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Metabolism and
Energy Production
Citric Acid Cycle
Electron Transport Chain
ATP Energy from Glucose
Oxidation of Fatty Acids
Metabolic Pathways for Amino Acids
Citric Acid Cycle
A reaction series that
• Operates under aerobic conditions only
• Oxidizes the 2 carbon atoms of acetyl CoA to
CO2
• Provides reduced coenzymes
O
||
CH3–C –CoA
acetyl CoA
2 CO2 , FADH2, 3 NADH, + ATP
Steps 1-3 in Citric Acid Cycle
COOCOO+ CH3
O C
O
C CoA
CH2
HO C COO
CH2
CH2
COO-
COO-
oxaloacetate
acetyl CoA
+ CoA
citrate
-
COO
NAD+
CH2
CH2
α-ketoglutarate
C O
COO-
+ CO2
+ NADH
Steps 4-5 of citric acid cycle
In the next reactions, α-ketoglutarate is oxidized
to succinate.
COO-
COO-
CH2
CH2
CH2
+ NAD+
C O
CH2
COO-
COO-
α-ketoglutarate
succinate
+ CO2 + NADH
Steps 6-8 of citric acid cycle
More oxidations convert succinate to oxaloacetate.
The C=C requires FAD.
COO-
COO-
CH2
+ FAD
CH2
CH
CH
COOsuccinate
COOHO C H + NAD+
CH
-
COO
malate
+ FADH2
COOfumarate
COOC O + NADH
CH
COOoxaloacetate
H 2O
Coenzymes Produced in the
Citric Acid Cycle
1.
Acetyl CoA (2C) + oxaloacetate (4C) to
citrate (6C)
2.
Citrate (6C) to α-ketoglutarate (5C) + CO2
3.
α-ketoglutarate (5C) to succinate (4C) +
CO2. GDP picks up Pi.
1 NADH
1 NADH
1 GTP
4.
Succinate(4C) to fumarate (C=C) to malate
1 FADH2
5.
Malate to oxaloacetate. Start again.
Total: 2CO2 + 3 NADH + 1 FADH2 + GTP
Coenzymes
1 NADH
Learning Check
Complete the following statements:
A. When 1 acetyl CoA enters the citric acid
cycle, the C atoms produce ____CO2.
B. In 1 cycle, a total of ____NADH are
produced.
C. In 1 cycle, a total of ____FADH2 are
produced.
Solution
Complete the following statements:
A. When 1 acetyl CoA enters the citric acid
cycle, the C atoms produce 2 CO2.
B. In 1 cycle, a total of 3 NADH are
produced.
C. In 1 cycle, a total of 1 FADH2 are
produced.
Regulation of Citric Acid Cycle
• Operates when ATP is needed
• High levels of ATP and/or NADH inhibit
citrate synthetase (first step in cycle)
• High levels of ADP and NAD+ activate
isocitrate dehydrogenase
• Low levels of ATP or high levels of acetyl
CoA speed up the cycle to give energy
ATP
Electron Transport Chain
• A series of electron carriers
• Transfers H+ and electrons from
coenzymes NADH and FADH2 (citric acid
cycle)
• Energy released along chain to make ATP
NADH + 3 ADP
FADH2 + 2 ADP
NAD+ + 3 ATP
FAD + 2 ATP
Electron Carriers
• Found in three protein complexes
• Attached to inner membrane of mitochondria
• H+ move into intermembrane space to create
proton gradient
• As H+ return to matrix, ATP synthase uses
energy to synthesize ATP
• Oxidation phosphorylation
ADP + Pi + Energy
ATP
Enzyme Complexes
1. NADH dehydrogenase
Coenzyme A
2. Cytochrome c reductase
Cytochrome c
3. Cytochrome c Oxidase
Chemiosmotic Model
Intermembrane space
Q
H+
H+
H+
H+
H+
H+
Cytc
eNADH + H+
Matrix
FADH2
H2O
ADP + P
ATP
Learning Check
Classify each as (1) a product of the citric
acid cycle, (2) a product of the electron
transport chain
A. CO2
B. FADH2
C. NAD+
D. NADH
E. ATP
Solution
Classify each as (1) a product of the citric
acid cycle, (2) a product of the electron
transport chain
A. 1 CO2
B. 1 FADH2
C. 2 NAD+
D. 1 NADH
E. 2 ATP
ATP Energy from Glycolysis
(Aerobic)
• In the electron transport system
NADH = 3 ATP
FADH2 = 2 ATP
• Glycolysis
Glucose
2 pyruvate + 2 ATP + 2 NADH
NADH in cytoplasm
FADH2 mitochondria
Glucose
2 pyruvate + 6 ATP
ATP Energy from Pyruvate
2 pyruvate
2 acetyl CoA + 2 CO2 + 2 NADH
2 pyruvate
2 acetyl CoA + 2 CO2 + 6 ATP
ATP Energy from Citric Acid
Cycle
One turn of the citric acid cycle
3 NADH x 3 ATP =
9 ATP
1 FADH2 x 2 ATP =
2 ATP
1 GTP
x 1 ATP =
1 ATP
Total
=
12 ATP
Glucose provides two acetyl COA
molecules for two turns of citric acid cycle
2 acetyl CoA
24 ATP + 4 CO2
ATP from Glucose
For 1 glucose molecule undergoing
complete oxidation
Glycolysis
6 ATP
2 Pyruvate to 2 Acetyl CoA
6 ATP
2 Acetyl CoA to 4 CO2
24 ATP
Glucose + 6 O2
6 CO2 + 6 H2O + 36 ATP