Download Chapter 10 - Clayton State University

Cellular Respiration
CHAPTER 10
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Chemotrophic Energy Metabolism:
Aerobic Respiration
• Some cells meet their energy needs through
anaerobic fermentation
• However, fermentation yields only modest
amounts of energy due to the absence of
electron transfer
• ATP yield is much higher in cellular
respiration
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Cellular Respiration: Maximizing
ATP Yields
• Cellular respiration (or respiration) uses an
external electron acceptor to oxidize substrates
completely to CO2
• External electron acceptor: one that is not a
by-product of glucose catabolism
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Cellular respiration defined
• Respiration is the flow of electrons through or
within a membrane, from reduced coenzymes
to an external electron acceptor usually
accompanied by the generation of ATP
• Coenzymes such as FAD (flavin adenine
dinucleotide) and coenzyme Q (ubiquinone)
are involved
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The terminal electron acceptor
• In aerobic respiration, the terminal electron
acceptor is oxygen and the reduced form is
water
• Other terminal electron acceptors (sulfur,
protons, and ferric ions) are used by other
organisms, especially bacteria and archaea
• These are examples of anaerobic respiration
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Mitochondria
• Most aerobic ATP production in eukaryotic
cells takes place in the mitochondrion
• In bacteria, the plasma membrane and
cyotoplasm are analogous to the
mitochondrial inner membrane and matrix
with respect to energy metabolism
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Figure 10-1
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Aerobic Respiration Yields Much More
Energy than Fermentation Does
• With O2 as the terminal electron acceptor, pyruvate
can be oxidized completely to CO2
• Aerobic respiration has the potential of generating
up to 38 ATP molecules per glucose
• Oxygen provides a means of continuous
reoxidation of NADH and other reduced coenzymes
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Respiration Includes Glycolysis,
Pyruvate Oxidation, the TCA Cycle,
Electron Transport, and ATP Synthesis
• Respiration will be considered in five stages
– Stage 1: the glycolytic pathway
– Stage 2: pyruvate is oxidized to generate acetyl CoA
– Stage 3: acetyl Co A enters the tricarboxylic acid
cycle (TCA cycle), where it is completely oxidized to
CO2
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The five stages, continued
• Respiration will be considered in five stages
– Stage 4: electron transport, the transfer of electrons
from reduced coenzymes to oxygen coupled to
active transport of protons across a membrane
– Stage 5: The electrochemical proton gradient
formed in step 4 is used to drive ATP synthesis
(oxidative phosphorylation)
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The Mitochondrion: Where the Action
Takes Place
• The mitochondrion is called the “energy
powerhouse” of the eukaryotic cell
• These organelles are thought to have arisen from
bacterial cells
• Mitochondria have been shown to carry out all the
reactions of the TCA cycle, electron transport, and
oxidative phosphorylation
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Video: Mitochondria in 3-D
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Mitochondria Are Often Present Where
the ATP Needs Are Greatest
• Mitochondria are found in virtually all aerobic cells
of eukaryotes
• They are present in both chemotrophic and
phototrophic cells
• Mitochondria are frequently clustered in regions of
cells with the greatest need for ATP, e.g., muscle
cells
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Are Mitochondria Interconnected
Networks Rather than Discrete
Organelles?
