Biology Times (^19)
March
Points in a nutshell:
m Occurs in mitochondrial matrix.
m Called as Kreb’s cycle because it was discovered by Sir Hans Krebs.
m Also called as citric acid cycle because first stable product of this cycle is citric acid.
m Also called as TCA cycle (Tricarboxylic acid cycle)because the first product citric acid has 3 carboxylic
groups (Tricarboxylic acid).
Acetyl CoA acts as a connecting link between glycolysis and kreb’s cycle.
Steps involved in Kreb’s cycle:
- Condensation: Acetyl CoA reacts with oxaloacetic acid (4C) (OAA) in the presence of citrate synthase to
form a tricarboxylic 6-carbon compound called citric acid, this is the first product of kreb’s cycle. - Dehydration: Citric acid undergoes re-organisation, in the presence of aconitase, forming Cis-aconitate
and water. - Hydration: Cis-aconitate undergoes hydration and forms iso-citrate in the presence of aconitase.
- Dehydrogenation(oxidation): Isocitrate is de-hydrogenated to oxalosuccinate, in the presence of Iso-
citrate dehydrogenase and Mn2+, NADH^2 are produced. - De-carboxylation: Oxalosuccinate is decarboxylated into a-ketoglutarate through enzyme decarboxylase
and Mn2+, CO^2 are liberated. - Dehydrogenation and decarboxylation: a-Ketoglutarate is both dehyrogenated and decarboxylated in the
presence of a-ketoglutarate dehydrogenase enzyme complex which contains TPP, Lipoic acid and Mg2+
Succinyl CoA is produced. - Substrate Level phosphorylation: In the presence of succinyl CoA synthase or succinic acid thio-Kinase,
succinyl CoA is split into succinic acid and CoA. In this step, high energy bond of succinyl CoA is
transferred into GDP, in the presence of ip and form GTP. This is the only step in kreb’s cycle, the high
energy phosphate bond is generated at the substrate level.
By action of phosphokinase, ATP may be formed from GTP.
- Dehydrogenation: Succinic acid is dehydrogenated in presence of succinic dehydrogenase into Fumarate.
one FADH 2 is produced.