Chapter
Twenty Three
Biochemical Energy
Production
Metabolism
• The sum total of all the biochemical reactions that
take place in a living organism
Ch 23 | # 2 of 51
Cell Structure
Metabolic Reactions occur in specific
sites within cells
Typical animal cell
• Nucleus
– Chromosomes in the nucleus contain
genetic material
• Cytoplasm is material between
nucleus and cell membrane
• Mitochondria are where energyproducing reactions occur
Ch 23 | # 3 of 51
Biochemical Energy Production
Ch 23 | # 4 of 51
Biochemical Energy Production
© R. Bhatnagar / Visuals
Unlimited
(a) Representation of a mitochondria. (b) micrograph of a
mitochondria crista.
Ch 23 | # 5 of 51
ATP
• Energy is released as food is oxidized
• Used to form ATP from ADP and Pi
ADP + Pi +
Energy
ATP
• In cells, energy is provided by the hydrolysis of ATP (31
kJ/mole of ATP)
ATP
ADP + Pi + Energy
Ch 23 | # 6 of 51
Biochemical Energy Production
Ch 23 | # 7 of 51
High-Energy Phosphate Compounds
• Phosphate containing compounds that have a
greater free energy of hydrolysis than that of a
typical compound
– The energy of hydrolysis is large because of strong
repulsive forces between electronegative atoms
– Enough energy is released by their hydrolysis to
compensate for the energy needed for ATP production
Ch 23 | # 8 of 51
Major Coenzymes in Metabolic Reactions
• NAD+/NADH
• FAD/FADH2
• Coenzyme A (CoA-SH)
Ch 23 | # 9 of 51
Major Coenzymes in Metabolic Reactions
(a) Flavin adenine nucleotide (b) nicotinamide adenine dinucleotide
Ch 23 | # 10 of 51
Major Coenzymes in Metabolic Reactions
Ch 23 | # 11 of 51
Biochemical Energy Production
Structural formula for coenzyme A
CoA-SH
Ch 23 | # 12 of 51
Coenzyme NAD+
• In cells, the oxidation of compounds provides 2H
as 2H+ and 2e- that reduce coenzymes
• NAD+ (nicotinamide adenine dinucleotide)
participates in reactions that produce a carbonoxygen double bond (C=O)
Oxidation
CH3-CH2-OH  CH3-CHO + 2H+ + 2eReduction
NAD+ + 2H+ + 2e-  NADH + H+
Ch 23 | # 13 of 51
Coenzyme FAD
• FAD participates in reactions that produce a
carbon-carbon double bond (C=C)
Oxidation
-CH2-CH2-  -CH=CH- + 2H+ + 2eReduction
FAD + 2H+ + 2e-  FADH2
Ch 23 | # 14 of 51
Biochemical Energy Production
Classification of metabolic intermediate compounds in terms of
function.
Ch 23 | # 15 of 51
Free Energies of
Hydrolysis of
Phosphate
Containing
Compounds
Ch 23 | # 16 of 51
Biochemical Energy Production
Hans Adolf Krebs
received the Nobel
Prize in medicine.
Hulton Archive / Getty Images
Ch 23 | # 17 of 51
Stages of Metabolism
Catabolic reactions are organized as stages
• In Stage 1, digestion breaks down large molecules
into smaller ones that enter the bloodstream.
• In Stage 2, molecules in the cells are broken down
to two- and three-carbon compounds
Ch 23 | # 18 of 51
Digestion of Foods
Digestion is the first step of catabolism
• Carbohydrates
glucose, fructose,
galactose
• Proteins
amino acids
• Lipids
glycerol
fatty acids
Ch 23 | # 19 of 51
Stages of Metabolism
• In Stage 3, compounds are oxidized in the citric
acid cycle to provide NADH and FADH2 molecules
(reduced forms of coenzymes)
• In Stage 4, NADH and FADH2 are oxidized in
order to provide energy for the production of ATP
Ch 23 | # 20 of 51
Ch 23 | # 21 of 51
Citric Acid Cycle
The citric acid cycle:
• Operates under aerobic conditions only
• Oxidizes the two-carbon acetyl group in acetyl
CoA to CO2
• Produces reduced coenzymes NADH and FADH2
and one ATP directly
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Ch 23 | # 23 of 51
Reaction 1: Formation of Citrate
• Oxaloacetate combines with the two carbon acetyl
group to form citrate
Ch 23 | # 24 of 51
Reaction 2: Formation of Isocitrate
• Citrate isomerizes to isocitrate
• The tertiary –OH group in citrate is converted to
a secondary –OH group that can be oxidized
Ch 23 | # 25 of 51
Reaction 3: Oxidative Decarboxylation (1)
• A decarboxylation removes a carbon as CO2 from
isocitrate.