• In electron micrographs, mitochondria usually
appear as oval structures
• However they can take various shapes and sizes,
depending on the cell type
• Their appearance under EM suggests that they are
large, and numerous discrete entities
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The Outer and Inner Membranes
Define Two Separate Compartments
and Three Regions
• A distinctive feature of mitochondria is the presence
of both outer and inner membranes
• The outer membrane contains porins that allow
passage of solutes with molecular weights up
to 5000
• The intermembrane space between the inner and
outer membranes is thus continuous with the cytosol
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The inner membrane
• The inner membrane of the mitochondria is
impermeable to most solutes, partitioning the
mitochondrion into two separate compartments
– The intermembrane space
– The interior of the organelle, or mitochondrial
matrix
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The inner boundary membrane and
cristae
• The portion of the inner membrane adjacent to
the intermembrane space is called the inner
boundary membrane
• The inner membrane is about 75% protein by
weight; the proteins include those involved in
solute transport, electron transport, and ATP
synthesis
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The cristae
• The inner membrane of most mitochondria has
many infoldings called cristae
• They increase surface area of the inner
membrane, and provide more space for electron
transport to take place
• The cristae provide localized regions, intracristal
spaces, where protons can accumulate during
electron transport
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The cristae (continued)
• The cristae are thought to be tubular structures
that associate in layers
• They have limited connections to the inner
boundary membrane through small openings,
crista junctions
• Cells with high metabolic activity seem to have
more cristae in their mitochondria
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The mitochondrial matrix
• The interior of the mitochondrion is filled with a
semi-fluid matrix
• The matrix contains many enzymes involved in
mitochondrial function as well as DNA molecules
and ribosomes
• Mitochondria contain proteins encoded by their own
DNA as well as some that are encoded by nuclear
genes
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Mitochondrial Functions Occur in
or on Specific Membranes and
Compartments
• Specific functions and pathways have been
localized within mitochondria by fractionation
studies
• Most of the enzymes involved in pyruvate
oxidation, the TCA cycle, and catabolism of fatty
acids and amino acids are found in the matrix
• Most electron transport intermediates are
integral inner membrane components
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Table 10-1
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Localization of Specific Mitochondrial
Functions
• Knoblike spheres called F1 complexes protrude
from the inner membrane into the matrix
• These are involved in ATP synthesis
• Each complex is an assembly of several
different polypeptides, and can be seen in an
electron micrograph using negative staining
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F1 complexes
• Each F1 complex is attached by a short protein
stalk to an Fo complex
• This is an assembly of hydrophobic polypeptides
embedded in the mitochondrial inner membrane
• This FoF1 complex is an ATP synthase that is
responsible for most of the ATP generation in the
mitochondria (and in bacterial cells as well)
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The Tricarboxylic Acid Cycle:
Oxidation in the Round
• In the presence of oxygen pyruvate is oxidized fully
to carbon dioxide with the released energy used to
drive ATP synthesis
• This involves the TCA (tricarboxylic acid) cycle, in
which citrate is an important intermediate
• The TCA cycle is also called the Krebs cycle after
Hans Krebs, whose lab played a key role in
elucidating the cycle
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The Tricarboxylic Acid Cycle
• The TCA cycle metabolized acetyl CoA, produced
from pyruvate decarboxylation
• Acetyl CoA can also arise from fatty acid oxidation
• Acetyl CoA transfers its acetate group to a fourcarbon acceptor called oxaloacetate, generating
citrate
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The fate of citrate
• After its formation, citrate undergoes two successive
decarboxylations
• It also goes through several oxidation steps
• Eventually oxaloacetate is regenerated, and can
accept two more carbons from acetyl CoA and the
cycle begins again
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The overall cycle
• Each round of the TCA cycle involves the entry of
two carbons, the release of two CO2, and the
regeneration of oxaloacetate
• Oxidation occurs at five steps, four in the cycle itself
and one when pyruvate is converted to acetyl CoA
• In each case, electrons are accepted by coenzymes
