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19
The Citric
Acid Cycle
Hans Krebs,
1900–1981
19-1
19 Learning Objectives
1.
2.
3.
4.
5.
6.
7.
8.
What Role Does the Citric Acid Cycle Play in Metabolism?
What Is the Overall Pathway of the Citric Acid Cycle?
How Is Pyruvate Converted to Acetyl-CoA?
What Are the Individual Reactions of the Citric Acid
Cycle?
What Are the Energetics of the Citric Acid Cycle, and How
Is It Controlled?
What Is the Glyoxylate Cycle?
What Role Does the Citric Acid Cycle Play in Catabolism?
What Role Does the Citric Acid Cycle Play in Anabolism?
19-2
19 The Citric Acid Cycle
• Three processes play central role in aerobic
metabolism
• the citric acid cycle
• electron transport
• oxidative phosphorylation
• Metabolism consists of
• catabolism: the oxidative breakdown of nutrients
• anabolism: the reductive synthesis of biomolecules
• The citric acid cycle is amphibolic; that is, it
plays a role in both catabolism and anabolism
• It is the central metabolic pathway
19-3
19 Mitochondrion
19-4
19 The Citric Acid Cycle
• TCA cycle= Krebs cycle= citric acid cycle
• In eukaryotes, cycle occurs in the mitochondrial matrix
mitochondrion
19-5
19 The Citric Acid Cycle
Pyruvate
N AD +
N AD H
Acetyl-CoA Coenzyme A
CO2
N AD H
N AD +
Citric
acid
cycle
(8 steps)
F AD H2
F AD
GTP
GDP
N AD +
N AD H
CO 2
N AD +
N AD H
CO 2
19-6
19
19-7
19 Pyruvate to Acetyl-CoA
• Oxidative decarboxylation reaction
• Occurs in the mitochondria
O
CH3 CCOO - + CoA -SH + N AD +
Pyruvate
pyruvate
dehydrogenase
complex
Coenzyme A
O
CH3 C-SCo A + CO 2 + N AD H
Acetyl -CoA
this reaction requires NAD+, FAD, Mg2+, thiamine
pyrophosphate, coenzyme A, and lipoic acid
• G°’ = -33.4 kJ•mol-1
19-8
19
Structure of the pyruvate
dehydrogenase complex
E1, pyruvate dehydrogenase (yellow) (; E2, dihydrolipoyl
transacetylase;(green) and E3,dihydrolipoyl dehydrogenase
(red). The lipoyl domain of E2 (blue)
19-9
19
19-10
19
Beriberi-
A vitamin-deficiency disease first described in 1630
by Jacob Bonitus, a Dutch physician working in
Java:
"A certain very troublesome affliction, which
attacks men, is called by the inhabitants Beriberi
(which means sheep). I believe those, whom this
same disease attacks, with their knees shaking
and the legs raised up, walk like sheep. It is a
kind of paralysis, or rather tremor: for it
penetrates the motion and sensation of the hands
and feet indeed sometimes of the whole body."
19-11
19 The Citric Acid Cycle
• Step 1: Formation of citrate by condensation of
acetyl-CoA with oxaloacetate; G°’= -32.8 kJ•mol-1
O
CH3 C- S Co A
C H2 - CO O Acetyl-CoA citrate
synthase
H O C - C O O - + Co A - S H
+
CH2 - CO O - Coenzyme A
O C- CO O
CH 2 - C O O Citrate
(3 carboxyl groups)
Oxaloacetate
 citrate synthase (condensing E) is an allosteric
enzyme, inhibited by NADH, ATP, and succinyl-CoA
19-12
19 The Citric Acid Cycle
• Step 2: dehydration and rehydration gives
isocitrate; catalyzed by aconitase
CH2 - CO O HO C - CO O -
CH2 - CO O Citrate
CH2 - CO O C- CO O CH- CO O -
Aconitate
CH2 - CO O H C- C O O H O CH- CO O Isocitrate
(3 carboxyl groups)
• 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
19-13
19 The Citric Acid Cycle
• Step 3: oxidation of isocitrate followed by
decarboxylation
CH2 - CO O - N AD +
H C- CO O H O C H - CO O
Isocitrate
-
N AD H
isocitrate
dehydrogenase
CH2 - CO O -
H C- CO O O C- CO O Oxalosuccinate
CO 2
CH2 - CO O H C- H
O C- CO O a -Ketoglutarate
(2 carboxyl groups)
• isocitrate dehydrogenase is an allosteric enzyme; it is
inhibited by ATP and NADH, activated by ADP and
NAD+
19-14
19 The Citric Acid Cycle
• Step 4: oxidative decarboxylation of
a-ketoglutarate to succinyl-CoA
Co A - S H
C H2 - CO O CH 2
O C- CO O a-Ketoglutarate
N AD +
N AD H
C H2 - C O O CH 2
a-ketoglutarate
dehydrogenase O C SCo A
Succinyl-CoA
complex
+ CO 2
(1 carboxyl groups)
• like pyruvate dehydrogenase, this enzyme is a
multienzyme complex and requires coenzyme A,
thiamine pyrophosphate, lipoic acid, FAD, and NAD+
