Download Metabolic Pathways and Energy Production

Chapter 18 Metabolic Pathways
and Energy Production
18.1
Metabolism and ATP Energy
1
Metabolism
Metabolism involves
• catabolic reactions
that break down
large, complex
molecules to provide
energy and smaller
molecules.
• anabolic reactions
that use ATP energy
to build larger
molecules.
2
Stages of Metabolism
Catabolic reactions are organized as
• Stage 1: Digestion and hydrolysis break down
large molecules to smaller ones that enter
the bloodstream.
• Stage 2: Degradation break down molecules to
two- and three-carbon compounds.
• Stage 3: Oxidation of small molecules in the citric
acid cycle and electron transport provides
ATP energy.
3
Stages of Metabolism
4
Cell Structure and Metabolism
Metabolic reactions occur in specific sites within cells.
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5
Cell Components and Function
6
ATP and Energy
In the body, energy is stored as
adenosine triphosphate (ATP).
7
Hydrolysis of ATP
• The hydrolysis of ATP to ADP releases 7.3 kcal.
ATP
ADP + Pi
+
7.3 kcal
8
ATP and Muscle Contraction
Muscle fibers
• contain the protein fibers actin and myosin.
• contract (slide closer together) when a nerve
impulse increases Ca2+.
• obtain the energy for contraction from the
hydrolysis of ATP.
• return to the relaxed position as Ca2+ and ATP
decrease.
9
ATP and Muscle Contraction
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10
Learning Check
Match the following:
1) ATP
2) ADP + Pi
A.
B.
C.
D.
used in anabolic reactions
the energy-storage molecule
coupled with energy-requiring reactions
hydrolysis products
11
Solution
Match the following:
1) ATP
2) ADP + Pi
A.
B.
C.
D.
1
1
1
2
used in anabolic reactions
the energy-storage molecule
coupled with energy-requiring reactions
hydrolysis products
12
Chapter 18 Metabolic Pathways
and Energy Production
18.2
Digestion of foods
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13
Stage 1: Digestion of
Carbohydrates
In Stage 1, the carbohydrates
• begin digestion in the mouth, where salivary amylase
breaks down polysaccharides to smaller polysaccharides
(dextrins), maltose, and some glucose.
• continue digestion in the small intestine, where
pancreatic amylase hydrolyzes dextrins to maltose and
glucose.
• maltose, lactose, and sucrose are hydrolyzed to
monosaccharides, mostly glucose, which enter the
bloodstream for transport to the cells.
14
Carbohydrate Digestion
15
Digestion of Fats
In Stage 1, the digestion of fats (triacylglycerols)
• begins in the small intestine, where bile salts break fat
globules into smaller particles called micelles.
• uses pancreatic lipases to hydrolyze ester bonds,
forming glycerol and fatty acids.
• ends as fatty acids bind with proteins for transport to
the cells of the heart, muscle, and adipose tissues.
16
Digestion of Triacylglycerols
17
Digestion of Proteins
In Stage 1, the digestion of proteins
• begins in the stomach, where HCl in stomach acid
activates pepsin to hydrolyze peptide bonds.
• continues in the small intestine, where trypsin and
chymotrypsin hydrolyze peptides to amino acids.
• ends as amino acids enter the bloodstream for
transport to cells.
18
Digestion of Proteins
19
Learning Check
Match the end products of digestion with the types of
food.
1) amino acids
2) fatty acids and glycerol
3) glucose
A. fats
B. proteins
C. carbohydrates
20
Solution
Match the end products of digestion with the types of
food.
A. fats
B. proteins
C. carbohydrates
2) fatty acids and glycerol
1) amino acids
3) glucose
21
Chapter 18 Metabolic Pathways
and Energy Production
18.3
Important Coenzymes in
Metabolic Pathways
22
Oxidation and Reduction
To extract energy from foods
• oxidation reactions
involve a loss of 2 H (2H+ and 2 e–).
compound
oxidized compound + 2 H
• reduction reactions
require coenzymes that pick up 2 H.
coenzyme + 2H
reduced coenzyme
23
Coenzyme NAD+
NAD+ (nicotinamide adenine dinucleotide)
• participates in reactions that produce a carbon-oxygen double
bond (C=O).
