19
19-1
The Citric
Acid Cycle
19
19-2
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
19-3
Mitochondrion
The Powerhouse of the Cell
19
19-4
The Citric Acid Cycle • TCA cycle= Krebs cycle= Citric acid cycle
• In Eukaryotes, cycle occurs in the mitochondrial matrix
• In Prokaryotes CAC occurs in the cytosol
mitochondrion
19
19-5
The Citric Acid Cycle Pyruvate
Acetyl -CoA
GDP GTP
F A D
F A D H 2
N A D +
N A D H
N A D +
N A D H
C O 2
N A D +
N A D H
C O 2
Citric acid cycle
(8 steps)
Coenzyme A
N A D +
N A D H CO2
19
19-6
Summary Of
Reactions Of CAC
Can I Keep Selling
Sex For
Money Officer
19
19-7
19
19-8
Pyruvate to Acetyl-CoA • Oxidative decarboxylation reaction
• Occurs in the mitochondria
this reaction requires NAD+, FAD, Mg2+, thiamine
pyrophosphate, coenzyme A, and lipoic acid
• G°’ = -33.4 kJ•mol-1
CH3 CCOO- + NAD+CoA-SH +
CH3 C-SCo A + CO2
pyruvatedehydrogenase
complex
Pyruvate Coenzyme A
Acetyl-CoA
O
O
+ NADH
19
19-9
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
19-10
19
19-11
Congenital Lactic Acidosis
• Absence of pyruvate decarboxylase activity in
man ( first enzyme ).
• characterised by progressive neuromuscular
deterioration and accumulation of lactate and
hydrogen ions in blood, urine and/or
cerebrospinal fluid, frequently resulting in early
death.
• It is an x-linked dominant disease affecting both
sexes.
19
19-12
Arsenic poisoning • It inhibits lipoic acid.
• Epidemiological evidence shows an association
between inorganic arsenic in drinking water and
increased risk of skin, lung and bladder cancers
19
19-13
Beriberi
A vitamin-deficiency disease first described in 1630
by Jacob Bonitus, a Dutch physician working in
Java
The term beriberi is derived from the Sinhalese
word meaning “extreme weakness.”
19
19-14
Beriberi • It is a nutritional disorder caused by a deficiency
of thiamin (vitamin B1) and characterized by
impairment of the nerves and heart.
• In the form known as dry beriberi, there is a
gradual degeneration of the long nerves, first of
the legs and then of the arms, with associated
atrophy of muscle and loss of reflexes.
• In wet beriberi, a more acute form, there is
edema resulting largely from cardiac failure and
poor circulation. In infants breast-fed by mothers
who are deficient in thiamin, beriberi may lead to
rapidly progressive heart failure.
19
19-15
Wernicke–Korsakoff syndrome
• This is found predominantly in alcoholics.
Chronic alcohol consumption can result in
thiamine deficiency by causing inadequate
nutritional thiamine intake, decreased absorption
of thiamine from the gastrointestinal tract, and
impaired thiamine utilization in the cells.
• People differ in their susceptibility to thiamine
deficiency, however, and different brain regions
also may be more or less sensitive to this
condition.
19
19-16
The Citric Acid Cycle
• Step 1: Formation of citrate by condensation of
acetyl-CoA with oxaloacetate; G°’= -32.8 kJ•mol-1
citrate synthase (condensing E) is an allosteric
enzyme, inhibited by NADH, ATP, and succinyl-CoA
C H 3 C - S C o A
Acetyl-CoA
C - C O O -
C H 2 - C O O -
Oxaloacetate
+ C - C O O - H O
C H 2 - C O O -
C H 2 - C O O -
+ C o A - S H
Coenzyme A
c itrate synthase
Citrate (3 carboxyl groups)
O
O
19
19-17
The Citric Acid Cycle • Step 2: dehydration and rehydration gives
isocitrate; catalyzed by aconitase(-by flouroacetate
rat poison).