• The –OH group is oxidized to a ketone, releasing
H+ and 2e- that form reduced coenzyme NADH
Ch 23 | # 26 of 51
Reaction 4: Oxidative Decarboxylation (2)
• In a second decarboxylation, a carbon is removed
as CO2 from a-ketoglutarate
• The 4-carbon compound bonds to coenzyme A,
providing H+ and 2e- to form NADH
Ch 23 | # 27 of 51
Reaction 5: Hydrolysis
• The hydrolysis of the thioester bond releases
energy to add phosphate to GDP and form GTP, a
high energy compound
Ch 23 | # 28 of 51
Reaction 6: Dehyrogenation
• In this oxidation, two H are removed from
succinate to form a double bond in fumarate
• FAD is reduced to FADH2
Ch 23 | # 29 of 51
Reaction 7: Hydration of Fumarate
• Water is added to the double bond in fumarate to
form malate
Ch 23 | # 30 of 51
Reaction 8: Dehyrogenation
• Another oxidation forms a C=O bond
• The hydrogens from the oxidation form NADH +
H+
Ch 23 | # 31 of 51
Summary of Products in 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
Ch 23 | # 32 of 51
Overall Chemical Reaction for the Citric
Acid Cycle
Acetyl CoA + 3NAD+ + FAD
+ GDP + Pi + 2H2O
2CO2 + 3NADH + 2H+ + FADH2
+ HS-CoA + GTP
Ch 23 | # 33 of 51
Regulation of Citric Acid Cycle
• Low levels of ATP stimulate the formation of acetyl
CoA for the citric acid cycle
• High ATP and NADH levels decrease the
formation of acetyl CoA and slow down the citric
acid cycle
Ch 23 | # 34 of 51
Regulation of Citric Acid Cycle
The citric acid cycle:
• Increases its reaction rate when low levels of ATP
or NAD+ activate isocitrate dehydrogenase
• Slows when high levels of ATP or NADH inhibit
citrate synthetase (first step in cycle)
Ch 23 | # 35 of 51
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
Ch 23 | # 36 of 51
Oxidation-Reduction
• Electron carriers are continuously oxidized and
reduced as hydrogen and/or electrons are
transferred from one to the next
Electron carrier A (reduced)
Electron carrier B (oxidized)
Electron carrier A (oxidized)
Electron carrier B (reduced)
Ch 23 | # 37 of 51
Electron Transport Chain
• A series of biochemical reactions in which
electrons and hydrogen ions from NADH and
FADH2 are passed to intermediate electron
carriers and then ultimately react with molecular
oxygen to produce water
• Most of the enzymes for the Electron Transport
Chain are found in the inner mitochondrial
membrane (found in the order in which they are
needed)
Ch 23 | # 38 of 51
Biochemical Energy Production
Ch 23 | # 39 of 51
Biochemical Energy Production
(a) The oxidized form
and reduced form of
the electron carrier
flavin mononucleotide.
(b) The oxidized form
and reduced form of
the electron carrier
coenzyme Q.
Ch 23 | # 40 of 51
Biochemical Energy Production
(a) CoQH2 carries
electrons from both
complexes I and II to
complex III. (b) NADH
is the substrate for the
complex I and FADH2
is the substrate for
complex II.
Ch 23 | # 41 of 51
Biochemical Energy Production
Electron movement through Complex III is initiated by
the electron carrier CoQH2.
Ch 23 | # 42 of 51
Biochemical Energy Production
The electrontransfer pathway
through Complex
IV.
Ch 23 | # 43 of 51
Biochemical Energy Production
Ch 23 | # 44 of 51
Biochemical Energy Production
Protein complexes I, III, and IV also act as proton pumps.
Ch 23 | # 45 of 51
Ch 23 | # 46 of 51
Chemiosmotic Model
In the chemiosmotic model:
• Complexes I, III, and IV pump protons into the
intermembrane space, creating a proton gradient.
• Protons must pass through ATP synthase to return
to the matrix
• The flow of protons through ATP synthase
provides the energy for ATP synthesis (oxidative
phosphorylation)
– ADP + Pi + Energy  ATP
Ch 23 | # 47 of 51
ATP Synthase
ATP Synthase has two portions:
• Protons flow back to the matrix through a channel
in the F0 complex.
• Proton flow provides the energy that drives ATP
synthesis by the F1 complex
Ch 23 | # 48 of 51
Ch 23 | # 49 of 51
ATP Production for the Common Metabolic
Pathway
• For every mole of NADH oxidized in the ETC, 2.5 moles of
ATP are formed
– 3 formed in one turn of citric acid cycle (7.5 ATP)
• For every mole of FADH2 oxidized in the ETC, 1.5 moles of
ATP are formed
– 1 formed in one turn of citric acid cycle (1.5 ATP)
• GTP is the equivalent of ATP
– 1 formed in one turn of the citric acid cycle (1 ATP)
10 ATP Overall!!!
Ch 23 | # 50 of 51
Biochemical Energy Production
The interconversion of ATP and ADP is the principal medium
for energy exchange in the biochemical processes.
Ch 23 | # 51 of 51