• is reduced when an oxidation provides 2H+ and 2 e–.
Oxidation
O
||
CH3—C—H + 2H+ + 2 e–
CH3—CH2—OH
Reduction
NAD+ + 2H+ + 2 e–
NADH + H+
24
Structure of Coenzyme NAD+
NAD+ (nicotinamide adenine
dinucleotide)
• contains ADP,
ribose, and
nicotinamide.
• is reduced to
NADH when
NAD+ accepts 2H+
and 2 e–.
25
Coenzyme FAD
FAD (flavin adenine dinucleotide)
• participates in reactions that produce a carbon-carbon
double bond (C=C).
• is reduced to FADH2.
Oxidation
—CH2—CH2—
—CH=CH— + 2H+ + 2 e–
Reduction
FAD + 2H+ + 2 e–
FADH2
26
Structure of Coenzyme FAD
FAD (flavin adenine
dinucleotide)
• contains ADP
and riboflavin
(vitamin B2).
• is reduced to
FADH2 when
flavin accepts
2H+ and 2 e–.
27
Coenzyme A
Coenzyme A (CoA) activates acyl groups such as the two
carbon acetyl group for transfer.
O
||
CH3—C— + HS—CoA
Acetyl group
O
||
CH3—C—S—CoA
Acetyl CoA
28
Structure of Coenzyme A
Coenzyme A (CoA) contains
• pantothenic acid (Vitamin B3).
• ADP.
• aminoethanethiol.
29
Learning Check
Match the following.
1) NAD+
2) FAD
4) FADH2
5) Coenzyme A
3) NADH + H+
A. coenzyme used in oxidation of carbon-oxygen
bonds
B. reduced form of flavin adenine dinucleotide
C. used to transfer acetyl groups
D. oxidized form of flavin adenine dinucleotide
E. the coenzyme after C=O bond formation
30
Solution
Match the following.
1) NAD+
2) FAD
4) FADH2
5) Coenzyme A
3) NADH + H+
A. 1 coenzyme used in oxidation of carbon-oxygen
bonds
B. 4 reduced form of flavin adenine dinucleotide
C. 5 used to transfer acetyl groups
D. 2 oxidized form of flavin adenine dinucleotide
E. 3 the coenzyme after C=O bond formation
31
Chapter 18 Metabolic Pathways
and Energy Production
18.4
Glycolysis Oxidation
of Glucose
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32
Stage 2: Glycolysis
Stage 2: Glycolysis
• is a metabolic
pathway that uses
glucose, a digestion
product.
• degrades six-carbon
glucose molecules to
three-carbon
pyruvate molecules.
• is an anaerobic (no
oxygen) process.
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33
Glycolysis: Energy-Investment
In reactions 1-5 of glycolysis,
• energy is required to add phosphate groups to glucose.
• glucose is converted to two three-carbon molecules.
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34
Glycolysis: Energy Investment
3
1
4
5
2
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35
Glycolysis: Energy Production
In reactions 6-10 of
glycolysis, energy is
generated as
• sugar phosphates are
cleaved to triose
phosphates.
• four ATP molecules are
produced.
36
Glycolysis: Reactions 6-10
8
6
9
7
10
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37
Glycolysis: Overall Reaction
In glycolysis,
• two ATP add phosphate to glucose and fructose-6phosphate.
• four ATP form as phosphate groups add to ADP.
• there is a net gain of 2 ATP and 2 NADH.
C6H12O6 + 2ADP + 2Pi + 2NAD+
Glucose
2 C3H3O3– + 2ATP + 2NADH + 4H+
Pyruvate
38
Learning Check
In glycolysis, what compounds provide phosphate
groups for the production of ATP?
39
Solution
In glycolysis, what compounds provide phosphate
groups for the production of ATP?
In reaction 7, phosphate groups from two 1,3bisphosphoglycerate molecules are transferred to
ADP to form two ATP.
In reaction 10, phosphate groups from two
phosphoenolpyruvate molecules are used to form
two more ATP.
40
Pyruvate: Aerobic Conditions
Under aerobic conditions (oxygen present),
• three-carbon pyruvate is decarboxylated.
• two-carbon acetyl CoA and CO2 are produced.
