Download Citric acid cycle • What are the functions of Citric Acid Cycle?

Citric acid cycle
• What are the functions of Citric Acid
Cycle?
• What are the reactions of Citric Acid
Cycle?
• How does a mitochondrion look like?
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The third step of the energy metabolism
The third step of the energy metabolism comprises the citric acid cycle and
the oxidative phosphorylation. Electrons are transferred from reduced
electron carriers to oxygen, and ATP is formed via a proton gradient.
The mitochondrion
The mitochondrion has two membranes of which the inner one is selective and
folded.
The degree of folding depends on the energy need of the cell
Outer membrane: All molecules with MW<5000 can pass
Inner membrane: Selective, contains electron transport chain and ATP-synthase
Matrix: Contains the enzymes of citric acid cycle and fatty acid oxidation
Outer membrane
Matrix
Inner membrane
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The pyruvate dehydrogenase complex catalyses the conversion of pyruvate
to acetyl-CoA
Pyruvate + CoA + NAD+
Acetyl-CoA + CO2 + NADH +H+
Citric acid cycle
An acetyl group is oxidised
in the citric acid cycle and
the electrons transferred to
NAD+ or FAD
In
Out
1 Acetyl group
2 CO2
3 NAD+
3 NADH
1 FAD
1 FADH2
1 GDP
1 GTP
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The citric acid cycle can be drained of intermediaries
The second purpose of the tricarboxylic acid cycle is to provide the cell with
building blocks for the biosynthesis of e.g. amino acids , heme and fatty acids. A
consequence is that a lack of intermediaries can impair the oxidative function.
REFILLING
Refilling using pyruvate
carboxylase
Pyruvat + CO2 +ATP
Oxaloacetate+ ADP + Pi +
H2O
Summary:
• The tricarboxylic acid cycle takes place in the
mitochondrion
• The tricarboxylic acid cycle generates reduced electron
carriers and GTP
• The tricarboxylic acid cycle has two purposes:
- Oxidise acetyl-CoA
- Provide the cell with building blocks for biosynthesis
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Oxidative phosphorylation
• How are electrons transported from NADH to
oxygen?
• How is ATP formed?
• How are electron transport and ATP formation
coupled?
• How much ATP is formed when one molecule
glucose is oxidised?
Principle of oxidative phosphorylation
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Oxidative phosphorylation (1)
Oxidative phosphorylation can be divided into three parts:
1. Electron transport from reduced electron carriers to oxygen
2. Creation of a membrane potential and a pH-gradient
3. Synthesis of ATP
These three parts are coupled
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Oxidative phosphorylation (2) – Electron transport chain
Electrons from NADH take the following path:
1. Complex I (Integral membrane protein, transfers protons)
2. CoQ (quinone, transfers electrons between complex I and complex III)
3. Complex III (Integral membrane protein, transfers protons)
4. Cytochrome c (protein, transports electrons between complex III and complex IV)
5 Complex IV (Integral membrane protein
5.
protein, transfers protons
protons, conveys electrons
to oxygen)
6. Oxygen is reduced to water
Electrons from FADH2 are delivered to CoQ via complex II (peripheral membrane
protein)
H+
H+
H+
H+
cyt c
C Q
CoQ
I 2e-
III
II
IV
V
2eO2 + 4e- + 4H+
NADH +
H+
2H2O
NAD+
FADH2
FAD
H+
Oxidative phosphorylation (3) - CoQ
CoQ is a quinone, which can take up two electrons. Its hydrophobic side chain
makes it comfortable in the inner membrane of the mitochondrion.
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Oxidative phosphorylation (4) – Prosthetic groups
Electron transport by a number of reductions and oxidations
NADH
NAD
C- I (red)
CoQ (red)
C- I (ox)
CoQ(ox)
C- III (red)
Cyt c (red)
C- III (ox)
Cyt c (ox)
C- IV (red)
C- IV (ox)
H2O
O2
Prosthetic groups in the complexes are reduced and oxidised:
Flavin mononucleotide:
Heme group:
Iron-Sulphur
Iron
Sulphur clusters:
Copper:
FMN
Fe3+
Fe3+
Cu2+
FMNH2
Fe2+
Fe2+
Cu+
• Prosthetic groups are organised in order to enable electron transfer
• Specificity in electron transfer: E.g. NADH can't reduce cytochrome c
• All components of the electron transfer diffuse in the membrane
Reactive intermediaries in the electron transport chain
Reactive oxygen species (ROS) like e.g. the superoxide radical are generated
during transport of electrons from NADH and FADH2 to oxygen. They can react with
other biomolecules and cause damage (and possibly diseases). We have some
protection from some enzymes, which take care of ROS.
Superoxide dismutase
-
.
2O2 + 2H+
O2 + H2O2
Catalase
2 H2O2
H 2 O + O2
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Uncoupler (1)
Electron transport, proton transfer and phosphorylation of ADP are usually
coupled processes but they can be uncoupled under certain circumstances.
Uncouplers can lead to formation of heat instead of ATP, e.g. hibernating
animals and newborn children use an uncoupling protein to lead the protons
b k inside
back
i id the
th mitochondrion.
it h d i
Uncoupler (2)
Also low molecular weight substances can act as uncouplers. In the
example below, protonated dinitrophenol can diffuse through the membrane
thereby ”destroying” the membrane potential.