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Figure 10-6
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Figure 10-5
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Figure 10-5A
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Figure 10-5B
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Figure 10-5C
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Pyruvate Is Converted to Acetyl
Coenzyme A by Oxidative
Decarboxylation
• The glycolytic pathway ends with pyruvate, which is
small enough to enter the intermembrane space of
the mitochondrion
• At the inner mitochondrial membrane, a specific
symporter transports pyruvate into the matrix, along
with a proton
• Then, pyruvate is converted to acetyl CoA by
pyruvate dehydrogenase complex (PDH)
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Conversion of pyruvate
• The conversion is a decarboxylation because one
carbon is liberated as CO2
• It is also an oxidation because two electrons (and
one proton) are transferred to NAD+ to form NADH
• Coenzyme A contains the B vitamin
• Coenzyme A has a SH group that makes it a good
carrier of acetate (and other organic acids)
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Conversion of pyruvate
• The conversion of pyruvate to acetyl CoA can be
summarized as follows:
–
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The TCA Cycle Begins with the
Entry of Acetate as Acetyl CoA
• With each round of the TCA cycle, two carbon
atoms enter in organic form as acetate and leave in
inorganic form as carbon dioxide
• In the first reaction, TCA-1, the two-carbon acetate
group is transferred from acetyl CoA to oxaloacetate
(4C) to form citrate (6C)
• This reaction is catalyzed by citrate synthetase
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Figure 10-10
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Summing Up: The Products of the TCA
Cycle Are CO2, ATP, NADH, and FADH2
• The TCA cycle accomplishes the following:
1. Two carbons enter the cycle as acetyl CoA, which
joins oxaloacetate to form the six-carbon citrate
2. Decarboxylation occurs at two steps to balance the
input of two carbons by releasing two CO2
3. Oxidation occurs at four steps, with NAD+ the
electron acceptor in three steps and FAD in one
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The Products of the TCA Cycle Are CO2,
ATP, NADH, and FADH2 (continued)
• The TCA cycle accomplishes the following:
4. ATP is generated at one point, with GTP as an
intermediate in the case of animal cells
5. One turn of the cycle is completed as oxaloacetate,
the original 4C acceptor, is regenerated
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Summing up the TCA cycle
• The TCA cycle can be summarized as follows:
• Including glycolysis, pyruvate decarboxylation, and
the TCA cycle
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Several TCA Cycle Enzymes Are
Subject to Allosteric Regulation
• Like all metabolic pathways, the TCA cycle must be
carefully regulated to meet cellular needs
• Most of the control of the cycle involves allosteric
regulation of four key enzymes by specific effector
molecules
• Effector molecules may be activators or inhibitors
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Regulation of PDH
• PDH is reversibly inactivated by phosphorylation
and activated by dephosphorylation of one of its
protein components
• PDH is inhibited by ATP, which is abundant where
energy is plentiful
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The TCA Cycle Also Plays a Central
Role in the Catabolism of Fats and
Proteins
• The TCA cycle represents the main conduit of
aerobic energy metabolism for a variety of
substrates besides sugar, in particular, fats and
proteins
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Fat as a Source of Energy
• Fats are highly reduced compounds that liberate
more energy per gram upon oxidation than do
carbohydrates
• They are a long-term energy storage form for many
organisms
• Most fat is stored as deposits of triacylglycerols,
neutral triesters of glycerol and long-chain fatty acids
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Catabolism of triacylglycerols
• Triacylglycerol catabolism begins with their
hydrolysis to glycerol and free fatty acids
• The glycerol is channeled into the glycolytic
pathway
• Fatty acids are linked to coenzyme A, to form fatty
acyl CoAs, then degraded by b-oxidation
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b-oxidation
• b-oxidation is a catabolic process that generates
acetyl CoA and the reduced coenzymes NADH
and FADH2
• b-oxidation occurs in different compartments in
different organisms
• Here, we will focus on the mitochondrion of animals
using saturated fatty acids with an even number of
carbons as an energy source
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Fatty acid degradation
• Most fatty acids are oxidatively converted to acetyl
CoA in the mitochondrion
• These can be further catabolized in the TCA cycle
• The fatty acids are degraded in a series of
repetitive cycles, which removes two carbons at a
