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The Citric Acid Cycle
The Citric Acid Cycle
• Three processes play central roles in aerobic
metabolism
•the citric acid cycle (this chapter)
•electron transport (Chapter 17)
•oxidative phosphorylation (Chapter 17)
• Metabolism consists of
•catabolism:
catabolism: the oxidative breakdown of nutrients
•anabolism:
anabolism: the reductive synthesis of biomolecules
• The citric acid cycle is amphibolic;
amphibolic that is, it plays
a role in both catabolism and anabolism
The Citric Acid Cycle
Pyruvate to Acetyl-CoA
Pyruvate
• Oxidation by NAD+ and formation of a thioester
N AD +
N AD H
Acetyl -CoA Coenzyme A
O
CH3 CCOO - + CoA -SH + N AD +
Pyruvate
NAD H
N AD
+
Citric
acid
cycle
(8 steps)
FAD H2
FAD
GTP
GDP
N AD +
N ADH
CO 2
N AD
N AD H
CO 2
+
Pyruvate dehydrogenase complex
pyruvate
dehydrogenase
complex
Coenzyme A
O
CH3 C-SCo A + CO 2 + N AD H
Acetyl-CoA
•this conversion requires NAD+, FAD, Mg2+, thiamine
pyrophosphate, coenzyme A, and lipoic acid
-1
•G°
’=-33.
4kJ•
mol
Pyruvate dehydrogenase complex
Pyruvate dehydrogenase
Dihydrolipoyl transacetylase
Dihydrolipoyl dehydrogenase
Pyruvate dehydrogenase kinase
Pyruvate dehydrogenase phosphatase
1
Pyruvate to Acetyl-CoA
• Step 1: pyruvate loses CO2 and HETPP is formed
O
CH 3 CCOO - + T PP
Pyruvate
pyruvate
dehydrogenase
OH
CO 2 + CH 3 CH- TPP
Hydroxyethyl-TPP
Pyruvate to Acetyl-CoA
• Step 2:
•the hydroxyethyl group is oxidized and transferred to a
sulfur atom of the reduced form of lipoamide
•lipoamide is reduced to dihydrolipoamide
O
• Step 2 requires lipoic acid
•the active form of lipoic acid is bound to the enzyme by
an amide bond to the amino group of a lysine
OH
CH3 CH- TPP
Hydroxyethyl-TPP
C-N H- Enz
+
S S
Lipoamide
dihydrolipoyl transacylase
COOH
reduction
oxidation
S S
Lipoic acid
COOH
HS
SH
Dihydrolipoic acid
T PP
Pyruvate to Acetyl-CoA
• Step 3: the acetyl group is transferred to the
O
CH3 C S
+
• Step 4: oxidation of dihydrolipoamide
O
C-N H- Enz
C-N H- Enz
SH
HS
Dihydrolipoamide
N AD +
N AD H
dihydrolipoyl transacylase
O
Acetyl-CoA
Dihydrolipoamide
O
O
SH
CoA -SH + CH3 C-S
Coenzyme A
Dihydrolipoamide
O
C-N H- Enz
+ HS
SH
Pyruvate to Acetyl-CoA
sulfhydryl group of coenzyme A
O
CoA -S-CCH3
O
C-N H- Enz
SH
C-N H- Enz
S S
Dihydrolipoamide
Where TCA cycle locate
Lipoamide
The Citric Acid Cycle
• Step 1: condensation of acetyl-CoA with
-1
oxaloacetate; G°
’=-32.