• 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
C - C O O - H O
C H 2 - C O O -
C H 2 - C O O -
Citrate
C - C O O -
C H 2 - C O O -
C - C O O - H
C H - C O O -
C H 2 - C O O -
Aconitate
H O
Isocitrate (3 carboxyl groups)
C H - C O O -
19
19-18
The Citric Acid Cycle
• Step 3: oxidation of isocitrate followed by
decarboxylation
• isocitrate dehydrogenase is an allosteric enzyme; it is
inhibited by ATP and NADH, activated by ADP and
NAD+
C - C O O - H
C H - C O O -
C H 2 - C O O -
H O
Isocitrate
C - C O O - H
C - C O O -
C H 2 - C O O -
C - H H
C - C O O -
C H 2 - C O O -
N A D H N A D +
a -Ketoglutarate (2 carboxyl groups)
C O 2
isocitrate dehydrogenase
O O
Oxalosuccinate
19
19-19
The Citric Acid Cycle
• Step 4: oxidative decarboxylation of
a-ketoglutarate to succinyl-CoA
• 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
C H 2
C - C O O -
C H 2 - C O O -
a -Ketoglutarate
O
C o A - S H
N A D H N A D +
a -ketoglutarate
dehydrogenase complex
C H 2
C
C H 2 - C O O -
S C o A O
Succinyl-CoA (1 carboxyl groups)
+ C O 2
19
19-20
The Citric Acid Cycle
• Step 5: formation of 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
Su ccin yl-CoA+ H2 O Su ccin ate + CoA-SH
GDP + Pi GTP + H2 O
G0'
(k J•mol-1)
-33.4
+30.1
-3.3Su ccin yl-CoA+ GDP + Pi Su ccin ate+ CoA-SH + GTP
C H 2
C
C H 2 - C O O -
S C o A O
Succinyl-CoA
G D P + P i C o A - S H
Succinate (2 carboxyl groups)
+ G T P +
succinyl-CoA synthetase
C H 2 - C O O -
C H 2 - C O O - +
19
19-21
The Citric Acid Cycle
• Step 6: oxidation of succinate to fumarate
F A D F A D H 2
C H 2 - C O O -
C H 2 - C O O -
Succinate
succinate Dehydrogenase +Fe, - heme
C
C H
H
C O O -
- O O C
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
19-22
The Citric Acid Cycle
• Step 7: hydration of fumarate
• Step 8: oxidation of malate
C - C O O -
C H 2 - C O O -
Oxaloacetate (2 carboxyl groups)
N A D + N A D H
malate dehydrogenase
C H - C O O - H O
C H 2 - C O O -
L-Malate
O
C
C H
H
C O O -
- O O C
Fumarate
H 2 O C H - C O O - H O
C H 2 - C O O -
L-Malate (2 carboxyl groups)
fumarase
19
19-23
From Pyruvate to CO2
CoA-SH +
Pyruvate dehydrogenase complex
Citric acid cycle
Pyruvate + NAD +
Acetyl-CoA + NADH + CO2 H++
Acetyl-CoA + 3NAD + + FAD G DP+ + Pi
2 CO2 + CoA-SH + 3NADH 3H++ + FADH2 + G TP
Pyruvate 4NAD + + FAD G DP+ + Pi
2 H2 O+
2 H2 O+
3 CO2 + 4NADH + FADH2 + G TP 4H++
+
19
19-24
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
19-25
Pyruvate
Acetyl-CoA +
+ CoA-SH + NAD+
Acetyl-CoA + NADH + CO2 + H+
Citrate Isocitrate
Isocitrate
+ Oxaloacetate H2 O
Citrate + CoA-SH + H+
+ NAD+
a-Ketoglutarate + NADH + CO2
1.
2.
G °' (kJ•mol -1)
-33.4
-32.2
+6.3
-7.13.
4 NAD+
G TPSuccinate
FAD FADH2
4.
CoA-SH
5.
6.
7.
a-Ketoglutarate + NAD+ + CoA-SH
Succinyl-CoA + NADH + CO2 + H+
Succinyl-CoA + G DP + Pi
+ +
Succinate + Fumarate +
Fumarate + H2 O Malate
8. Malate
+
Oxaloacetate + NADH+ NAD+
+ FAD + G DP + Pi
3 CO2 4 NADH+ + FADH2 + G TP + 4 H+
Pyruvate
-33.4
-3.3
~0
-3.8
+29.2
-77.7
19
19-26
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
19-27
Control of the CA Cycle
Conversion of pyruvate to acetyl-CoA
19
19-28
Cells in a resting
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
19-29
Why Is the Oxidation of Acetate
So Complicated?
19
19-30
Because …………… 1. Besides its role in the oxidative
catabolism of carbohydrates, fatty acids,
and amino acids, the cycle provides
precursors for many biosynthetic
pathways.
2. It is also important for plants and bacteria
19
19-31
Glyoxalate Cycle…..
19
19-32
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.
19
19-33
19
19-34
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.
19
19-35
End
Chapter 19