time until the fatty acid is completely degraded
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Steps of b-oxidation
• Each cycle involves
–
–
–
–
1.Oxidation (ATP hydrolysis)
2. Hydration (add water)
3. Reoxidation (NADH production)
4. Thiolysis (conversation to acetyl CoA)
• The result is the production of one FADH2, one
NADH, and one acetyl CoA per cycle
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Protein as a Source of Energy and
Amino Acids
• Besides their other numerous functions, proteins
can be catabolized to produce ATP if necessary
when carbohydrate and lipid stores are depleted
• Eventually cells undergo turnover of proteins and
protein-containing structures
• The resulting amino acids can be used to generate
new proteins or degraded for energy
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Protein catabolism
• Protein catabolism begins with hydrolysis of the
peptide bonds that link amino acids together
• This is called proteolysis and the enzymes
responsible for it are called proteases
• The products of proteolysis are small peptides
and free amino acids
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Endopeptidases and exopeptidases
• Further digestion of peptides is catalyzed by
peptidases
• Endopeptidases hydrolyze internal peptide bonds
• Exopeptidases remove successive amino acids
from the end of the peptide
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Amino acid catabolism
• Free amino acids can be catabolized for energy
• These are converted into intermediates of
mainstream catabolism in as few steps as possible
• The pathways differ for individual amino acids, but
all eventually lead to acetyl CoA, pyruvate, or a few
key TCA cycle intermediates
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Electron Transport: Electron Flow
from Coenzymes to Oxygen
• Chemotrophic energy metabolism through the TCA
cycle accounts for synthesis of 4 ATP per glucose
(2 from glycolysis and 2 from the TCA cycle)
• This accounts for only a small portion of the energy
in the original glucose molecule
• The remainder is stored in NADH and FADH2
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The Electron Transport System
Conveys Electrons from Reduced
Coenzymes to Oxygen
• Coenzyme reoxidation by transfer of electrons to
oxygen is called electron transport
• Electron transport and ATP generation are not
independent processes; they are functionally
linked to each other
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Electron Transport and Coenzyme
Oxidation
• Electron transport involves the highly exergonic
oxidation of NADH and FADH2 with O2 as the
terminal electron acceptor and so accounts for the
formation of water
•
•
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The Electron Transport System
• Electron transfer is carried out as a multistep
process involving an ordered series of reversibly
oxidized electron carriers functioning together
• This is called the electron transport system, ETS
• The ETS contains a number of integral membrane
proteins that are found in the inner mitochondrial
membrane (or plasma membrane of bacteria)
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Focus
• What are the major electron carriers?
• What is the sequence of these carriers?
• Understand the role of the organization of
carriers in the flow of electrons from reduced
coenzymes to oxygen is coupled to pumping
of proton across the membrane and ATP
synthesis.
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The Electron Transport System
Consists of Five Kinds of Carriers
• Flavoproteins
• Iron-sulfur proteins
• Cytochromes
• Copper-containing cytochromes
• Coenzyme Q
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Features of electron carriers
• Flavoproteins and coenzyme Q pump protons and
electrons.
• All except coenzyme Q are proteins with prosthetic
groups capable of being reversibly oxidized and
reduced
– Most are hydrophobic
– Most of these intermediates occur in the
membrane as large assemblies of proteins called
respiratory complexes
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Flavoproteins
• Membrane-bound flavoproteins use either (FAD)
or flavin adenine mononucleotide (FMN) as the
prosthetic group
– For example, NADH dehyrogenase
• What is the function?
• Flavoproteins transfer both electrons and protons
and are reversibly oxidized and reduced
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Figure 10-9
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Iron-Sulfur Proteins
• Iron-sulfur proteins are also called nonheme iron
proteins and have an iron-sulfur (Fe-S) center
complexed with cysteine groups of a protein
– The iron atoms in the center of the proteins are
the actual electron carriers; these alternate
between the Fe2+ (oxidized) and Fe3+ (reduced)
states
• They transfer one electron at a time, and no
protons
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Cytochromes
• Cytochromes also contain iron, but as part of a porphyrin
prosthetic group, heme.
– Where else in the body do you fine heme?
