Download KREBS CYCLE Definition Krebs cycle (aka tricarboxylic acid cycle

KREBS CYCLE
Definition
Krebs cycle (aka tricarboxylic acid cycle / citric acid cycle) is the final pathway followed by all carbon
atoms derived from carbohydrates, lipids and proteins when they are oxidised during respiration. The
main function of the Krebs cycle is to oxidize acetyl-CoA through decarboxylation and
dehydrogenation.
Acetyl-CoA (2C)
H2O
CoASH
Citrate (6C)
Oxaloacetate (4C)
Citrate synthase
NAD+
NADH + H+
H2O
Cis-aconitate (6C)
L-Malate (4C0
H2O
H2O
Fumarase
Succinate dehydrogenase
Isocitrate dehydrogenase
NAD+
NADH + H+
Oxalosuccinate (6C)
Succinate (4C0
GDP + Pi
GTP
Aconitase
Isocitrate (6C)
Fumarate (4C)
FAD
FADH2
Aconitase
Malate dehydrogenase
CoASH
Isocitrate dehydrogenase
Succinyl-CoA synthetase
CoASH
CO2
CO2
α-Ketoglutarate (5C)
Succinyl-CoA (4C)
NAD+ NADH + H+
α-Ketoglutarate dehydrogenase
Explanation
MAIN STAGES OF
EXPLANATION
KREBS CYCLE
Condensation
1.
Acetyl-CoA and oxaloacetate undergo a condensation reaction to form
citrate. A coenzyme is released.
Isomerization
2. Before the oxidation reactions can begin, the hydroxyl group of citrate
must be repositioned. This is done by removing a water molecule from
one carbon and the water is added to different carbon. The product is
an isomer of citrate called isocitrate.
First oxidation
3. Isocitrate is oxidised to form oxalosuccinate. NAD+ is reduced to
NADH.
4. Oxalosuccinate is decarboxylated, loses a CO2 and is converted to αketoglutarate.
Second oxidation
5. Second oxidative-decarboxylation takes place. α-ketoglutarate is
converted to succinyl-CoA. CO2 and NADH are produced.
Substrate-level
6. Succinyl-CoA is converted to succinate. The energy released is used for
phosphorylation
phosphorylation of GDP forming GTP. GTP transfers its phosphate group
to ADP forming ATP.
Third oxidation
7. Succinate is oxidised to fumarate, two hydrogen atoms are transferred
to FAD to form FADH2,
Regeneration of
oxaloacetate
8. Fumarate becomes hydrated by addition of water is converted to Lmalate.
9. L-malate is oxidised regenerating oxaloacetate, and NAD + is reduced to
NADH. Oxaloacetate can be used to combine with acetyl-CoA and the
cycle is repeated.
Summary
1.
2 molecules of ATP are produced. (From GTP)
2. 6 molecules of reduced NAD are produced.
3. 2 molecules of reduced FAD is produced.
4. 4 molecules of CO2 are produced.
5. 2 oxaloacetate molecules are regenerated, which are used for the Krebs cycle.