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Citric Acid Cycle

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Updated 2009-03-04 08:54

Citric Acid Cycle

The Citric Acid Cycle is also known as the Tricarboxylic Acid Cycle (TCA cycle),
the Krebs cycle, or, rarely, the Szent-Györgyi–Krebs cycle.




Citric Acid/Citrate is not Vitamin C. Vitamin C is Ascorbic Acid/Ascorbate.

 

Step/Reaction
Enzyme (Prosthetic Group)
(1) Acetyl CoA + Oxaloacetate + H2O ->
Citrate + CoA + H+
Citrate
Synthetase
(2) Citrate <=> cis-Aconitate + H2OAconitase (Fe-S)
(3) cis-Aconitate + H2O <=> IsocitrateAconitase (Fe-S)
(4) Isocitrate + NAD+ <=>
α-Ketoglutarate + CO2 + NADH
Isocitrate
Dehydrogenase
(Lipoic Acid, FAD)
(5) α-Ketoglutarate + NAD+ + CoA <=>
Succinyl CoA + CO2 + NADH
α-Ketoglutarate
Dehydrogenase
Complex (TPP)
(6) Succinyl CoA + Pi + GDP <=>
Succinate + GTP + CoA
Succinyl CoA
Synthetase
(7) Succinate +FAD (enzyme-bound) <=>
Fumarate + FADH2 (enzyme-bound)
Succinate
Dehydrogenase
(FAD, Fe-S)
(8) Fumarate + H2O <=> L-MalateFumarase
(9) L-Malate + NAD+ <=>
Oxaloacetate + NADH + H+
Malate
Dehydrogenase
memorize

Pyruvate Dehydrogenase Complex of E. Coli

Reaction
Enzyme (Prosthetic Group)
Oxidative Decarboxylation of PyruvatePyruvate Dehydrogenase Component (TPP)
Transfer of the Acetyl Group to CoADihydrolipoyl Transacetylase (Lipoamide)
Regeneration of the Oxidized Form of LipoamideDihydrolipoyl Dehydrogenase (FAD)
memorize

References:

pp.378-9 of Lubert Stryer's "Biochemistry" 3rd Edition, 1988, W. H. Freeman, New York (ISBN=0-7167-1843-X).
http://en.wikipedia.org/wiki/Citric_acid_cycle.
http://en.wikipedia.org/wiki/Citric_acid.
http://en.wikipedia.org/wiki/Vitamin_C.