Transcript OXIDATIVE PHOSPHORYLATION

1
1.
2.
3.
Carbon fuels are oxidized in the citric acid cycle to
yield electrons with high transfer potential.
This electron-motive force is converted into a
proton-motive force.
The proton-motive force is converted into phosphoryl
transfer potential.

Oxidation and ATP synthesis are coupled by
transmembrane proton fluxes.
1.
2.
The intermembrane space between the outer and the
inner membranes
 Oxidative phosphorylation
The matrix which is bounded by the inner membrane:
 most of the reactions of the citric acid cycle
and fatty acid oxidation
1.
Large multi subunit integral membrane protein
complexes or coupling sites: couple electron transfer
with H+ gradient generation
› Complex I: NADH-Q oxidoreductase (MW =880
kd) Coupling Site 1
› Complex II: succinate-Q reductase complex
(MW =140 kd)
› Complex III: Q-cytochrome c oxido-reductase
(MW = 250 kd). Coupling Site 2
› Complex IV: cytochrome c oxidase (MW = 160
kd) Coupling Site 3.
Small Mobile Electron Carriers:
2.
›
Ubiquinone/Ubiquinol (Q/QH2): small
hydrophobic electron carriers which shuttle
electrons between the large complexes
and back and forth across the lipid bilayer.
›
Cytochrome c: is a small water soluble
protein which is a mobile electron carrier
and carries electrons between cytochrome
bc1 complex and cytochrome c oxidase.
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7





Electron flow within these transmembrane
complexes leads to the transport of
protons across the inner mitochondrial
membrane.
Electrons are carried from NADH-Q
oxidoreductase to Q-cytochrome c
oxidoreductase by the reduced form of
coenzyme Q (Q).
Q also carries electrons from FADH2,
generated in succinate dehydrogenase or
(succinate-Q reductase) in the citric acid
cycle, to Q-cytochrome c oxidoreductase
Cytochrome c, a small, soluble protein,
shuttles electrons from Q-cytochrome c
oxidoreductase to cytochrome c oxidase
(complex IV), which catalyzes the
reduction of O2.
Succinate-Q reductase (Complex II), in
contrast with the other complexes, does
not pump protons.
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

Q is a hydrophobic quinone that diffuses rapidly
within the inner mitochondrial membrane.
Q is a quinone derivative with a long isoprenoid tail.
› The number of five-carbon isoprene units in coenzyme
Q depends on the species.
› The most common form in mammals contains 10
isoprene units (coenzyme Q10).
The reduction of ubiquinone (Q) to ubiquinol (QH2)
proceeds through a semiquinone anion intermediate
(Q.-).
In the fully oxidized state
(Q), coenzyme Q has two
keto groups.
The semiquinone form is
relatively easily deprotonated
to form a semiquinone radical
anion (Q·-).
The addition of one
electron and one proton
results in the semiquinone
form (QH·).
The addition of a second
electron and proton
generates ubiquinol
(QH2),
10

Thus, electron-transfer reactions of
quinones are coupled to proton binding
and release, a property that is key to
transmembrane proton transport.
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
Oxidation States of Flavins:
› The reduction of flavin mononucleotide (FMN)
to FMNH2 proceeds through a semiquinone
intermediate.
Fe-S clusters in iron-sulfur proteins (nonheme
iron proteins) play a critical role in a wide
range of reduction reactions in biological
systems.
 Iron ions in these Fe-S complexes cycle
between:

› Fe2+ (reduced state)
› Fe3+ (oxidized state)

Unlike quinones and flavins, iron-sulfur
clusters generally undergo oxidationreduction reactions without releasing or
binding protons.
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Several types of Fe-S clusters are known:
A.
B.
C.

A single iron ion bound by four cysteine residues.
2Fe-2S cluster with iron ions bridged by sulfide ions.
4Fe-4S cluster.
Each of these clusters can undergo oxidation-reduction
reactions.

NADH-Q
oxidoreductase (also
called NADH
dehydrogenase):
› an enormous enzyme
(880 kd)
› consists of at least 34
polypeptide chains.
› consists of a membranespanning part and a long
arm that extends into the
matrix.
› Contains FMN and Fe-S
prosthetic groups.