Pyruvate
O O
dehydrogenase
|| ||
CH3—C—C—O– + HS—CoA + NAD+
Pyruvate
O
||
CH3—C—S—CoA + CO2 + NADH
Acetyl CoA
41
Pyruvate: Anaerobic Conditions
Under anaerobic conditions (without oxygen),
• pyruvate is reduced to lactate.
• NADH oxidizes to NAD+, allowing glycolysis to
continue.
O O
|| ||
CH3—C—C—O–+ NADH + H+
Pyruvate
Lactate
dehydrogenase
OH O
|
||
CH3—CH—C—O– + NAD+
Lactate
42
Lactate in Muscles
During strenuous exercise,
• anaerobic conditions are produced in muscles.
• oxygen is depleted.
• lactate accumulates.
OH
│
C6H12O6 + 1ADP + 2Pi 2CH3–CH –COO– + 2ATP
Glucose
Lactate
• muscles tire and become painful.
After exercise, a person breathes heavily to repay the
oxygen debt and reform pyruvate in the liver.
43
Pathways for Pyruvate
44
Learning Check
Match the following terms with the descriptions.
1) catabolic reactions
2) coenzymes
3) glycolysis
4) lactate
A. produced during anaerobic conditions
B. reaction series that converts glucose to pyruvate
C. metabolic reactions that break down large molecules to
smaller molecules + energy
D. substances that remove or add H atoms in oxidation and
reduction reactions
45
Solution
Match the following terms with the descriptions.
1) catabolic reactions
2) coenzymes
3) glycolysis
4) lactate
A. 4 produced during anaerobic conditions
B. 3 reaction series that converts glucose to pyruvate
C. 1 metabolic reactions that break down large molecules
to smaller molecules + energy
D. 2 substances that remove or add H atoms in oxidation
and reduction reactions
46
Chapter 18 Metabolic Pathways
and Energy Production
18.5
The Citric Acid Cycle
47
Citric Acid Cycle
In Stage 3, the citric acid cycle
• operates under aerobic conditions only.
• oxidizes the two-carbon acetyl group in acetyl CoA to
2CO2.
• produces reduced coenzymes NADH and FADH2,
and one ATP directly.
48
Citric Acid Cycle Overview
In the citric acid cycle,
• acetyl (2-carbon)
bonds to oxaloacetate
(4-carbon) to form
citrate (6-carbon).
• oxidation and
decarboxylation
reactions convert
citrate to oxaloacetate.
• oxaloacetate bonds
with another acetyl to
repeat the cycle.
49
Citric Acid Cycle
50
Reaction 1: Formation of Citrate
Oxaloacetate combines with the two-carbon acetyl
group to form citrate.
–
–
COO
C O
CH2
COO
O
CH3 C SCoA
–
COO
Oxaloacetate Acetyl CoA
CH2
–
HO C COO
CH2
–
COO
Citrate
51
Reaction 2: Isomerization to
Isocitrate
Citrate
• isomerizes to isocitrate.
• converts the tertiary –OH group in citrate to a
secondary –OH in isocitrate that can be oxidized.
–
–
COO
COO
CH2
CH2
–
HO C COO
CH2
–
COO
Citrate
–
H C COO
HO C H
–
COO
Isocitrate
52
Summary of Reactions 1 and 2
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53
Reaction 3: Oxidative
Decarboxylation (1)
Isocitrate undergoes decarboxylation (carbon removed
as CO2).
• The –OH oxidizes to a ketone releasing H+ and 2 e–.
• Coenzyme NAD+ is reduced to NADH.
COO
–
CH2
H C C OO
HO C H
COO
–
Isocitrate
C OO
–
NAD+
–
C H2
H C H
C O + C O 2 + N AD H
C OO
–
α-Ketoglutarate
54
Reaction 4: Oxidative
Decarboxylation (2)
α-Ketoglutarate
• undergoes decarboxylation to form succinyl CoA.
• produces a 4-carbon compound that bonds to CoA.
• provides H+ and 2 e– to form NADH.
–
–
COO
COO
CH2
CH2
NAD+
CH2
C O
CoA-SH
–
COO
α-Ketoglutarate
CH2 + CO2 + NADH
C O
S
CoA
Succinyl CoA
55
Summary Reactions 3 and 4
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56
Reaction 5: Hydrolysis
Succinyl CoA undergoes hydrolysis, adding a phosphate
to GDP to form GTP, a high-energy compound.