• There are five types: b, c, c1, a, and a3
– The iron atom of the heme group serves as the electron
carrier and transfers one electron at a time and no
protons
– Cytochrome c is a peripheral membrane protein loosely
associated with the outer surface of the membrane. It is
not part of a large complex and can diffuse more rapidly
• Important for transferring electrons between complexes
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Figure 10-13
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Copper-Containing Cytochromes
• In addition to their iron atoms, cytochromes a and
a3 contain a single copper atom bound to their
heme group
• It associates with the iron atom to form a bimetallic
iron-copper (Fe-Cu) center
– Copper ions can be reversibly converted from the
oxidized (Cu2+) to the reduced form (Cu+) by accepting or
donating single electrons
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Copper-Containing Cytochromes
(continued)
• The iron-copper center plays a critical role in
keeping an O2 molecule bound to the cytochrome
oxidase complex
• The oxygen is held there until it has picked up four
electrons and four protons, at which point two water
molecules are released
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Coenzyme Q
• The only nonprotein component of the ETS is
coenzyme Q (CoQ), a quinone
• Because of its ubiquitous occurrence in nature, it is
also called ubiquinone
• CoQ is reduced in two successive (one electron
plus one proton) steps to semiquinone (CoQH) and
then dihydroquinone (CoQH2)
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Coenzyme Q (continued)
• Unlike the proteins of the ETS, most of the CoQ is
freely mobile in the inner mitochondrial membrane
• CoQ is the most abundant electron carriers in the
membrane and occupy a central position in the ETS
– They serve as a collection point for electrons from the
reduced FMN and FAD-linked dehydrogenases in the
membrane
– A portion of the CoQ is tightly bound to specific
respiratory complexes
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Coenzyme Q (continued)
• CoQ accepts both protons and electrons when it is
reduced and releases both protons and electrons
when it is oxidized
• This is vital to its role in the active transport of
protons across the inner mitochondrial membrane
– It accepts protons on one side of the membrane,
diffuses across and releases the protons- proton
pump
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Most of the Carriers Are Organized into
Four Large Respiratory Complexes
• Although many electron carriers are part of the
ETS, most are organized into multiprotein
complexes
• Most are thought to be organized into four different
kinds of respiratory complexes
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Figure 10-15
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Table 10-3
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Properties of the Respiratory
Complexes
• Each respiratory complex consists of distinctive
assembly of polypeptides and prosthetic groups
• Complex I transfers electrons from NADH to CoQ
and is called the NADH-coenzyme Q oxidation
complex (or NADH dehydrogenase complex)
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The respiratory complexes
• Complex II transfers to CoQ the electrons derived
from succinate (FAD) in Reaction TCA-6 and it is
called the succinate-coenzyme Q oxidoreductase
complex, or succinate dehydrogenase
• Complex III is called the coenzyme Q-cytochrome
oxidoreductase complex because it accepts
electrons from coenzyme Q and passes them to
cytochrome c
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The respiratory complexes (continued)
• Complex III is also called cytochrome b/c1 complex
• Complex IV transfers electrons from cytochrome c
to oxygen and is called cytochrome c oxidase
• For each pair of electrons transported through
complexes I through IV, 10 protons are pumped
from the matrix to the intermembrane space
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The Role of Cytochrome c Oxidase
• Cytochrome c oxidase (complex IV) is the terminal
oxidase, transferring electrons directly to oxygen
• Cyanide and azide are toxic to most aerobic cells
because they bind the Fe-Cu center of cytochrome
c oxidase, blocking electron transport
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Incomplete reduction of oxygen
• Complexes I and III can also transfer electrons to
oxygen, resulting in its incomplete reduction
• This can generate toxic superoxide anion (O2) or
hydrogen peroxide (H2O2), both of which contribute
to cellular aging
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The Respiratory Complexes Move
Freely Within the Inner Membrane