8kJ•
mol
O
CH3 C-SCo A
citrate
Acetyl-CoA
synthase
+
O C-COO
CH2 -COO -
CH2 -COO HO C-COO CH2 -COO -
+
CoA -SH
Coenzyme A
Citrate
Oxaloacetate
•citrate synthase is an allosteric enzyme, inhibited by
NADH, ATP, and succinyl-CoA
2
The Citric Acid Cycle
The Citric Acid Cycle
• Step 2: dehydration and rehydration gives
• Step 3: oxidation of isocitrate followed by
isocitrate; catalyzed by aconitase
CH2 -COO HO C-COO
decarboxylation
CH2 -COO -
-
C-COO
-
H C-COO
CH- COO -
CH2 -COO Citrate
CH 2 -COO - N AD +
CH 2 -COO -
H C-COO HO CH- COO Isocitrate
-
HO CH- COO Isocitrate
Aconitate
•citrate is achiral; it has no stereocenter
•isocitrate is chiral; it has 2 stereocenters and 4
stereoisomers are possible
•only one of the 4 stereoisomers of isocitrate is formed
in the cycle
CH2 -COO -
• isocitrate dehydrogenase is an allosteric enzyme; it is
inhibited by ATP and NADH, activated by ADP and
NAD+
The Citric Acid Cycle
• Step 4: oxidative decarboxylation of -
• Step 5: formation of succinate
ketoglutarate to succinyl-CoA
CH 2 -COO -
CoA-SH
CH2
O C-COO -Ketoglutarate
N AD +
CH2 -COO
CH2
-ketoglutarate
dehydrogenase
complex
+ CO 2
O C SCoA
Succinyl-CoA
CH2 -COO -
succinate
dehydrogenase
H
FAD H 2
-
OOC
C
C
+ GT P + CoA -SH
Succinate
•the two CH2-COO- groups of succinate are equivalent
•this is the first energy-yielding step of the cycle
•the overall reaction is slightly exergonic
G 0'
Succinyl-CoA + H2 O
GDP + Pi
Succinyl-CoA + GDP + Pi
Succinate + CoA-SH
GTP + H2 O
(kJ ol
-33.4
Succinate + CoA -SH + GTP
-1 )
+30.1
-3.3
The Citric Acid Cycle
• Step 6: oxidation of succinate to fumarate
FAD
CH 2 -COO CH 2 -COO -
Succinyl-CoA
The Citric Acid Cycle
CH2 -COO -
succinyl-CoA
synthetase
O C SCoA
-
•like pyruvate dehydrogenase, this enzyme is a
multienzyme complex and requires coenzyme A,
thiamine pyrophosphate, lipoic acid, FAD, and NAD+
-1
•G0’= -33.
4kJ•
mol
Succinate
+ GD P + Pi
CH 2
N ADH
CH2 -COO -
CO 2
H C-H
O C-COO -Ketoglutarate
H C-COO O C-COO Oxalosuccinate
The Citric Acid Cycle
CH 2 -COO -
N AD H
isocitrate
dehydrogenase
• Step 7: hydration of fumarate
COOH
Fumarate
H
-
C
C
COO-
OOC
H
Fumarate
H2 O HO CH- COO
CH 2 -COO fumarase
L-Malate
3
From Pyruvate to CO2
The Citric Acid Cycle
Pyruvate dehydrogenase complex
Pyruvate + CoA-SH + NAD +
Acetyl-CoA + NADH + CO 2 + H +
• Step 8: oxidation of malate
N AD + N AD H
HO CH- COO CH2 -COO
L-Malate
-
malate
dehydrogenase
O C-COO
Citric acid cycle
Acetyl-CoA + 3NAD + + FAD + GDP + Pi + 2 H2 O
2 CO 2 + CoA-SH + 3NADH + 3H + + FAD H2 + GTP
CH 2 -COO Oxaloacetate
Pyruvate + 4NAD + + FAD + GDP + Pi + 2 H 2 O
3 CO 2 + 4NADH + FAD H2 + GTP + 4H +
G? (kJol
Pyruvate + CoA-SH + N AD +
Acetyl-CoA + N AD H + CO 2 + H+
1. Acetyl-CoA + Oxaloacetate + H2 O
Citrate + CoA-SH + H+
Isocitrate
2. Citrate
+
3. Isocitrate + N AD
-Ketoglutarate + N AD H + CO 2
-33.4
4. -Ketoglutarate + N AD + + CoA-SH
Succinyl-CoA + N AD H + CO 2 + H+
5. Succinyl-CoA + GDP + Pi
Succinate + GTP + CoA-SH
6. Succinate + FAD
Fumarate + FAD H2
Malate
7. Fumarate + H2 O
8. Malate + N AD +
Oxaloacetate + N AD H
-33.4
-32.2
+6.3
-7.1
-3.3
~0
-3.8
+29.2
-1
)
Control of the TCA Cycle
• Three control points within the cycle
•citrate synthase: inhibited by ATP, NADH, and succinyl
CoA; also product inhibition by citrate
•isocitrate dehydrogenase:
dehydrogenase: activated by ADP and NAD+,
inhibited by ATP and NADH
•-ketoglutarate dehydrogenase complex:
complex: inhibited by
ATP, NADH, and succinyl CoA; activated by ADP and
NAD+
• One control point outside the cycle
•pyruvate dehydrogenase: inhibited by ATP and NADH;
also product inhibition by acetyl-CoA
Pyruvate + 4 N AD + + FAD + GDP + Pi