COO-
COO-
CH2
CH2
+ GDP + Pi
C O
S CoA
Succinyl CoA
CH2
CH2 +
CoA + GTP
COOSuccinate
57
Reaction 6: Dehydrogenation
Succinate undergoes dehydrogenation
•
by losing 2 H and forming a double bond.
•
providing 2 H to reduce FAD to FADH2.
–
COO
CH2
CH2
+ FAD
–
COO
Succinate
–
COO
C H
+ FADH2
H C
–
COO
Fumarate
58
Summary of Reactions 5 and 6
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59
Reaction 7: Hydration of Fumarate
Fumarate forms malate when water is added to the
double bond.
–
–
COO
COO
C H
HO C
+
H C
–
COO
fumarate
Fumarate
H2O
H
H C H
–
COO
malate
Malate
60
Reaction 8: Dehydrogenation
Malate undergoes dehydrogenation
• to form oxaloacetate with a C=O double bond.
• providing 2 H for reduction of NAD+ to NADH + H+.
CO O
–
HO C H +
m
alate
Malate
NAD+
–
NA D H + H +
C O
CH 2
H C H
CO O
CO O
–
CO O
–
oOxaloacetate
xaloacetate
61
Summary of Reactions 7 and 8
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62
Summary of the Citric Acid Cycle
In the citric acid cycle,
•
•
•
•
oxaloacetate bonds with an acetyl group to form
citrate.
two decarboxylations remove two carbons as 2CO2.
four oxidations provide hydrogen for 3NADH and one
FADH2.
a direct phosphorylation forms GTP.
63
Overall Chemical Reaction for the
Citric Acid Cycle
Acetyl CoA + 3NAD+ + FAD + GDP + Pi + 2H2O
2CO2 + 3NADH + 2H+ + FADH2 + CoA + GTP
64
Learning Check
How many of each are produced in one turn of the
citric acid cycle?
A ___ CO2
B. ___ NADH
C. ___ FADH2
D. ___ GTP
65
Solution
How many of each are produced in one turn of the
citric acid cycle?
A 2 CO2
B. 3 NADH
C. 1 FADH2
D. 1 GTP
66
Chapter 18 Metabolic Pathways
and Energy Production
18.6
Electron Transport
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67
Electron Carriers
Electron carriers
• accept hydrogen and
electrons from the
reduced coenzymes
NADH and FADH2.
• are oxidized and reduced
to provide energy for the
synthesis of ATP.
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68
Electron Transport
Electron transport
• uses electron carriers.
• transfers hydrogen ions and electrons from NADH and
FADH2 until they combine with oxygen.
• forms H2O.
• produces ATP energy.
69
Electron Carriers
Electron carriers
• are oxidized and reduced as hydrogen and/or electrons
are transferred from one carrier to the next.
• are FMN, Fe-S, Coenzyme Q, and cytochromes.
electron carrier AH2(reduced)
electron carrier A(oxidized)
electron carrier B(oxidized)
electron carrier BH2(reduced)
70
Iron-Sulfur (Fe-S) Clusters
Fe-S clusters
• are groups of proteins containing iron ions and sulfide.
• accept electrons to reduce Fe3+ to Fe2+, and lose
electrons to re-oxidize Fe2+ to Fe3+.
71
FMN (Flavin Mononucleotide)
FMN coenzyme
• contains flavin,
ribitol, and a
phosphate.
• accepts 2H+ +
2 e– to form
reduced
coenzyme
FMNH2.
72
Coenzyme Q (Q or CoQ)
Coenzyme Q (Q or CoQ) is
• a mobile electron carrier derived from quinone.
• reduced when the keto groups accept 2H+ and 2 e–.
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73
Cytochromes
Cytochromes (cyt)
are
• proteins containing
heme groups with
iron ions.
Fe2+
Fe3+ + 1 e–
• abbreviated as:
cyt a, cyt a3, cyt b,
cyt c, and cyt c1.
74
Learning Check
Write the abbreviation for each.