• G0’ = -33.4 kJ•mol-1
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19 The Citric Acid Cycle
• Step 5: formation of succinate
CH2 - CO O CH2
+ GD P + Pi
succinyl-CoA
synthetase C H 2 - C O O -
O C SC o A
CH2
- CO O -
+ GT P + C o A - S H
Succinate
Succinyl-CoA
(2 carboxyl groups)
• 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•mol -1)
-33.4
+30.1
Succinate + CoA -SH + GTP -3.3
19-16
19 The Citric Acid Cycle
• Step 6: oxidation of succinate to fumarate
CH2 - CO O -
F AD
CH2 - CO O -
succinate
Dehydrogenase
Succinate
F AD H2
+Fe, - heme
H
-OOC
C
C
C O OH
Fumarate
(2 carboxyl groups)
• Note: succinate dehydrogenase is the only TCA
enzyme that is located in the inner mitochondrial
membrane and linked directly to ETC
19-17
19 The Citric Acid Cycle
• Step 7: hydration of fumarate
H
-
C
C
C O O-
OOC
H
Fumarate
H 2 O H O C H - CO O
CH2 - CO O fumarase
L-Malate
(2 carboxyl groups)
• Step 8: oxidation of malate
H O C H - CO O CH2 - CO O L-Malate
N AD + N AD H
malate
dehydrogenase
O C- CO O
CH2 - CO O Oxaloacetate
(2 carboxyl groups)
19-18
19 From Pyruvate to CO2
Pyruvate dehydrogenase compl ex
Pyruvate + CoA-SH + NAD +
Acetyl -CoA + NADH + CO 2 + H +
Citric acid cycle
Acetyl -CoA + 3NAD
+
+ FAD + GDP + Pi + 2 H2 O
2 CO 2 + CoA-SH + 3NADH + 3H + + FADH2 + GTP
Pyruvate + 4NAD + + FAD + GDP + Pi + 2 H2 O
+
3 CO 2 + 4NADH + FADH2 + GTP + 4H
19-19
19 Summary
• The two-carbon unit needed at the start of the
citric acid cycle is obtained by converting
pyruvate to acetyl-CoA
• This conversion requires the three primary
enzymes of the pyruvate dehydogenase complex,
as well as, the cofactors TPP, FAD, NAD+, and
lipoic acid
• The overall reaction of the pyruvate
dehydogenase complex is the conversion of
pyruvate, NAD+, and CoA-SH to acetyl-CoA,
NADH + H+, and CO2
19-20
19
-1
G°' (kJ•mol )
Pyruvate + CoA-SH + N AD +
Acetyl -CoA + N AD H + CO 2 + H +
1. Acetyl -CoA + Oxaloacetate + H 2 O
Citrate + CoA-SH + H +
Isoci trate
2. Citrate
+
3. Isoci trate + N AD
a-Ketoglutarate + N AD H + CO 2
-33.4
4. a-Ketoglutarate + N AD + + CoA-SH
Succi nyl-CoA + N AD H + CO 2 + H +
5. Succi nyl-CoA + GDP + Pi
Succi nate + GTP + CoA-SH
6. Succi nate + FAD
Fumarate + FAD H2
Malate
7. Fumarate + H 2 O
8. Malate + N AD +
Oxaloacetate + N AD H
-33.4
-32.2
+6.3
-7.1
-3.3
~0
-3.8
+29.2
Pyruvate + 4 N AD + + FAD + GDP + Pi
-77.7
3 CO 2 + 4 N AD H + FAD H 2 + GTP + 4 H +
19-21
19 Control of the CA Cycle
• Three control points within the cycle
• citrate synthase: inhibited by ATP, NADH, and succinyl
CoA; also product inhibition by citrate
• isocitrate dehydrogenase: activated by ADP and NAD+,
inhibited by ATP and NADH
• a-ketoglutarate dehydrogenase 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
19-22
19 Control of the CA Cycle
Conversion of pyruvate to acetyl-CoA
19-23
Cells in a resting
19
metabolic state
Cells in an active
metabolic state
need and use
comparatively little energy
need and use more energy
than resting cells
high ATP, low ADP imply
high ATP/ADP ratio
low ATP, high ADP imply
low ATP/ADP ratio
high NADH, low NAD+
imply high NADH/NAD+
ratio
low NADH, high NAD +
imply low NAHDH/NAD+
ratio
19-24
19
Why Is the Oxidation of Acetate
So Complicated?
19-25
19 Because ……………
1. Citric Acid Cycle Components Are
Important Biosynthetic Intermediates
2. Besides its role in the oxidative
catabolism of carbohydrates, fatty acids,
and amino acids, the cycle provides
precursors for many biosynthetic
pathways.
3. It is also important for plants and bacteria
19-26
19 Glyoxalate Cycle…..
19-27
19 Glyoxalate Cycle
Bacteria and plants can synthesize acetyl CoA
from acetate and CoA by an ATP-driven
reaction that is catalyzed by
acetyl CoA synthetase.
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19
19-29
19
Biosynthetic Roles of the Citric Acid Cycle. Intermediates
drawn off for biosyntheses (shown by red arrows) are
replenished by the formation of oxaloacetate from
pyruvate.
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19
End
Chapter 19
19-31