-77.7
3 CO 2 + 4 N AD H + FAD H2 + GTP + 4 H+
Control of the CA Cycle
Metabolic rate in different state
• Cells in a resting metabolic state
•need and use comparatively little energy
•high ATP, low ADP imply high ATP/ADP ratio
•high NADH, low NAD+ imply high NADH/NAD+ ratio
• Cells in an active metabolic state
•need and use more energy than resting cells
•low ATP, high ADP imply low ATP/ADP ratio
•low NADH, high NAD + imply low NADH/NAD+ ratio
Figure 16.7 Conversion of pyruvate to acetyl-CoA
4
The Glyoxylate Cycle
Glyoxylate Cycle
CH 2 -COO -
• Plants and some bacteria, but not animals, use a
modification of the citric acid cycle to produce
four-carbon dicarboxylic acids and eventually
glucose
•the glyoxylate cycle bypasses the two oxidative
decarboxylations of the citric acid cycle
•instead, it routes isocitrate via glyoxylate to malate
•key enzymes in this cycle are isocitrate lyase and
malate synthetase
isocitrate
lyase
H C-COO HO CH- COO Isocitrate
O
O
CH3 C-SCo A + HC- COO Acetyl-CoA
CH 2 -COO CH 2 -COO Succinate
malate
synthetase
Glyoxylate
+
O= C-COO H
Glyoxylate
OH
CH- COO - + CoA -SH
Coenzyme A
CH 2 -COO Malate
Glyoxylate Cycle
The Glyoxylate Cycle
•
The glyoxylate cycle takes place
• in plants:
in glyoxysomes, specialized organelles devoted to this cycle
• in yeast and algae: in the cytoplasm
5
The Glyoxylate Cycle
TCA Cycle in Catabolism
• The catabolism of proteins, carbohydrates, and
• Helps plants grow in the dark
•seeds are rich in lipids, which contain fatty acids
•during germination, plants use the acetyl-CoA
produced in fatty acid oxidation to produce
oxaloacetate and other intermediates for
carbohydrate synthesis
•once plants begin photosynthesis and can fix CO 2,
glyoxysomes disappear
fatty acids all feed into the citric acid cycle at one
or more points
Carbohydrates
Proteins
Amino Acids
Fatty Acids
Pyruvate
Acetyl-CoA
-Ketoglutarate
Succinyl-CoA
Fumarate
Malate
Oxaloacetate
CA Cycle in Catabolism
intermediates
of the citric
acid cycle
CA Cycle in Anabolism
• The citric acid cycle is the source of starting
materials for the biosynthesis of other
compounds
•examples:
O
-
OOCCH 2 CH2 CCOO
-
transamination
N H3 +
-
OOCCH 2 CH2 CH COO -
-Ketoglutarate
Glutamate
O
-
OOCCH2 CCOO
Oxaloacetate
CA Cycle and Anabolism
-
transamination
N H3 +
-
OOCCH2 CHCOO Aspartate
The CA Cycle in Anabolism
• If a component of the citric acid cycle is taken out
for biosynthesis, it must be replaced
•oxaloacetate, for example, is replaced by the
carboxylation of pyruvate
O
CH3 CCOO - + CO 2 + A TP
Pyruvate
biotin
pyruvate
carboxylase
O
CH2 CCOO - + A DP + Pi
COOOxaloacetate
6
The CA Cycle in Anabolism
The CA Cycle in Anabolism
The CA Cycle in Anabolism
The CA Cycle in Anabolism
• Lipid anabolism begins with acetyl-CoA and
takes place in the cytosol
•acetyl-CoA is produced mainly in mitochondria from
catabolism of fatty acids and carbohydrates
•an indirect transfer mechanism exists involving
citrate
Citrate + CoA-SH + ATP
Acetyl-CoA + Oxaloacetate + ATP + Pi
• the oxaloacetate thus formed provides a means for
the production of the NADPH needed for
biosynthesis
Oxaloacetate
+ N AD H + H +
Malate + N AD P+
Malate + N AD +
Pyruvate + CO 2 + N AD PH + H +
•the net effect of these two reactions is replacement of
NADH by NADPH
•while there is some NADPH produced by this means,
its principal source is the pentose phosphate pathway
• The anabolic reactions that produce amino acids
and many other biomolecules begin with CA
cycle molecules that are transported into the
cytosol
CA Cycle and Anabolism
7
細胞內所有代謝路徑
Nelson et al (2002) Principle of biochemistry (4e) p.560
8