A. reduced form of coenzyme Q
B. oxidized form of flavin mononucleotide
C. reduced form of iron in cytochrome c
75
Solution
Write the abbreviation for each.
A. reduced form of coenzyme Q
CoQH2, or QH2
B. oxidized form of flavin mononucleotide
FMN
C. reduced form of cytochrome c
Cyt c (Fe2+)
76
Learning Check
Indicate whether the electron carrier in each is oxidized
or reduced.
A. FMNH2
FMN
B. Cyt b (Fe3+)
Cyt b (Fe2+)
C. Q
QH2
D. Cyt c (Fe2+)
Cyt c (Fe3+)
77
Solution
Indicate whether the electron carrier in each is oxidized
or reduced.
A. FMNH2
FMN
B. Cyt b (Fe3+)
Cyt b (Fe2+) reduced
C. Q
QH2
D. Cyt c (Fe2+)
Cyt c (Fe3+) oxidized
oxidized
reduced
78
Electron Transport System
In the electron transport system, the electron carriers
are
• attached to the inner membrane of the mitochondrion.
• organized into four protein complexes.
Complex I
Complex II
Complex III
Complex IV
NADH dehydrogenase
Succinate dehydrogenase
CoQ-Cytochrome c reductase
Cytochrome c oxidase
79
Electron Transport Chain
80
Complex I: NADH Dehydrogenase
At Complex I,
• hydrogen and electrons are transferred from NADH to
FMN.
NADH + H+ + FMN
NAD+ + FMNH2
• FMNH2 transfers hydrogen to Fe-S clusters and then to
coenzyme Q, reducing Q and regenerating FMN.
FMNH2 + Q
QH2 + FMN
• QH2, a mobile carrier, transfers hydrogen to Complex III.
81
Complex II:
Succinate Dehydrogenase
At Complex II, with a lower energy level than Complex I,
• FADH2 transfers hydrogen and electrons to coenzyme
Q, reducing Q and regenerating FAD.
FADH2 + Q
QH2 + FAD
• QH2, a mobile carrier, transfers hydrogen to
Complex III.
82
Complex III:
CoQ-Cytochrome c reductase
At Complex III, electrons are transferred
• from QH2 to two Cyt b, which reduces Cyt b and
regenerates Q.
2Cyt b (Fe3+) + QH2
2Cyt b (Fe2+) + Q + 2H+
• from Cyt b to Fe-S clusters and to Cyt c, the second
mobile carrier.
2Cyt c (Fe3+) + 2Cyt b (Fe2+)
2Cyt c (Fe2+) + 2Cyt b (Fe3+)
83
Complex IV:
Cytochrome c Oxidase
• At Complex IV, electrons are transferred from
• Cyt c to Cyt a..
2Cyt a (Fe3+) + 2Cyt c (Fe2+)
2Cyt a (Fe2+) + 2Cyt c (Fe3+)
• Cyt a to Cyt a3, which provides the electrons to
combine H+ and oxygen to form water.
4H+ + O2 + 4 e- (from Cyt a3)
2H2O
84
Complex IV
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85
Learning Check
Match each with their function.
1) FMN
2) Q
3) Cyt c
A. accepts H and electrons from NADH + H+
B. a mobile carrier between Complex II and III
C. carries electrons from Complex I and II to
Complex III
D. accepts H and electrons from FADH2
86
Solution
Match each with their function.
1) FMN
2) Q
3) Cyt c
A. 1 accepts H and electrons from NADH + H+
B. 3 a mobile carrier between Complex II and III
C. 2 carries electrons from Complex I and II to
Complex III
D. 2 accepts H and electrons from FADH2
87
Learning Check
Classify each as a product of the
1) citric acid cycle or 2) electron transport chain.
A.
B.
C.
D.
E.
CO2
FADH2
NAD+
NADH
H2O
88
Solution
Classify each as a product of the
1) citric acid cycle or 2) electron transport chain.
A.
B.
C.
D.
E.
1
1
2
1
2
CO2
FADH2
NAD+
NADH
H2O
89
Chapter 18 Metabolic Pathways
and Energy Production
18.7
Oxidation Phosphorylation and ATP
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90
Chemiosmotic Model
In the chemiosmotic model
• protons (H+) from Complexes I, III, and IV move
into the intermembrane space.