• The protein complexes of the mitochondrial inner
membrane are mobile, free to diffuse within the
membrane
• The inner membrane has no cholesterol and is
very fluid, so the protein mobility is high
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Respirasomes
• Recent work suggests that the multiprotein
respiratory complexes are organized into
supercomplexes called respirasomes
• The association of several of the TCA cycle
dehydrogenases within the respirasomes suggests
that they function to minimize diffusion distances
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Coenzyme Q and cytochrome c
• Coenzyme Q is the “funnel” that collects electrons
from virtually every oxidation reaction in the cell
• Coenzyme Q and cytochrome c are both small
molecules that can diffuse rapidly within the
membrane (coenzyme Q) or on its surface
(cytochrome c)
• They are both quite numerous, which accounts for
observed rates of electron transfer
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Electron Transport and ATP Synthesis
Are Coupled Events
• The crucial link between electron transport and ATP
production is an electrochemical proton gradient
• It is established by the directional pumping of
protons across the membrane in which electron
transport is occurring
• ATP synthesis is coupled to electron transport
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ATP synthesis is dependent on
electron transport
• Certain chemicals known as uncouplers can abolish
the interdependence of the two processes
• These allow continued electron transport and O2
consumption in the absence of ATP synthesis
• Treatments that stop electron transport also inhibit
ATP synthesis, so ATP synthesis is dependent on
electron transport but the reverse is not true
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Respiratory Control of Electron
Transport
• The availability of ADP regulates the rate of
oxidative phosphorylation and thus of electron
transport
• This is called respiratory control
• Electron transport and ATP generation will be
favored when ADP concentration is high and
inhibited when ADP concentration is low
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The Chemiosmotic Model: The
“Missing Link” Is a Proton Gradient
• In 1961 Peter Mitchell proposed the chemiosmotic
coupling model
• The essential feature of the model is that the link
between electron transport and ATP formation is the
electrochemical potential across a membrane
• The electrochemical potential is created by the
pumping of protons across a membrane as electrons
are transferred through the respiratory complexes
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Coenzyme Oxidation Pumps Enough
Protons to Form 3 ATP per NADH and
2 ATP per FADH2
•
The transfer of two electrons from NADH is
accompanied by the pumping of a total of 10
protons (12 if the Q cycle is operating)
•
The number of protons required per molecule of
ATP is thought to be 3 or 4, with 3 regarded as
most likely
•
So, about 3 molecules of ATP are synthesized per
NADH oxidized
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Number of ATP generated is an estimate
• FADH2 donates electrons to complex II with higher
reduction potential, pumping 6 protons (8 if the
Q cycle is operating)
• So about 2 ATP are synthesized per FADH2
• These values are estimates, affected by an
organism’s specific ATP synthase and other factor
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The Chemiosmotic Model Is Affirmed by
an Impressive Array of Evidence
• Since its initial formulation, the chemiosmotic model
has become universally accepted as the link
between electron transport and ATP synthesis
• Several lines of evidence support the mode
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1. Electron Transport Causes Protons
to Be Pumped Out of the Mitochondrial
Matrix
• Mitchell and Moyle demonstrated experimentally that
the flow of electrons through the ETS is
accompanied by the unidirectional pumping of
protons across the inner mitochondrial membrane
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5. Uncoupling Agents Abolish Both the
Proton Gradient and ATP Synthesis
• Dinitrophenol (DNP) is known to uncouple ATP
synthesis from electron transport
• When membranes are treated with DNP, they allow
protons to cross the membrane freely, so that no
proton gradient can be formed
• ATP synthesis is abolished as well
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6. The Proton Gradient Has Enough
Energy to Drive ATP Synthesis
• The electrochemical proton gradient across the
membrane involves both a membrane potential
and a concentration gradient
• A mitochondrion actively respiring has a membrane
potential of about 0.16V (positive on the
