Tricarboxylic Acid (TCA) Cycle
Citric Acid Cycle
Krebs Cycle
• Pathway by which acetyl coenzyme A (acetyl
CoA; AcCoA) is oxidised aerobically
• The products of oxidation (of acetyl component) are
CO2, H2O and ATP
• Acetyl CoA entering the cycle can come from
several sources:
from glucose metabolism via pyruvate
from fatty acid oxidation
• The TCA cycle is the major energy generating
pathway in cells
• It generates 12 ATP for every 1 acetyl CoA
entering the cycle
• ATP is produced by both substrate level
phosphorylation and oxidative phosphorylation
• In eukaryotic cells it takes place in
mitochondria
• It occupies a central place in cell metabolism
with other pathways leading from it or feeding
into it
Glycolysis
TCA Cycle
FA oxidation
TCA cycle: Reaction 1
• Acetyl CoA joins with oxaloacetate (OA) to
form citrate
• The reaction is catalysed by citrate synthase
• Citryl CoA is an unstable intermediate
• Citric acid contains 3 -COOH groups - a
tricarboxylic acid
• At the pH within cells each of the –COOH
groups on citric acid, and all subsequent
compounds, ionises by releasing H+ and forming
–COO- to give the anion citrate
• Oxaloacetate is regenerated 7 reactions later
Citrate synthase
O
CO OH
CH 2 C
C O
S CoA
HO C CO OH
CH 2
O
CO OH
CH 3 C
Oxaloacetate
Citrate
contains 2 C
atoms from
acetyl CoA
and 4 from
oxaloacetate
CH 2
S CoA
Acetyl CoA
CO OH
Citryl CoA
H 2O
CO O H
CH 2
H O C COO
CH 2
CO O H
Citrate
CoA SH
CoA
TCA cycle: Reaction 2
• The reaction is a rearrangement reaction
• Citrate is converted to iso-citrate by
successive dehydration/hydration reactions
• The overall reaction is catalysed by aconitase
• Aconitase shows stereospecificity
• Fe2+ acts as a cofactor
COOH
CH2
HO C COOH
CH2
COOH
Citrate
Aconitase
Hydration
HOH
HOH
Dehydration
COOH
CH2
C COOH
CH
COOH
HOH
Hydration
Dehydration
HOH
COOH
CH2
H C COOH
H C OH
COOH
Isocitrate
TCA cycle: Reaction 3
• An oxidative decarboxylation reaction to
convert isocitrate to 2-oxo-glutarate (a-ketoglutarate)
• 2 H are transferred to NAD+ and then CO2 is
released
• The NADH+H+ feeds into the etc to generate
3 ATP by oxidative phosphorylation
• The reaction is catalysed by isocitrate
dehydrogenase
• Oxalo-succinate is an unstable intermediate
• The C atom lost as CO2 was derived from
oxaloacetate not acetyl CoA
COOH
CH2 Isocitrate
H C COOH
H C OH
COOH
NAD+
Isocitrate dehydrogenase
COOH
CH2
H C COOH
C
O
+
NADH+ H
COOH
Oxalo-succinate
CO2
Mn2+ acts as cofactor
COOH
CH2
H C H
C O
COOH
2-oxo-glutarate
TCA cycle: Reaction 4
• An oxidative decarboxylation reaction which
converts 2-oxo-glutarate to succinyl CoA
• The reaction is catalysed by 2-oxo-glutarate
dehydrogenase
• The reaction is very similar to that catalysed
by pyruvate dehydrogenase
• The same 6 coenzymes/cofactors are required:
TPP, Lipoic acid, CoA, FAD, NAD+ and Mg2+
• The NADH+H+ feeds into the etc to generate
3 ATP by oxidative phosphorylation
• The C atom lost as CO2 was derived from
oxaloacetate not acetyl CoA
2-oxo-glutarate dehydrogenase