• a proton gradient is created.
• protons return to matrix through ATP synthase, a
protein complex.
• the flow of protons provides energy for ATP
synthesis (oxidative phosphorylation).
ADP + Pi + Energy
ATP
91
ATP Synthase
At ATP synthase,
• protons flow back to
the matrix through a
channel in the
protein complex.
• energy is generated
to drive ATP
synthesis.
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92
Chemiosmotic Model of Electron
Transport
93
Electron Transport and ATP
In electron transport, sufficient energy is provided from
• NADH (Complex I) oxidation for 3 ATPs.
NADH + 3 ADP + 3Pi
NAD+ + 3 ATP
• FADH2 (Complex II) oxidation for 2 ATPs.
FADH2 + 2 ADP + 2Pi
FAD + 2 ATP
94
ATP from Electron Transport
95
ATP Energy from Glucose
The complete
oxidation of
glucose yields
• 6CO2,
• 6H2O, and
• 36 ATP.
96
ATP from Glycolysis
In glycolysis
• glucose forms 2 pyruvate, 2 ATP, and 2 NADH.
• NADH produced in the cytoplasm cannot enter the
mitochondria.
• a shuttle compound (glycerol-3-phosphate) moves
hydrogen and electrons into the mitochondria to FAD,
which forms FADH2.
• each FADH2 provides 2 ATP.
Glucose
2 pyruvate + 6 ATP
97
ATP from Glycolysis
Reaction Pathway
ATP for One Glucose
ATP from Glycolysis
Activation of glucose
-2 ATP
Oxidation of 2 NADH (as FADH2)
4 ATP
Direct ADP phosphorylation (two triose)
4 ATP
6 ATP
Summary:
2 pyruvate + 2H2O + 6 ATP
C6H12O6
glucose
98
ATP from Two Pyruvate
Under aerobic conditions
• 2 pyruvate are oxidized to 2 acetyl CoA and 2 NADH.
• 2 NADH enter electron transport to provide 6 ATP.
Summary:
2 Pyruvate
2 Acetyl CoA + 6 ATP
99
ATP from Citric Acid Cycle
• One turn of the citric acid cycle provides
3 NADH x 3 ATP
=
9 ATP
1 FADH2 x 2 ATP
=
2 ATP
1 GTP
x 1 ATP
=
1 ATP
Total
=
12 ATP
Acetyl CoA
2CO2 + 12 ATP
• Because each glucose provides two acetyl CoA, two
turns of the citric acid cycle produce 24 ATP.
2 Acetyl CoA
4CO2 + 24 ATP
100
ATP from Citric Acid Cycle
Reaction Pathway
ATP for One Glucose
ATP from Citric Acid Cycle
Oxidation of 2 isocitrate (2 NADH)
6 ATP
Oxidation of 2 α-ketoglutarate (2 NADH)
6 ATP
2 direct substrate phosphorylations (2 GTP)
2 ATP
Oxidation of 2 succinate (2 FADH2)
4 ATP
Oxidation of 2 malate (2 NADH)
6 ATP
Total 24 ATP
Summary: 2 Acetyl CoA
4CO2 + 2H2O + 24 ATP
101
Total ATP from Glucose
One glucose molecule undergoing complete oxidation
provides:
From glycolysis
6 ATP
From 2 Pyruvate
6 ATP
From 2 Acetyl CoA
24 ATP
Overall ATP Production for One Glucose:
C6H12O6 + 6O2 + 36 ADP + 36Pi
Glucose
6CO2 + 6H2O + 36 ATP
102
Learning Check
Indicate the ATP yield for each under aerobic conditions.
A.
B.
C.
D.
E.
Complete oxidation of glucose
FADH2
Acetyl CoA in citric acid cycle
NADH
Pyruvate decarboxylation
103
Solution
Indicate the ATP yield for each under aerobic conditions.
A.
B.
C.
D.
E.
Complete oxidation of glucose
FADH2
Acetyl CoA in citric acid cycle
NADH
Pyruvate decarboxylation
36 ATP
2 ATP
12 ATP
3 ATP
3 ATP
104
Chapter 18 Metabolic Pathways
and Energy Production
18.8
Oxidation of Fatty Acids
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105
β-Oxidation of Fatty Acids
In reaction 1, oxidation
• removes H atoms from
the α and β carbons.
