KEGG   PATHWAY: ko00020Help
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
BRITE hierarchy
Pathway map
Citrate cycle (TCA cycle)
ko00020

All organismsOrtholog table
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]
Disease
H00073  
Pyruvate carboxylase deficiency
H00469  
Mitochondrial DNA depletion syndrome (MDS)
H00804  
Multiple cutaneous and uterine leiomyomata
H01022  
Diseases of the tricarboxylic acid cycle
H01222  
Cowden-like syndrome
H01225  
D-2-hydroxyglutaric aciduria (D-2-HGA)
Other DBs
BSID: 
GO: 
Orthology
K01647  
citrate synthase [EC:2.3.3.1]
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]
K01682  
aconitate hydratase 2 / 2-methylisocitrate dehydratase [EC:4.2.1.99 4.2.1.3]
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]
K00382  
dihydrolipoamide dehydrogenase [EC:1.8.1.4]
K00174  
2-oxoglutarate ferredoxin oxidoreductase subunit alpha [EC:1.2.7.3]
K00175  
2-oxoglutarate ferredoxin oxidoreductase subunit beta [EC:1.2.7.3]
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:6.2.1.5 6.2.1.4]
K01900  
succinyl-CoA synthetase beta subunit [EC:6.2.1.5 6.2.1.4]
K18118  
succinyl-CoA:acetate CoA-transferase [EC:2.8.3.18]
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 b556 subunit
K00242  
succinate dehydrogenase membrane anchor subunit
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
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]
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 alpha subunit [EC:1.2.4.1]
K00162  
pyruvate dehydrogenase E1 component beta subunit [EC:1.2.4.1]
K00627  
pyruvate dehydrogenase E2 component (dihydrolipoamide 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]
K00171  
pyruvate ferredoxin oxidoreductase, delta subunit [EC:1.2.7.1]
K00172  
pyruvate ferredoxin oxidoreductase, gamma subunit [EC:1.2.7.1]
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
(map 2)
  Authors
Nishizuka Y (ed).
  Title
[Metabolic Maps] (In Japanese)
  Journal
Tokyo Kagaku Dojin (1980)
Reference
(map 3)
  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)

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