intermembrane space side) and a pH gradient of
about 1.0 (higher on the matrix side)
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Proton motive force
• The electrochemical gradient exerts a proton
motive force (pmf), that tends to drive protons
back down their concentration gradient (back into
the matrix)
•
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ATP Synthesis: Putting It All Together
• Some of the energy of glucose is transferred to
reduced coenzymes during glycolysis and the
TCA cycle
• This energy is used to generate an electrochemical
proton gradient across the inner mitochondrial
membrane
• The pmf of that gradient is harnessed to make ATP
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Uncoupling synthesis of ATP and electron
transport
• F1 particles and membranous vesicles were
separated by centrifugation
• The membranes could still carry out electron
transport, but could not synthesize ATP
• The isolated F1 particles could synthesize ATP but
could not carry out electron transport
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The FoF1 Complex: Proton
Translocation Through Fo Drives ATP
Synthesis by F1
• The F1 complex is not directly membranebound, but is attached to the Fo complex that
is embedded in the inner membrane
• Fo acts as a proton translocator, the
channel through which protons flow across
the membrane
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The FoF1 ATP synthase
• Fo provides a channel for exergonic flow of protons
across the membrane
• F1 carries out the ATP synthesis, driven by the
energy of the proton gradient
• Together, they form a complete ATP synthase
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Video: Rotation of ATP Synthase
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Rotation of a and b with respect to g
• Boyer proposed that at any time, each of the active
sites is in a different conformation and the
hexagonal ring of a and b subunits rotates relative
to the central stalk containing the g subunit
• The rotation was thought to be driven by the flow of
protons through Fo
• It is now known that it is the g subunit that actually
rotates
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The Chemiosmotic Model Involves
Dynamic Transmembrane Proton Traffic
• There is continuous, dynamic two-way proton traffic
across the inner membrane
• NADH sends 10 protons across via complexes I, III,
and IV; FADH2 sends 6 across, via complexes II, III,
and IV
• Assuming that 3 protons must return through FoF1
per ATP generated, this means 3 ATP per NADH
and 2 per FADH2 are generated
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Aerobic Respiration: Summing It All Up
• As carbohydrates and fats are oxidized to generate
energy, coenzymes are reduced
• These reduced coenzymes represent a storage
form of the energy released during oxidation
• This energy can be used to drive ATP synthesis as
the enzymes are reoxidized by the ETS
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Summing it all up (continued)
• As electrons are transported from NADH or FADH2
to O2, they pass through respiratory complexes
where proton pumping is coupled to electron
transport
• The resulting electrochemical gradient exerts a pmf
that serves as the driving force for ATP synthesis
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Video: ATP Synthase 3-D Structure
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The Maximum Yield of Aerobic
Respiration Is 38 ATPs per glucose
• The maximum ATP yield per glucose under
aerobic conditions:
• Including the summary reactions of glycolysis and
the TCA cycle to this gives:
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1. Why Does the Maximum ATP Yield
in Eukaryotic Cells Vary Between 36
and 38 ATPs Per Glucose?
• Glycolysis produces two NADH per glucose in the
cytosol, and catabolism of pyruvate produces eight
more in the mitochondrial matrix
• NADH in the cytosol cannot enter the matrix to
deliver its electrons to complex I
• Instead the electrons and H+ ions are passed
inward by an electron shuttle system
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Aerobic Respiration Is a Highly
Efficient Process
• To determine efficiency of respiration, we need to
determine how much of the energy of glucose is
preserved in the resulting 36–38 ATP
• DGo for glucose  CO2 + H2O is 686 kcal/mol
• ATP hydrolysis under cellular conditions is about
10 to 14 kcal/mol
© 2012 Pearson Education, Inc.
Efficiency of aerobic respiration
• For 36–38 ATP, assuming a value of 10 kcal/mol,
the energy per mole of glucose is about
360–380 kcal conserved
• This efficiency of 52–55% is well above that
obtainable from the most efficient machines created
© 2012 Pearson Education, Inc.