NADH is oxidized in the arm, and the electrons are
transferred to reduce Q in the membrane.
The reaction catalyzed by this enzyme appears to
be:
Cytosol
1.
The initial step is
the binding of
NADH and the
transfer of its two
high-potential
electrons to the
flavin
mononucleotide
(FMN) prosthetic
group of this
complex to give
the reduced form,
FMNH2.
FMNH2
FMN
NAD+
NADH + H+
Matrix
17
Electrons are then
transferred from FMNH2
to a series of three
4Fe4S, the second type
of prosthetic group in
NADH-Q
oxidoreductase.
 NADH-Q
oxidoreductase
contains:
2.
›
›

2Fe-2S cluster
4Fe-4S cluster
Two protons move back
to the matrix
Fe3+
Fe2+
FMNH2
FMN
NAD+
NADH + H+
Matrix
2H+
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Cytosol
3.

Electrons in the 4Fe4S
of NADH-Q
oxidoreductase are
shuttled to coenzyme
Q.
Two hydrogen ions are
pumped out of the
matrix of the
mitochondrion.
UQH2
UQ
Fe3+
Fe2+
FMNH2
FMN
NAD+
NADH + H+
Matrix
2H+
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2H+
2H+
Cytosol
UQH2
4.
5.
The pair of electrons on
bound QH2 are
transferred to a 4Fe-4S
center and the protons
are released on the
cytosolic side.
These electrons are
transferred to a mobile
Q in the hydrophobic
core of the membrane,
resulting in the uptake
of two additional
protons from the matrix.
UQ
Fe2+
Fe3+
UQH2
UQ
Fe3+
Fe2+
FMNH2
FMN
NAD+
NADH + H+
Matrix
2H+
2H+
2H+
1.
2.
3.

NADH binds to a site on the
vertical arm and transfers its
electrons to FMN.
These electrons flow within the
vertical unit to three 4Fe-4S
centers.
Then they flow to a bound Q.
The reduction of Q to QH2
results in the uptake of two
protons from the matrix.
4 Protons are pumped this way
which is still under investigation


It is an integral membrane protein of the
inner mitochondrial membrane.
Contains three different kinds of Fe-S
clusters:
1. 2Fe-2S
2. 3Fe-3S
3. 4Fe-4S.
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
FADH2 is generated in the citric acid cycle
by the enzyme succinate dehydrogenase,
with the oxidation of succinate to
fumarate.
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Two electrons are transferred from FADH2 directly
to Fe-S clusters of succinate dehydrogenase.
 The electrons are then passed to (Q) for entry into
the electron-transport chain.
 FADH2 is generated by other reactions (such as:
Glycerol phosphate dehydrogenase and fatty acyl
CoA dehydrogenase).
 This FADH2 also transfers its electrons to (Q), to form
(QH2).

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

The succinate-Q reductase complex and other
enzymes that transfer electrons from FADH2 to Q, in
contrast with NADH-Q oxidoreductase, do not
transport protons.
Consequently, less ATP is formed from the oxidation of
FADH2 than from NADH.


The second of the three proton pumps in the
respiratory chain (also known as cytochrome
reductase).
The function of Q-cytochrome c oxidoreductase is to
catalyze the transfer of electrons from QH2 to oxidized
cytochrome c (cyt c), a water-soluble protein, and
concomitantly pump protons out of the mitochondrial
matrix.
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This enzyme is a homodimer
with 11 distinct polypeptide
chains.
The major prosthetic groups,
three hemes and a 2Fe-2S
cluster, mediate the
electron-transfer reactions
between quinones in the
membrane and
cytochrome c in the
intermembrane space.

Q-cytochrome c oxidoreductase contains:
›
›
›
›
›
Cytochrome b562
Cytochrome b566
Cytochrome c1
An iron sulfur protein
At least six other subunits.
A cytochrome is an electron-transferring
protein that contains a heme prosthetic
group.
 The iron ion of a cytochrome alternates
between a reduced ferrous (+2) state and
an oxidized ferric (+3) state during electron
transport.

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
The enzyme contains three heme prosthetic groups
contained within two cytochrome subunits:
› Two b-type hemes within cytochrome b:
 Heme bL (L for low affinity)
 Heme bH (H for high affinity)
› One c-type heme within cytochrome c1 (Heme c1).