• forms a trans C=C bond.
• reduces FAD to FADH2.
β α
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106
β-Oxidation of Fatty Acids
In reaction 2, hydration
• adds water across the
trans C=C bond.
• forms a hydroxyl group
(—OH) on the β
carbon.
βα
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107
β-Oxidation of Fatty Acids
In reaction 3, a second
oxidation
• oxidizes the hydroxyl
group.
• forms a keto group on
the β carbon.
• reduces NAD+ to
NADH.
β α
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108
β-Oxidation of Fatty Acids
In reaction 4, fatty
acyl CoA is split
• between the α and β
carbons.
• to form acetyl CoA
and a shortened fatty
acyl CoA that
repeats Steps 1-4 of
β-oxidation.
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109
Learning Check
Match the reactions of β-oxidation with each.
1) oxidation 1
2) hydration
3) oxidation 2
4) fatty acyl CoA cleaved
A.
B.
C.
D.
E.
Water is added.
FADH2 forms.
A two-carbon unit is removed.
A hydroxyl group is oxidized.
NADH forms.
110
Solution
Match the reactions of β-oxidation with each:
1) oxidation 1
2) hydration
3) oxidation 2
4) fatty acyl CoA cleaved
A.
B.
C.
D.
E.
2
1
4
3
3
Water is added.
FADH2 forms.
A two-carbon unit is removed.
A hydroxyl group is oxidized.
NADH forms.
111
Cycles of β-Oxidation
The number of β-oxidation cycles
• depends on the length of a fatty acid.
• is one less than the number of acetyl CoA groups
formed.
Carbons in
Acetyl CoA
β-Oxidation Cycles
Fatty Acid
(C/2)
(C/2 –1)
12
6
5
14
7
6
16
8
7
18
9
8
112
β-Oxidation of Myristic (C14) Acid
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113
β-Oxidation of Myristic (C14) Acid
(continued)
6 cycles
7 Acetyl
CoA
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114
Learning Check
A. The number of acetyl CoA groups produced by the
complete β-oxidation of palmitic acid (C16) is
1) 16.
2) 8.
3) 7.
B. The number of oxidation cycles to completely
oxidize palmitic acid (C16 ) is
1) 16.
2) 8.
3) 7.
115
Solution
A. The number of acetyl CoA groups produced by the
complete β-oxidation of palmitic acid (C16 ) is
2) 8.
B. The number of oxidation cycles to completely oxidize
palmitic acid (C16) is
3) 7.
116
β-Oxidation and ATP
• Activation of a fatty acid requires
2 ATP
• One cycle of oxidation of a fatty acid produces
1 NADH
3 ATP
1 FADH2
2 ATP
• Acetyl CoA entering the citric acid cycle produces
1 Acetyl CoA
12 ATP
117
ATP for Lauric Acid C12
ATP production for lauric acid (12 carbons):
Activation of lauric acid
-2 ATP
6 acetyl CoA
6 acetyl CoA x 12 ATP/acetyl CoA
72 ATP
5 oxidation cycles
5 NADH x 3 ATP/NADH
5 FADH2 x 2 ATP/FADH2
15 ATP
10 ATP
Total
95 ATP
118
Learning Check
The total ATP produced from the β-oxidation of
stearic acid (C18) is.
1) 108 ATP.
2) 146 ATP.
3) 148 ATP.
119
Solution
The total ATP produced from the β-oxidation of
stearic acid (C18) is:
2) 146 ATP
Activation
9 Acetyl CoA x 12 ATP
8 NADH x 3 ATP
8 FADH2 x 2 ATP
-2 ATP
108 ATP
24 ATP
16 ATP
146 ATP
120
Ketone Bodies
If carbohydrates
are not available,
• body fat breaks
down to meet
energy needs.
• compounds
called ketone
bodies form.
Ketone
bodies
121
Formation of Ketone Bodies
Ketone bodies form
• if large amounts of acetyl CoA accumulate.
• when two acetyl CoA molecules form acetoacetyl CoA.
• when acetoacetyl CoA hydrolyzes to acetoacetate.
• when acetoacetate reduces to β-hydroxybutyrate or
loses CO2 to form acetone, both ketone bodies.
