4.2.1.99] K00031 isocitrate dehydrogenase [EC:1.1.1.42] K00030 isocitrate dehydrogenase (NAD+) [EC:1.1.1.41] K17753 isocitrate--homoisocitrate dehydrogenase [EC:1.1.1.286] K00164 2-oxoglutarate dehydrogenase E1 component [EC:1.2.4.2] K00658 2-oxoglutarate dehydrogenase E2 component (dihydrolipoamide succinyltransferase) [EC:2.3.1.61] K01616 multifunctional 2-oxoglutarate metabolism enzyme [EC:2.2.1.5 4.1.1.71 1.2.4.2 2.3.1.61] K00382 dihydrolipoyl dehydrogenase [EC:1.8.1.4] K00174 2-oxoglutarate/2-oxoacid ferredoxin oxidoreductase subunit alpha [EC:1.2.7.3 1.2.7.11] K00175 2-oxoglutarate/2-oxoacid ferredoxin oxidoreductase subunit beta [EC:1.2.7.3 1.2.7.11] K00177 2-oxoglutarate ferredoxin oxidoreductase subunit gamma [EC:1.2.7.3] K00176 2-oxoglutarate ferredoxin oxidoreductase subunit delta [EC:1.2.7.3] K01902 succinyl-CoA synthetase alpha subunit [EC:6.2.1.5] K01903 succinyl-CoA synthetase beta subunit [EC:6.2.1.5] K01899 succinyl-CoA synthetase alpha subunit [EC:6.2.1.4 6.2.1.5] K01900 succinyl-CoA synthetase beta subunit [EC:6.2.1.4 6.2.1.5] K18118 succinyl-CoA:acetate CoA-transferase [EC:2.8.3.18] K00234 succinate dehydrogenase (ubiquinone) flavoprotein subunit [EC:1.3.5.1] K00235 succinate dehydrogenase (ubiquinone) iron-sulfur subunit [EC:1.3.5.1] K00236 succinate dehydrogenase (ubiquinone) cytochrome b560 subunit K00237 succinate dehydrogenase (ubiquinone) membrane anchor subunit K25801 succinate dehydrogenase (ubiquinone) subunit 4 K00239 succinate dehydrogenase flavoprotein subunit [EC:1.3.5.1] K00240 succinate dehydrogenase iron-sulfur subunit [EC:1.3.5.1] K00241 succinate dehydrogenase cytochrome b subunit K00242 succinate dehydrogenase membrane anchor subunit K18859 succinate dehydrogenase subunit D K18860 succinate dehydrogenase subunit D K00244 succinate dehydrogenase flavoprotein subunit [EC:1.3.5.1] K00245 succinate dehydrogenase iron-sulfur subunit [EC:1.3.5.1] K00246 succinate dehydrogenase subunit C K00247 succinate dehydrogenase subunit D K00233 succinate dehydrogenase flavoprotein subunit [EC:1.3.5.1 7.1.1.12] K25995 succinate dehydrogenase iron-sulfur subunit [EC:1.3.5.1 7.1.1.12] K25996 succinate dehydrogenase subunit C K26318 fumarate reductase (cytochrome) [EC:1.3.2.4] K01676 fumarate hydratase, class I [EC:4.2.1.2] K01677 fumarate hydratase subunit alpha [EC:4.2.1.2] K01678 fumarate hydratase subunit beta [EC:4.2.1.2] K01679 fumarate hydratase, class II [EC:4.2.1.2] K01675 fumarate hydratase D [EC:4.2.1.2] K01774 fumarate hydratase E [EC:4.2.1.2] K00025 malate dehydrogenase [EC:1.1.1.37] K00026 malate dehydrogenase [EC:1.1.1.37] K00024 malate dehydrogenase [EC:1.1.1.37] K00116 malate dehydrogenase (quinone) [EC:1.1.5.4] K01958 pyruvate carboxylase [EC:6.4.1.1] K01959 pyruvate carboxylase subunit A [EC:6.4.1.1] K01960 pyruvate carboxylase subunit B [EC:6.4.1.1] K01610 phosphoenolpyruvate carboxykinase (ATP) [EC:4.1.1.49] K01596 phosphoenolpyruvate carboxykinase (GTP) [EC:4.1.1.32] K00163 pyruvate dehydrogenase E1 component [EC:1.2.4.1] K00161 pyruvate dehydrogenase E1 component subunit alpha [EC:1.2.4.1] K00162 pyruvate dehydrogenase E1 component subunit beta [EC:1.2.4.1] K00627 pyruvate dehydrogenase E2 component (dihydrolipoyllysine-residue acetyltransferase) [EC:2.3.1.12] K00169 pyruvate ferredoxin oxidoreductase alpha subunit [EC:1.2.7.1] K00170 pyruvate ferredoxin oxidoreductase beta subunit [EC:1.2.7.1] K00172 pyruvate ferredoxin oxidoreductase gamma subunit [EC:1.2.7.1] K00189 2-oxoisovalerate/pyruvate ferredoxin oxidoreductase gamma subunit [EC:1.2.7.7 1.2.7.1] K00171 pyruvate ferredoxin oxidoreductase delta subunit [EC:1.2.7.1] K03737 pyruvate-ferredoxin/flavodoxin oxidoreductase [EC:1.2.7.1 1.2.7.-] COMPOUND C00022 Pyruvate C00024 Acetyl-CoA C00026 2-Oxoglutarate C00036 Oxaloacetate C00042 Succinate C00068 Thiamin diphosphate C00074 Phosphoenolpyruvate C00091 Succinyl-CoA C00122 Fumarate C00149 (S)-Malate C00158 Citrate C00311 Isocitrate C00417 cis-Aconitate C05125 2-(alpha-Hydroxyethyl)thiamine diphosphate C05379 Oxalosuccinate C05381 3-Carboxy-1-hydroxypropyl-ThPP C15972 Enzyme N6-(lipoyl)lysine C15973 Enzyme N6-(dihydrolipoyl)lysine C16254 [Dihydrolipoyllysine-residue succinyltransferase] S-succinyldihydrolipoyllysine C16255 [Dihydrolipoyllysine-residue acetyltransferase] S-acetyldihydrolipoyllysine REFERENCE AUTHORS Nishizuka Y (ed). TITLE [Metabolic Maps] (In Japanese) JOURNAL Tokyo Kagaku Dojin (1980) REFERENCE AUTHORS Nishizuka Y, Seyama Y, Ikai A, Ishimura Y, Kawaguchi A (eds). TITLE [Cellular Functions and Metabolic Maps] (In Japanese) JOURNAL Tokyo Kagaku Dojin (1997) REFERENCE AUTHORS Michal G. TITLE Biochemical Pathways JOURNAL Wiley (1999) REL_PATHWAY ko00010 Glycolysis / Gluconeogenesis ko00053 Ascorbate and aldarate metabolism ko00061 Fatty acid biosynthesis ko00062 Fatty acid elongation ko00071 Fatty acid degradation ko00190 Oxidative phosphorylation ko00220 Arginine biosynthesis ko00250 Alanine, aspartate and glutamate metabolism ko00280 Valine, leucine and isoleucine degradation ko00350 Tyrosine metabolism ko00470 D-Amino acid metabolism ko00630 Glyoxylate and dicarboxylate metabolism ///