Hemes in the 3 classes of cytochrome (a, b, c) differ
slightly in substituents on the porphyrin ring system.
Only heme c is covalently linked to the protein via
thioether bonds to cysteine residues
The enzyme also contains an iron-sulfur
protein with an 2Fe-2S center (Rieske
center).
 This center is unusual in that one of the iron
ions is coordinated by two his residues
rather than two cysteine residues.
› This coordination stabilizes the center in its
reduced form, raising its reduction
potential.
 Finally, Q-cytochrome c oxidoreductase
contains two distinct binding sites for
ubiquinone termed (Qo) and (Qi), with the
Qi site lying closer to the inside of the matrix.

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The mechanism for the coupling of
electron transfer from Q to cytochrome
c to transmembrane proton transport.
 Facilitates the switch from the twoelectron carrier ubiquinol to the oneelectron carrier cytochrome c.

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
Cyt c
1.
2.
3.
Cyt c1

QH2
The first electron:
The second electron:
1.
2.
3.
+ H+
Q
QHoQH
site
Cyt bL
flows first to the Rieske 2Fe-2S
cluster
then to cytochrome c1
finally to a molecule of oxidized
cytochrome c, converting it into its
reduced form.

transferred first to cytochrome bL
then to cytochrome bH
finally to an oxidized uniquinone
bound in the Qi site.
This quinone (Q) molecule is
reduced to a semiquinone anion
(Q·-).
Cyt bH
Q
.QiQsite
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Cyt c
Cyt c1
QH2
QoQ
site
Cyt bL
Cyt bH
.QQ
i site
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Cyt c
Cyt c1
+ site
Q
HQH2
H+
oQ
Cyt bL
Cyt bH
.Q
QQH2
site
i
H+ H+
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


It catalyzes the coupled oxidation of the reduced
cyt c generated by Complex III, and reduction of
O2 to two molecules of H2O.
The four-electron reduction of oxygen directly to
water without the release of intermediates is quite
thermodynamically favorable.
DG°´ = -231.8 kJ mol-1
As much of this free energy as possible must be
captured in the form of a proton gradient for
subsequent use in ATP synthesis.
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

It consists of 13 subunits, 3 of which (subunits I, II, and
III) are encoded by the mitochondrial genome.
Cytochrome c oxidase contains:
›
Three copper ions, arranged as two copper centers,
designated A and B:
1. CuA/CuA: contains two copper ions linked by two bridging
cysteine residues. This center initially accepts electrons from
reduced cytochrome c.
2. CuB: is coordinated by three histidine residues, one of which is
modified by covalent linkage to a tyrosine residue.
›
Two heme A groups:
1. Heme a: functions to carry electrons from CuA/CuA
2. heme a3: passes electrons to CuB, to which it is directly adjacent.

Together, heme a3 and CuB form the active center at
which O2 is reduced to H2O.
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Cytochrome C Oxidase.
The major prosthetic groups include:
1. CuA/CuA
2. heme a
3. heme a3-CuB.
.
Heme a3-CuB is the site of the reduction of oxygen to water.
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FIRST ELECTRONE
Cyt c
Cyt c
Electrone transfer
to CuB
Heme A
Heme a3
CuB
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SECOND ELECTRONE
Cyt c
Cyt c
Electrone transfer
to Fe in Heme a3
Both CuB and Fe in
Heme a3 are in
reduced form
Heme a3
Binding of O2
Heme A
O O
CuB
Formation of
peroxide bridge
O O
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THIRD ELECTRONE
Cyt c
Cyt c
Heme A
Heme a3
O O
CuB
Cleavage of O-O
bond
H+
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FOURTH ELECTRONE
Cyt c
Cyt c
Heme A
H+
Heme a3
O O
CuB
Reduction of the
Ferryl group
H+
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Cyt c
Heme A
H+
H+
Heme a3
O O
H+ H+ CuB
Release of water
H+ H+
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Cyt c
Cyt c
Heme A
H+
H+
Heme a3
O O
H+ H+ CuB
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The four protons in this reaction come
exclusively from the matrix.
 Thus, the consumption of these four
protons contributes directly to the proton
gradient.
 Each proton contributes (21.8 kJ mol-1)
to the free energy associated with the
proton gradient.

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


Four "chemical" protons are
taken up from the matrix side to
reduce one molecule of O2 to
two molecules of H2O.
Four additional "pumped"
protons are transported out of
the matrix and released on the
Cytosolic side in the course of
the reaction.
The pumped protons double
the efficiency of free-energy
storage in the form of a proton
gradient for this final step in the
electron-transport chain.
Still under study.
However, two effects contribute to the
mechanism:
1. Charge neutrality tends to be maintained in
the interior of proteins.


