KEGG   PATHWAY: ko00020
ko00020                     Pathway                                

Citrate cycle (TCA cycle)
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].
Metabolism; Carbohydrate metabolism
Pathway map
ko00020  Citrate cycle (TCA cycle)

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]
Other DBs
GO: 0006099
K01647  citrate synthase [EC:]
K01648  ATP citrate (pro-S)-lyase [EC:]
K15230  ATP-citrate lyase alpha-subunit [EC:]
K15231  ATP-citrate lyase beta-subunit [EC:]
K05942  citrate (Re)-synthase [EC:]
K01681  aconitate hydratase [EC:]
K01682  aconitate hydratase 2 / 2-methylisocitrate dehydratase [EC:]
K00031  isocitrate dehydrogenase [EC:]
K00030  isocitrate dehydrogenase (NAD+) [EC:]
K17753  isocitrate--homoisocitrate dehydrogenase [EC:]
K00164  2-oxoglutarate dehydrogenase E1 component [EC:]
K00658  2-oxoglutarate dehydrogenase E2 component (dihydrolipoamide succinyltransferase) [EC:]
K01616  multifunctional 2-oxoglutarate metabolism enzyme [EC:]
K00382  dihydrolipoamide dehydrogenase [EC:]
K00174  2-oxoglutarate/2-oxoacid ferredoxin oxidoreductase subunit alpha [EC:]
K00175  2-oxoglutarate/2-oxoacid ferredoxin oxidoreductase subunit beta [EC:]
K00177  2-oxoglutarate ferredoxin oxidoreductase subunit gamma [EC:]
K00176  2-oxoglutarate ferredoxin oxidoreductase subunit delta [EC:]
K01902  succinyl-CoA synthetase alpha subunit [EC:]
K01903  succinyl-CoA synthetase beta subunit [EC:]
K01899  succinyl-CoA synthetase alpha subunit [EC:]
K01900  succinyl-CoA synthetase beta subunit [EC:]
K18118  succinyl-CoA:acetate CoA-transferase [EC:]
K00234  succinate dehydrogenase (ubiquinone) flavoprotein subunit [EC:]
K00235  succinate dehydrogenase (ubiquinone) iron-sulfur subunit [EC:]
K00236  succinate dehydrogenase (ubiquinone) cytochrome b560 subunit
K00237  succinate dehydrogenase (ubiquinone) membrane anchor subunit
K00239  succinate dehydrogenase / fumarate reductase, flavoprotein subunit [EC:]
K00240  succinate dehydrogenase / fumarate reductase, iron-sulfur subunit [EC:]
K00241  succinate dehydrogenase / fumarate reductase, cytochrome b subunit
K00242  succinate dehydrogenase / fumarate reductase, membrane anchor subunit
K18859  succinate dehydrogenase / fumarate reductase, subunit D
K18860  putative succinate dehydrogenase / fumarate reductase, subunit D
K00244  fumarate reductase flavoprotein subunit [EC:]
K00245  fumarate reductase iron-sulfur subunit [EC:]
K00246  fumarate reductase subunit C
K00247  fumarate reductase subunit D
K01676  fumarate hydratase, class I [EC:]
K01677  fumarate hydratase subunit alpha [EC:]
K01678  fumarate hydratase subunit beta [EC:]
K01679  fumarate hydratase, class II [EC:]
K01675  fumarate hydratase D [EC:]
K01774  fumarate hydratase E [EC:]
K00025  malate dehydrogenase [EC:]
K00026  malate dehydrogenase [EC:]
K00024  malate dehydrogenase [EC:]
K00116  malate dehydrogenase (quinone) [EC:]
K01958  pyruvate carboxylase [EC:]
K01959  pyruvate carboxylase subunit A [EC:]
K01960  pyruvate carboxylase subunit B [EC:]
K01610  phosphoenolpyruvate carboxykinase (ATP) [EC:]
K01596  phosphoenolpyruvate carboxykinase (GTP) [EC:]
K00163  pyruvate dehydrogenase E1 component [EC:]
K00161  pyruvate dehydrogenase E1 component alpha subunit [EC:]
K00162  pyruvate dehydrogenase E1 component beta subunit [EC:]
K00627  pyruvate dehydrogenase E2 component (dihydrolipoamide acetyltransferase) [EC:]
K00169  pyruvate ferredoxin oxidoreductase alpha subunit [EC:]
K00170  pyruvate ferredoxin oxidoreductase beta subunit [EC:]
K00172  pyruvate ferredoxin oxidoreductase gamma subunit [EC:]
K00189  2-oxoisovalerate/pyruvate ferredoxin oxidoreductase gamma subunit [EC:]
K00171  pyruvate ferredoxin oxidoreductase delta subunit [EC:]
K03737  pyruvate-ferredoxin/flavodoxin oxidoreductase [EC: 1.2.7.-]
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
(map 2)
Nishizuka Y (ed).
[Metabolic Maps] (In Japanese)
Tokyo Kagaku Dojin (1980)
(map 3)
Nishizuka Y, Seyama Y, Ikai A, Ishimura Y, Kawaguchi A (eds).
[Cellular Functions and Metabolic Maps] (In Japanese)
Tokyo Kagaku Dojin (1997)
Michal G.
Biochemical Pathways
Wiley (1999)
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

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