Outside
In the membrane
O
OH
NO2
O
NO2
+
NO2
Inside
NO2
H+
+
NO2
H+
NO2
Electron transfer driven transport of protons
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10
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Summary
• Oxidative phosphorylation takes place in the mitochondrial
inner membrane
• The membrane is an integral part of the mechanism
• The electron transport chain brings electrons from
NADH and FADH2 to oxygen
• A proton gradient is established when electrons passes the chain
• ADP is phosphorylated to ATP when the protons return to
the mitochondrion
• NADH (the electrons) from glycolysis enter the mitochondrion
by a shuttle
• Uncouplers generate heat
Metabolism of Fatty Acids
• Oxidation of fatty acids
• Ketone bodies and their use
• Synthesis
S th i off fatty
f tt acids
id
Fatty acids
•
•
•
•
Phospholipids
osp o p ds in membranes
e b a es
Glycolipids in membranes
Fatty acid derivatives as hormones
Fuel for the cell, energy store
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Fat (Triacylglycerol)
A triacylglycerol is a glycerol esterified with three fatty acids. The fatty
acids can be saturated, mono-unsaturated or poly-unsaturated. Usually
they have an even number of carbon atoms (12-20).
H
H
C
H
C
O
O
O
C
O
C
O
H
C
O
C
H
Digestion, mobilization and transport of dietary triacylglycerols
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Chylomicron
Lipase reactions
The enzyme lipase catalyses the hydrolysis of two of the fatty acids in the
intestine.
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Uptake of fat in the intestine
Fat is first hydrolysed in the intestine and then taken up through the cell wall.
The triacylglycerol is then regenerated before its transport as part of a
chylomicron (a lipoprotein) to fat cells where it can be stored.
Triacylglycerol in fat cells is degraded to fatty acids
and glycerol, which can be used in other cells
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Degradation of fatty acid has three steps:
1. Activation
2. Transport into mitochondrion
3. β-oxidation
Activation of the fatty acid consists of esterification to CoA at the
expense of ATP
O
O
C O -
CoA + R
ATP
R
C
S
CoA
AMP + PP i
2P
i
Transport of fatty acid into mitochondrion
The acyl group is transported into
mitochondrion using carnitine. A
translocase brings in acylcarnitine and expels carnitine.
A l C A iis regenerated
Acyl-CoA
t d iinside
id
mitochondrion.
Carnitine
CH3
H
H3C
+
C C C COO
H2
H2
CH3
OH
N
-
Position of the
fatty acid
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β-oxidation
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How much ATP can be formed by oxidation of fatty acid?
It is now possible to calculate the amount of ATP formed during complete oxidation of a
fatty acid, e.g. palmitic acid
8 Acetyl-CoA
y
in the tricarboxylic
y acid cycle:
y
8 x 3 = 24 NADH
8 x 1 = 8 FADH2
8 x 1 = 8 GTP
β-Oxidation yields: 7 NADH
7 FADH2
In total: 7 + 24 = 31 NADH corresponds to 31 x 2,5 = 77,5 ATP
7 + 8 = 15 FADH2 corresponds to 15 x 1,5 = 22,5 ATP
8 GTP corresponds
d to
t 8 ATP
Summarised: 8 + 77,5 + 22,5 = 108 ATP
Two ATP are used during the initial activation of the fatty acid
The total ATP-yield will be 106
Formation and use of ketone bodies
Succinyl CoA Succinate
Acetoacetate
CoA
Acetoacetyl CoA
2 Acetyl-CoA
Citric acid cycle
Ketone bodies are formed in
the liver from surplus of acetylCoA when degradation
g
of fat
dominates and the supply of
carbohydrates is limited, as
during starvation and fasting.
The ketone bodies are
exported to other organs
where they can be oxidised.
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Fatty acid synthesis
• Fat is an efficient way to store energy
• It is usually surplus of carbohydrates that are converted to fat
• The reactions are in principle a reversed β-oxidation
The synthesis of a fatty acid starts with the carboxylation of acetyl-CoA to
malonyl-CoA. This is the committed step, malonyl CoA can only be used for
synthesis of fatty acid.
O
H3C
C
O
O
S
CoA
+ ATP + HCO 3
C
O
Acetyl-CoA
C
H2
C
S
Malonyl-CoA
O
H3C
C
O
O
S
+
ACP
C
C
H2
O
Reaction sequence
during fatty acid
synthesis
+ ADP + Pi + H+
CoA
C
S
ACP
Condensation
ACP + CO2
O
H3C
O
C
C
H2
C
S
ACP
Acetoacetyl-ACP
NADPH
Reduction
NADP+
OH
H3C
C
O
C
H2
C
S
ACP
D-3-Hydroxybutyryl-ACP
H
Dehydration
H2O
H3C
C
H
O
C
C
S
ACP
Crotonyl-ACP
H
NADPH
Reduction
NADP+
O
H3C
C
H2
C
H2
C
S
ACP Butyryl-ACP
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Summary:
•
•
•
•
•
It is efficient to store energy as fat
The fatty acid is bound to albumin during transport to target cells
The fatty acid is activated in the cytoplasm
The fatty acid is transported into the mitochondrion using carnitine
The fatty acid is oxidised by β-oxidation yielding NADH, FADH2 and
acetyl-CoA
• Ketone bodies are formed when acetyl-CoA is in surplus
• Acetyl-CoA (from carbohydrates) is starting material for fatty acid
synthesis
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