› Thus, the addition of an electron to a site inside
a protein tends to favor the binding of a proton
to a nearby site.
2.
Conformational changes take place,
particularly around the heme a3-CuB
center, in the course of the reaction cycle.
› These changes must be used to allow protons to
enter the protein exclusively from the matrix side
and to exit exclusively to the cytosolic side.
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
Thus, the overall process catalyzed
by cytochrome c oxidase is.

molecular oxygen is an ideal terminal
electron acceptor, because its high
affinity for electrons provides a large
thermodynamic driving force.
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 The
reduction of O2 is safe because:
 Cytochrome c oxidase does not release partly
reduced intermediates by holding O2 tightly
between Fe and Cu ions.
 However,
partial reduction generates
small amounts of hazardous
compounds (ROS).
› Superoxide anion
› peroxide.
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
These TOXIC derivatives of molecular
oxygen are scavenged by protective
enzymes e.g. superoxide dismutase.

This enzyme catalyzes the conversion of two
of the superoxide radicals into hydrogen
peroxide and molecular oxygen.

The H2O2 formed by superoxide dismutase
and by other processes is scavenged by
catalase, that catalyzes the dismutation of
H2O2 into H2O and O2.

So far, we have considered the flow of
electrons from NADH to O2, an exergonic
process.

Next, we consider how this process is
coupled to the synthesis of ATP, an
endergonic process?
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54

The synthesis of ATP is carried out by A
molecular assembly in the inner
mitochondrial membrane called:
› ATP synthase
› mitochondrial ATPase
› F1F0 ATPase
› Complex V
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Chemiosmotic Hypothesis:
 Complex I, Complex III and Complex IV
pump protons across the inner
mitochondrial membrane.
› Pumping uses the energy liberated from the
oxidation of NADH and FADH2
› Pumping generates a membrane potential
because it generates an electrochemical
gradient
 Negative inside, positive outside
 Alkaline inside, acidic outside
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 The difference in proton conc. (pH) and the electrical potential together
represent a form of stored energy called (proton-motive force).
This proton-motive force drives the synthesis of ATP by ATP synthase.
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Outer side
X+
Conc. C1
Charge Z
+
+
+
- Inner side
-
X+
Conc. C2
DV = potential across the
membrane (V)

DG is given by:
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
Under typical mitochondrial conditions:
› pH outside is 1.4 units lower than inside. (log10(c2/c1))
› The membrane potenial is 0.14V. (outside +ve)
› Z=+1


DG = 5.2 kcal mol-1 (21.8 kJ mol-1).
Thus, each proton that is transported out of the
matrix to the cytosolic side corresponds to 5.2 kcal
mol-1 of free energy.
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Testing the Chemiosmotic
Hypothesis. ATP is synthesized
when reconstituted
membrane vesicles
containing
bacteriorhodopsin (a lightdriven proton pump) and ATP
synthase are illuminated.
 The orientation of ATP
synthase in this reconstituted
membrane is the reverse of
that in the mitochondrion


The F1 sector contains the
catalytic sites of the synthase.

F0 sector acts as a proton
translocator.
Consists of five types of polypeptide
chains (a3, b3, g, d, and e).
 The a and b subunits, make up the
bulk of the F1, and are arranged
alternately in a hexameric ring.


Both bind nucleotides but only the b
subunits participate directly in
catalysis.

The g subunit includes a long ahelical coil that extends into the
center of the a3b3 hexamer.

g breaks the symmetry of the a3b3
hexamer:
› each of the b subunits is
distinct by virtue of its
interaction with a different
face of g.

Distinguishing the three b subunits
is crucial for the mechanism of
ATP synthesis.
Consists of a ring comprising from
10 to 15 c subunits that are
embedded in the membrane.
 A single a subunit binds to the
outside of this ring.
 The proton translocation depends
on a subunit and the c ring.
 The F0 and F1 sectors are connected
in two ways:

› by the internal ge stalk.
› by peripheral stalk.

The peripheral stalk consists of b2 + d
subunit.
1.
2.
A moving unit, or rotor,
consisting of the c ring
and the ge stalk.
A stationary unit, or stator,
composed of the
remainder of the
molecule.