COOH
CH2
H C H
C O
COOH
2-oxo-glutarate
NADH+H+
+
NAD
TPP, Mg2+
Lipoic acid
FAD
CoA
SH
CO2
COOH
CH2
H C H
C O
S CoA
Succinyl CoA
The only reaction within the cycle that is
irreversible
TCA cycle: Reaction 5
• A hydrolysis reaction linked to the
phosphorylation of GTP
• GTP formed by substrate level phosphorylation
- the energy released in the hydrolysis reaction
being used to drive the phosphorylation of GDP
to GTP
• Exchange of phosphoryl groups between GTP
and ADP is energy neutral. 1 ATP produced in
the reaction
• The reaction is catalysed by succinyl thiokinase
• Mg2+ is a cofactor for the reaction
COOH
CH2
H C H
C O
S CoA
Succinyl CoA
P i + GDP
GTP
Mg2+
H2O
CoA SH
COOH
CH2
H C H
C O
OHSuccinate
Succinyl thiokinase
Succinate is a truly symmetrical
molecule so the 2 C atoms derived
from acetyl CoA can no longer be
distinguished from the C atoms
from oxaloacetate
COOH
CH2
CH2
COOH
TCA cycle: Reactions 6,7 and 8
• An oxidation reaction catalysed by succinate
dehydrogenase to convert succinate to
fumarate
• The hydrogen carrier is FAD (Complex II of
etc) which is reduced to FADH2
• Oxidation of FADH2 via the etc generates 2
ATP by oxidative phosphorylation
• A hydration reaction catalysed by fumarase to
convert fumarate to malate
• An oxidation reaction catalysed by malate
dehydrogenase to convert malate to
oxaloacetate
• NAD+ acts as the hydrogen carrier
• 3 ATP are generated by oxidative
phosphorylation when NADH+H+ is oxidised
via the etc
Succinate dehydrogenase
COOH
COOH
FAD
CH2
CH
Fumarate
Succinate
CH2
CH
FADH2
COOH
COOH
H2O
Oxaloacetate
Fumarase
COOH
NAD+ COOH
C O
CH OH
Malate
+
CH2
NADH+H
CH2
COOH
COOH
Malate dehydrogenase
Pyruvate
Acetyl CoA
TCA Cycle
CO2
CoA
Link reaction
Joining
Oxaloacetate
Citrate
Oxidation
Rearrangement
Malate
Isocitrate
Hydration
Oxidation/decarboxylation
Fumarate
2-oxo-glutarate
Oxidation/decarboxylation
Oxidation
Succinate
Succinyl CoA
Hydrolysis/
phosphorylation
Energy production during TCA Cycle
Reaction
Isocitrate to 2-oxoglutarate
2-oxo-glutarate to
succinyl CoA
Succinyl CoA to succinate
Succinate to fumarate
Malate to oxaloacetate
Total per Acetyl CoA
Reduced
Coenzyme
NADH
ATP
produced
3
Mechanism
NADH
3
etc
FADH2
NADH
1
2
3
12
slp
etc
etc
etc
TCA Cycle
Pyruvate
3 ATP
Acetyl CoA
CO2
(2C)
Citrate synthase
Oxaloacetate
(4C)
Malate
Fumarate
12 ATP
Succinate
Link reaction
Pyruvate
dehdrogenase
(6C)
Aconitase
Isocitrate
3 ATP
2 ATP
1 ATP
Fumarase
Succinate
dehydrogenase
Citrate
3 ATP
3 ATP
Malate
dehydrogenaase
(3C)
CoA
CO2
Isocitrate
dehydrogenase
2-oxo-glutarate
Succinyl CoA
Succinyl thiokinase
CO2
2-oxo-glutarate
dehydrogenase
Aerobic energy production from glucose
Glycolysis
• Substrate level phosphorylation
2 ATP
• Oxidative phosphorylation
6 ATP
Link reaction x 2
• Oxidative phosphorylation
6 ATP
Oxidation of acetyl CoA (TCA cycle x 2) 24 ATP
Total
38 ATP
(38 ATP is maximum yield. Yield may be reduced to 36 ATP depending on mechanism for
transfer of NADH+H+ from cytoplasm (glycolysis) into mitochondrion)