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] M00982 Methylcitrate cycle [PATH:ko00020] DBLINKS GO: 0006099 ORTHOLOGY K00024 mdh; malate dehydrogenase [EC:1.1.1.37] K00025 MDH1; malate dehydrogenase [EC:1.1.1.37] K00026 MDH2; malate dehydrogenase [EC:1.1.1.37] K00030 IDH3; isocitrate dehydrogenase (NAD+) [EC:1.1.1.41] K00031 IDH1, IDH2, icd; isocitrate dehydrogenase [EC:1.1.1.42] K00116 mqo; malate dehydrogenase (quinone) [EC:1.1.5.4] K00161 PDHA; pyruvate dehydrogenase E1 component subunit alpha [EC:1.2.4.1] K00162 PDHB; pyruvate dehydrogenase E1 component subunit beta [EC:1.2.4.1] K00163 aceE; pyruvate dehydrogenase E1 component [EC:1.2.4.1] K00164 OGDH, sucA; 2-oxoglutarate dehydrogenase E1 component [EC:1.2.4.2] K00169 porA; pyruvate ferredoxin oxidoreductase alpha subunit [EC:1.2.7.1] K00170 porB; pyruvate ferredoxin oxidoreductase beta subunit [EC:1.2.7.1] K00171 porD; pyruvate ferredoxin oxidoreductase delta subunit [EC:1.2.7.1] K00172 porC, porG; pyruvate ferredoxin oxidoreductase gamma subunit [EC:1.2.7.1] K00174 korA, oorA, oforA; 2-oxoglutarate/2-oxoacid ferredoxin oxidoreductase subunit alpha [EC:1.2.7.3 1.2.7.11] K00175 korB, oorB, oforB; 2-oxoglutarate/2-oxoacid ferredoxin oxidoreductase subunit beta [EC:1.2.7.3 1.2.7.11] K00176 korD, oorD; 2-oxoglutarate ferredoxin oxidoreductase subunit delta [EC:1.2.7.3] K00177 korC, oorC; 2-oxoglutarate ferredoxin oxidoreductase subunit gamma [EC:1.2.7.3] K00189 vorG, porG; 2-oxoisovalerate/pyruvate ferredoxin oxidoreductase gamma subunit [EC:1.2.7.7 1.2.7.1] K00233 frdA, fdrA; succinate dehydrogenase flavoprotein subunit K00234 SDHA, SDH1; succinate dehydrogenase (ubiquinone) flavoprotein subunit [EC:1.3.5.1] K00235 SDHB, SDH2; succinate dehydrogenase (ubiquinone) iron-sulfur subunit [EC:1.3.5.1] K00236 SDHC, SDH3; succinate dehydrogenase (ubiquinone) cytochrome b560 subunit K00237 SDHD, SDH4; succinate dehydrogenase (ubiquinone) membrane anchor subunit K00239 sdhA, frdA; succinate dehydrogenase flavoprotein subunit [EC:1.3.5.1] K00240 sdhB, frdB; succinate dehydrogenase iron-sulfur subunit [EC:1.3.5.1] K00241 sdhC, frdC; succinate dehydrogenase cytochrome b subunit K00242 sdhD, frdD; succinate dehydrogenase membrane anchor subunit K00244 frdA; succinate dehydrogenase flavoprotein subunit [EC:1.3.5.1] K00245 frdB; succinate dehydrogenase iron-sulfur subunit [EC:1.3.5.1] K00246 frdC; succinate dehydrogenase subunit C K00247 frdD; succinate dehydrogenase subunit D K00382 DLD, lpd, pdhD; dihydrolipoyl dehydrogenase [EC:1.8.1.4] K00627 DLAT, aceF, pdhC; pyruvate dehydrogenase E2 component (dihydrolipoyllysine-residue acetyltransferase) [EC:2.3.1.12] K00658 DLST, sucB; 2-oxoglutarate dehydrogenase E2 component (dihydrolipoamide succinyltransferase) [EC:2.3.1.61] K01596 E4.1.1.32, pckA, PCK; phosphoenolpyruvate carboxykinase (GTP) [EC:4.1.1.32] K01610 pckA; phosphoenolpyruvate carboxykinase (ATP) [EC:4.1.1.49] K01616 kgd; multifunctional 2-oxoglutarate metabolism enzyme [EC:2.2.1.5 4.1.1.71 1.2.4.2 2.3.1.61] K01647 CS, gltA; citrate synthase [EC:2.3.3.1] K01648 ACLY; ATP citrate (pro-S)-lyase [EC:2.3.3.8] K01659 prpC; 2-methylcitrate synthase [EC:2.3.3.5] K01675 fumD; fumarate hydratase D K01676 E4.2.1.2A, fumA, fumB; fumarate hydratase, class I [EC:4.2.1.2] K01677 E4.2.1.2AA, fumA; fumarate hydratase subunit alpha [EC:4.2.1.2] K01678 E4.2.1.2AB, fumB; fumarate hydratase subunit beta [EC:4.2.1.2] K01679 E4.2.1.2B, fumC, FH; fumarate hydratase, class II [EC:4.2.1.2] K01681 ACO; aconitate hydratase [EC:4.2.1.3] K01682 acnB; aconitate hydratase 2 / 2-methylisocitrate dehydratase [EC:4.2.1.3 4.2.1.99] K01774 fumE; fumarate hydratase E K01899 LSC1; succinyl-CoA synthetase alpha subunit [EC:6.2.1.4 6.2.1.5] K01900 LSC2; succinyl-CoA synthetase beta subunit [EC:6.2.1.4 6.2.1.5] K01902 sucD; succinyl-CoA synthetase alpha subunit [EC:6.2.1.5] K01903 sucC; succinyl-CoA synthetase beta subunit [EC:6.2.1.5] K01958 PC, pyc; pyruvate carboxylase [EC:6.4.1.1] K01959 pycA; pyruvate carboxylase subunit A [EC:6.4.1.1] K01960 pycB; pyruvate carboxylase subunit B [EC:6.4.1.1] K03737 por, nifJ; pyruvate-ferredoxin/flavodoxin oxidoreductase [EC:1.2.7.1 1.2.7.-] K05942 E2.3.3.3; citrate (Re)-synthase [EC:2.3.3.3] K15230 aclA; ATP-citrate lyase alpha-subunit [EC:2.3.3.8] K15231 aclB; ATP-citrate lyase beta-subunit [EC:2.3.3.8] K17753 HICDH; isocitrate--homoisocitrate dehydrogenase [EC:1.1.1.286] K18118 aarC, cat1; succinyl-CoA:acetate CoA-transferase [EC:2.8.3.18] K18859 sdhD, frdD; succinate dehydrogenase subunit D K18860 sdhD, frdD; succinate dehydrogenase subunit D K25801 SDH4; succinate dehydrogenase (ubiquinone) subunit 4 K25995 frdB, fdrB; succinate dehydrogenase iron-sulfur subunit K25996 frdC, fdrC; succinate dehydrogenase subunit C K26318 fccA, fcc3; fumarate reductase (cytochrome) K27797 MCSA; 2-methylcitrate synthase K27802 acnA; aconitate hydratase A / 2-methylisocitrate dehydratase 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 ///