ATP synthesis mechanism:

The rate of incorporation of O18 into Pi showed that
about equal amounts of bound ATP and ADP are in
equilibrium at the catalytic site, even in the absence
of a proton gradient

Changes in the properties of the three b
subunits allow:
1. ADP and Pi binding
2. ATP synthesis
3. ATP release

The concepts of this initial proposal is
refined by more recent crystallographic
data:
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
1.
Interactions with the g subunit make the three b subunits
inequivalent:
One b subunit can be in the T, or tight, conformation.
›
›
›
2.
A second subunit will be in the L, or loose, conformation.
›
›
3.
Binds ATP with great avidity.
Its affinity for ATP is so high that it will convert bound ADP and Pi
into ATP with an equilibrium constant near 1,
The conformation of this subunit is sufficiently constrained that it
cannot release ATP.
This conformation binds ADP and Pi.
It, too, is sufficiently constrained that it cannot release bound
nucleotides.
The final subunit will be in the O, or open, conformation.
›
›
This form can exist with a bound nucleotide in a structure that is
similar to those of the T and L forms,
But it can also convert to form a more open conformation and
release a bound nucleotide.
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
Suppose g subunit is rotated by 120 degree
counterclockwise.
1. The T conformation will convert into O conformation:

ATP will be released
2. The L conformation will convert into T conformation:

The bound ADP+Pi will be converted into ATP.
3. The O conformation will convert into L conformation:

It will trap the bound ADP+Pi. So that it will not escape.





A simple experimental system consisting of cloned a3b3g subunits
only
The b were engineered to contain N-terminal polyhistidine tags,
The tags allowed the a3b3 assembly to be immobilized on a glass
surface that had been coated with nickel ions.
The g subunit was linked to a fluorescently labeled actin filament
The addition of ATP caused the g to rotate in a counterclockwise
direction, which could be seen directly under fluorescent
microscope.


When Asp 61 of C subunit is in
contact with the hydrophobic
part of the membrane, the
residue must be in the neutral
aspartic acid form, rather than in
the charged, aspartate form.
Protons can pass into either of
these channels, but they cannot
move completely across the
membrane.
Suppose that the Asp 61
residues of the two c subunits
that are in contact with a halfchannel have given up their
protons so that they are in the
charged aspartate form.
 The c ring cannot rotate in
either direction, because such
a rotation would move a
charged aspartate residue into
the hydrophobic part of the
membrane.

A proton can move through
either half-channel to
protonate one of the
aspartate residues.
 It is much more likely to pass
through the channel that is
connected to the cytosolic
side of the membrane.
 The entry of protons into the
cytosolic half-channel is further
facilitated by the membrane
potential of +0.14 V (positive
on the cytoplasmic side)


If the aspartate residue is protonated to its
neutral form, the c ring can now rotate, but
only in a clockwise direction.
› The newly protonated aspartic acid
residue becomes in contact with the
membrane.
› The charged aspartate residue moves
from contact with the matrix halfchannel to the cytosolic half-channel
› A different protonated aspartic acid
residue moves from contact with the
membrane to the matrix half-channel.

The proton can then dissociate
from aspartic acid and move
through the half-channel into
the proton-poor matrix to
restore the initial state.
76

The difference in proton
concentration and potential
on the two sides of the
membrane leads to
different probabilities of
protonation through the two
half-channels, which yields
directional rotational
motion.
78




Recall that the number of c subunits in the c ring
appears to range between 10 and 15.
Each 360-degree rotation of the g subunit leads to
the synthesis and release of three molecules of ATP
(1/120o).
Thus, if there are 10 c subunits in the ring (as was
observed in a crystal structure of yeast
mitochondrial ATP synthase), each ATP generated
requires the transport of 10/3 = 3.33 protons. If there
are 12 c ring, then 12/3 = 4 H+
For simplicity, we will assume that 3 protons must
flow into the matrix for each ATP formed, but we
must keep in mind that the true value may differ.
79
NAD+ must be regenerated for glycolysis
to continue.
 How is cytosolic NADH reoxidized under
aerobic conditions?
 The inner mitochondrial membrane is
impermeable to NADH and NAD+.
 Thus electrons from NADH, rather than
NADH itself, are carried across the
mitochondrial membrane.

80
81

FADH2 transfers its electrons to Q, which then enters
the respiratory chain as QH2.

1.5 rather than 2.5 ATP are formed (FAD is the elect.
Acceptor)

The use of FAD enables electrons from cytosolic
NADH to be transported into mitochondria.

The price of this transport is one molecule of
ATP/2e-.

