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Entry 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 metabolismBRITE hierarchy
Pathway map Module M00003 Gluconeogenesis, oxaloacetate => fructose-6P [PATH:ko00020 ]
M00010 Citrate cycle, first carbon oxidation, oxaloacetate => 2-oxoglutarate [PATH:ko00020 ]
M00011 Citrate cycle, second carbon oxidation, 2-oxoglutarate => oxaloacetate [PATH:ko00020 ]
Other DBs Orthology 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 ]
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
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 ]
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 ]
K01774 fumE; fumarate hydratase E
K03737 por, nifJ; pyruvate-ferredoxin/flavodoxin oxidoreductase [EC:1.2.7.1 1.2.7.-]
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 C05125 2-(alpha-Hydroxyethyl)thiamine diphosphate
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)
Related ko00010 Glycolysis / Gluconeogenesis
ko00053 Ascorbate and aldarate metabolism
ko00250 Alanine, aspartate and glutamate metabolism
ko00280 Valine, leucine and isoleucine degradation
ko00630 Glyoxylate and dicarboxylate metabolism
