KEGG   PATHWAY: rn00020
rn00020                     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
rn00020  Citrate cycle (TCA cycle)

M00003  Gluconeogenesis, oxaloacetate => fructose-6P [PATH:rn00020]
M00009  Citrate cycle (TCA cycle, Krebs cycle) [PATH:rn00020]
M00010  Citrate cycle, first carbon oxidation, oxaloacetate => 2-oxoglutarate [PATH:rn00020]
M00011  Citrate cycle, second carbon oxidation, 2-oxoglutarate => oxaloacetate [PATH:rn00020]
M00307  Pyruvate oxidation, pyruvate => acetyl-CoA [PATH:rn00020]
M00740  Methylaspartate cycle [PATH:rn00020]
Other DBs
GO: 0006099
R00014  pyruvate:thiamin diphosphate acetaldehydetransferase (decarboxylating)
R00268  oxalosuccinate carboxy-lyase (2-oxoglutarate-forming)
R00341  ATP:oxaloacetate carboxy-lyase (transphosphorylating;phosphoenolpyruvate-forming)
R00342  (S)-malate:NAD+ oxidoreductase
R00344  Pyruvate:carbon-dioxide ligase (ADP-forming)
R00351  acetyl-CoA:oxaloacetate C-acetyltransferase (thioester-hydrolysing)
R00352  acetyl-CoA:oxaloacetate C-acetyltransferase [(pro-S)-carboxymethyl-forming, ADP-phosphorylating]
R00361  (S)-malate:quinone oxidoreductase
R00405  Succinate:CoA ligase (ADP-forming)
R00431  GTP:oxaloacetate carboxy-lyase (adding GTP;phosphoenolpyruvate-forming)
R00432  Succinate:CoA ligase (GDP-forming)
R00709  isocitrate:NAD+ oxidoreductase (decarboxylating)
R00726  ITP:oxaloacetate carboxy-lyase (adding ITP; phosphoenolpyruvate-forming)
R00727  Succinate:CoA ligase (IDP-forming)
R01082  (S)-malate hydro-lyase (fumarate-forming)
R01196  pyruvate:ferredoxin 2-oxidoreductase (CoA-acetylating)
R01197  2-oxoglutarate:ferredoxin oxidoreductase (decarboxylating)
R01325  citrate hydro-lyase (cis-aconitate-forming)
R01899  Isocitrate:NADP+ oxidoreductase
R01900  isocitrate hydro-lyase (cis-aconitate-forming)
R02164  succinate:quinone oxidoreductase
R02569  acetyl-CoA:enzyme N6-(dihydrolipoyl)lysine S-acetyltransferase
R02570  succinyl-CoA:enzyme N6-(dihydrolipoyl)lysine S-succinyltransferase
R07618  enzyme N6-(dihydrolipoyl)lysine:NAD+ oxidoreductase
R10343  succinyl-CoA:acetate CoA-transferase
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
Nishizuka Y (ed).
[Metabolic Maps] (In Japanese)
Tokyo Kagaku Dojin (1980)
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)
rn00010  Glycolysis / Gluconeogenesis
rn00053  Ascorbate and aldarate metabolism
rn00061  Fatty acid biosynthesis
rn00062  Fatty acid elongation
rn00071  Fatty acid degradation
rn00220  Arginine biosynthesis
rn00250  Alanine, aspartate and glutamate metabolism
rn00280  Valine, leucine and isoleucine degradation
rn00350  Tyrosine metabolism
rn00470  D-Amino acid metabolism
rn00630  Glyoxylate and dicarboxylate metabolism
KO pathway

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