This glycerol 3-phosphate shuttle is especially
prominent in muscle and enables it to sustain a
very high rate of oxidative phosphorylation.
82

In heart and liver

This shuttle, in contrast with the glycerol 3phosphate shuttle, is readily reversible.

Consequently, NADH can be brought into
mitochondria by the malate- aspartate shuttle only
if the NADH/NAD+ ratio is higher in the cytosol than
in the mitochondrial matrix.

This versatile shuttle also facilitates the exchange of
key intermediates between mitochondria and the
cytosol.
84



Also called adenine nucleotide translocase or ANT
The highly charged ATP and ADP do not diffuse freely
across the inner mitochondrial membrane.
The in and out flows of ATP and ADP are coupled by
the translocase, which acts as an antiporter.

The translocase, contains a single nucleotide-binding
site that alternately faces the matrix and cytosolic
sides of the membrane


ATP and ADP (both devoid of Mg2+) are bound with
nearly the same affinity.
In actively respiring mitochondrion the membrane
potential is positive.

The rate of binding-site eversion from the matrix to
the cytosolic side is more rapid for ATP than for ADP
because ATP has one more negative charge.

The membrane potential and hence the protonmotive force are decreased by the exchange of
ATP for ADP, which results in a net transfer of one
negative charge out of the matrix.

ATP-ADP exchange is energetically expensive;
about a quarter of the energy yield from electron
transfer by the respiratory chain is consumed to
regenerate the membrane potential.
87

Transporters (also called carriers) are transmembrane
proteins that move ions and charged metabolites
across the inner mitochondrial membrane.

Many mitochondrial transporters consist of three
similar 100-residue units. These proteins contain six
putative membrane-spanning segments
The best current estimates for the number of protons
pumped out of the matrix per electron pair is:
› 4 by NADH-Q oxidoreductase
› 2 by Q-cytochrome c oxidoreductase
› 4 by cytochrome c oxidase
 Three protons are pumbed through the ATP synthase for
each molecule of ATP.
 One proton is consumed (neutralized) in transporting
ATP from the matrix to the cytosol.
 Hence, about 2.5 molecules of ATP are generated as a
result of the flow of a pair of electrons from NADH to O2.

90



Electrons do not usually flow through the electrontransport chain to O2 unless ADP is simultaneously
phosphorylated to ATP.
Oxidative phosphorylation requires a supply of
NADH (or FADH2), O2, ADP, and Pi.
The most important factor in determining the rate
of oxidative phosphorylation is the level of ADP.
91

The rate of oxygen consumption by
mitochondria increases markedly when
ADP is added and then returns to its initial
value when the added ADP has been
converted into ATP
93

Specific inhibitors of electron transport
were invaluable in revealing the
sequence of electron carriers in the
respiratory chain.
94
rotenone and amytal block electron
transfer in NADH-Q oxidoreductase
Antimycin A interferes with electron
flow from cytochrome bH in Qcytochrome c oxidoreductase
Prevents utilization of electrons
from NADH but not FADH2
Prevents utilization of electrons
from NADH and FADH2
cyanide (CN-), azide (N3-), and
carbon monoxide (CO) block
electron flow in cytochrome c
oxidase
95



Oligomycin and dicyclohexylcarbodiimide (DCCD)
prevent the influx of protons through ATP synthase.
If actively respiring mitochondria are exposed to an
inhibitor of ATP synthase, the electron-transport chain
ceases to operate.
Indeed, this observation clearly illustrates that
electron transport and ATP synthesis are normally
tightly coupled.


The coupling of electron transport and
phosphorylation in mitochondria can be disrupted
(uncoupled) by 2,4-dinitrophenol and certain other
acidic aromatic compounds.
These substances carry protons across the inner
mitochondrial membrane.

Uncoupling is a means of generating heat to
maintain body temperature in hibernating animals,
in some newborn animals (including human
beings), and in mammals adapted to cold.

Brown adipose tissue, which is very rich in
mitochondria is specialized for this process.

The inner mitochondrial membrane of these
mitochondria contains a large amount of
uncoupling protein (UCP).
98



UCP-1 forms a pathway for the flow of protons from
the cytosol to the matrix.
In essence, UCP-1 generates heat by shortcircuiting the mitochondrial proton battery. UCP-2
& UCP-3 also have been identified and may have
a role in energy homeostasis.
This dissipative proton pathway is activated by free
fatty acids liberated from triacylglycerols in
response to hormonal signals, such as b-adrenergic
agonists