122
Ketosis
Ketosis occurs
• in diabetes, diets high in
fat, and starvation.
• as ketone bodies
accumulate.
• when acidic ketone
bodies lower blood pH
below 7.4 (acidosis).
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123
Ketone Bodies and Diabetes
In diabetes
• insulin does not
function properly.
• glucose levels are
insufficient for energy
needs.
• fats are broken down
to acetyl CoA.
• ketone bodies form.
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124
Chapter 18 Metabolic Pathways
and Energy Production
18.9
Degradation of Amino Acids
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125
Proteins in the Body
Proteins provide
• amino acids for
protein synthesis.
• nitrogen atoms for
nitrogen-containing
compounds.
• energy when
carbohydrate and
lipid resources are
not available.
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126
Transamination
In transamination
• amino acids are degraded in the liver.
• an amino group is transferred from an amino acid to
an α-keto acid, usually α-ketoglutarate.
• a new amino acid, usually glutamate, is formed.
• a new α-keto acid is formed.
127
A Transamination Reaction
+
NH3
|
CH3—CH—COO– +
Pyruvate
(new α-ketoacid)
O
||
–OOC—C—CH —CH —COO–
2
2
α-Ketoglutarate
Alanine
O
||
CH3—C—COO–
Alanine
transaminase
+
NH3+
|
–OOC—CH—CH —CH —COO–
2
2
Glutamate
(new amino acid)
128
Synthesis of Amino Acids
In humans, transamination of compounds from
glycolysis or the citric acid cycle produces nonessential
amino acids.
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129
Oxidative Deamination
Oxidative deamination
• removes the amino group as an ammonium ion from
glutamate.
• provides α-ketoglutarate for transamination.
NH3+
Glutamate
│
dehydrogenase
–OOC—CH—CH —CH —COO– + NAD+ + H O
2
2
2
Glutamate
O
||
–OOC—C—CH —CH —COO– + NH + + NADH
2
2
4
α-Ketoglutarate
130
Learning Check
Write the structures and names of the products for the
transamination of α-ketoglutarate by aspartate.
NH3+
|
–OOC—CH—CH —COO–
2
aspartate
O
||
–OOC—C—CH —CH —COO–
2
2
α-ketoglutarate
131
Solution
Write the structures and names of the products for the
transamination of α-ketoglutarate by aspartate.
O
||
–OOC—C—CH —COO–
2
Oxaloacetate
NH3+
|
–OOC—CH—CH —CH —COO–
2
2
Glutamate
132
Urea Cycle
The urea cycle
• removes toxic ammonium ions from amino acid
degradation.
• converts ammonium ions to urea in the liver.
O
||
+
H2N—C—NH2
2NH4 + CO2
Ammonium ion
Urea
• produces 25-30 g of urea daily for urine formation
in the kidneys.
133
Carbon Atoms from Amino Acids
Carbon skeletons of amino acids
• form intermediates of the citric acid cycle.
• produce energy.
Three-carbon skeletons:
alanine, serine, and cysteine
Four-carbon skeletons:
aspartate, asparagine
Five-carbon skeletons:
glutamine, glutamate, proline,
arginine, histidine
pyruvate
oxaloacetate
glutamate
134
Intermediates of the Citric Acid
Cycle from Amino Acids
135
Learning Check
Match each the intermediate with the amino acid that
provides its carbon skeleton.
1) pyruvate 2) fumarate 3) α-ketoglutarate
A.
B.
C.
D.
cysteine
glutamine
aspartate
serine
136
Solution
Match each the intermediate with the amino acid that
provides its carbon skeleton.
1) pyruvate 2) fumarate 3) α-ketoglutarate
A. 1
B. 3
C. 2
D. 1
cysteine
glutamine
aspartate
serine
137
Overview of Metabolism
In metabolism
• catabolic pathways degrade large molecules.
• anabolic pathway synthesize molecules.
• branch points determine which compounds are
degraded to acetyl CoA to meet energy needs or
converted to glycogen for storage.
• excess glucose is converted to body fat.
• fatty acids and amino acids are used for energy when
carbohydrates are not available.
• some amino acids are produced by transamination.
138
Overview of Metabolism
139