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Lecture #13
Electron Transport and Oxidative
Phosphorylation
Slide 1. Oxidative phosphorylation. Oxidative phosphorylation is a
composite of two biochemical processes--electron transport driven proton
pumping and proton driven ATP synthesis. These two processes work in
tandem to produce ATP from reduced nucleotides such as NADH and
FADH2.
Slide 2. Overview of Electron Transport and ATP synthesis This
simplified scheme shows how electron transport creates a proton gradient
across the inner mitochondrial membrane, and how the the ATP synthase
uses the proton gradient to drive the synthesis of ATP from ADP and Pi.
1. Electron transport is a process in which the transport of protons out of
the mitochondrial matrix is energized by the flow of electrons through
various protein complexes within the mitochondrial inner membrane.
Electron transport leads to the formation of a proton gradient with high
proton concentration in the intermembrane space and low proton
concentration in the mitochondrial matrix.
2. Proton driven ATP synthesis. The ATP synthase enzyme uses the
proton gradient formed by electron transport to drive the synthesis of ATP.
Protons flow downhill from the intermembrane space through the ATP
synthase protein. The downhill flow of protons (from an area of high
concentration in the intermembrane space to an area of low concentration in
the mitochondrial matrix) through the ATP synthase energizes the synthesis
of ATP from ADP and inorganic phosphate.
Slide 3. Components of the oxidative phosphorylation system. This
diagram gives a more detailed breakdown of the process of oxidative
phosphorylation.
- In the electron transport phase, reduced nucleotides pass their electrons
through a series of enzyme bound cofactors. At various stages of the electron
transport system the downhill transfer of electrons is coupled to the uphill
transport of hydrogen ions. The hydrogen ions are pumped from the inner
matrix of the mitochondria to the region between the inner and outer
mitochondrial membranes.
-In the phosphorylation phase the hydrogen ion gradient, created by the
oxidative proton pumps, is used to drive the synthesis of ATP from ADP and
Pi. In this process the protons flow through an ATPase down their
concentration gradient back across the inner mitochondrial membrane.
The respiratory chain consists of four complexes: Three proton pumps and
a physical link to the citric acid cycle
-High potential electron from NADH enter the system at NADH-Q
oxidoreductase (Complex I)
-Electrons flow from NADH to coenzyme Q through complex I. The
flow of electrons through complex I is coupled to the pumping of four
protons out of the matrix of the mitochondrion into the space between the
inner and outer mitochondrial membrane.
-Electrons from FADH2 (which have a lower potential than those from
NADH) flow to coenzyme Q through complex II. This complex does not
pump any protons.
-Two electrons are carried through the mitochondrial membrane from
complex I or complex II to Q-cytochrome c oxidoreductase by reduced
coenzyme Q (QH2). This coenzyme is lipid soluble and always stays in
the membrane.
-Two electrons flow from QH2 through Q-cytochrome c oxidoreductase
(Complex III) to the water soluble protein cytochrome c. The flow of
electrons through complex III is coupled to the net transport of four
protons into the space between the inner and outer mitochondrial
membrane and the uptake of two protons from the mitochondrial matrix.
-Each molecule of reduced cytochrome c carries one electron from Qcytochrome c oxidoreductase to cytochrome c oxidase (complex IIII).
-Four electrons from four molecules of cytochrome c flow through
complex IIII to react with oxygen and form water as the final product of
the respiratory chain. This process is coupled to the uptake of four
protons from the matrix, which react with oxygen to form two water
molecules, and the transport of four additional protons from matrix to the
space between the inner and outer mitochondrial membrane.
Slide 4. Electron transport participants. There are a number of cofactors
that participate in electron transport.
-NADH
-FADH2
-FMN
-Non-heme iron sulfur complexes
-Ubiquinone
-Cytochromes b, c1, c, a, a3
Slide 5. Absorption Spectra of NAD+ and NADH. There is a significant
difference in the absorbance spectrum of NAD+ and NADH. Such alterations
in the absorption spectra occur in various cofactors depending on their
oxidation states. These changes are useful in following the progress of a
biochemical reaction.
Slide 6. Structure of Flavin Mononucleotide (FMN). The first acceptor
of electrons from NADH in complex I is FMN. The reduction of FMN to
FMNH2 occurs on the same isoalloxizine ring and has essentially the same
chemistry as we have previously seen with the conversion of FAD to
FADH2.
Slide 7. Reduction of FMN. When the FMN cofactor is reduced
to FMNH2 it accepts two electrons and two protons on the
isoalloxizine ring. The protons occupy sites where there were paired
electrons on two of the ring nitrogen atoms. There is a shift in the double
bond pattern on two of the isoalloxizine rings when the additional electrons
are added to the system.
Slide 8. Iron-sulfur clusters in non-heme iron proteins. There are three
types of iron-sulfur clusters involved in the electron transport scheme.
Actually, non-heme iron proteins containing these types of clusters are found
in complexes I, II, and III.
The least complicated variety of iron-sulfur cluster contains one iron atom
and sulfur atoms of four cysteine residues. The iron ion participates in
oxidation-reduction reactions by accepting or donating electrons. In the
process that iron ion can change its charge from +2 to +3. The proximity of
the four protein sulfur atoms aids in the oxidation-reduction reaction and
helps determine the energy levels of the two oxidation states.
The next most complicated version of the iron-sulfur cluster contains two
iron atoms, four cysteine sulfur atoms, and two inorganic sulfur atoms. The
two inorganic sulfur atoms are not connected directly to the protein. The
addition of these sulfur atoms to the protein complex involves posttranslational protein modification mechanisms. As with the simpler one iron
cluster, the proximity of the protein sulfur atoms and the inorganic sulfur
atoms influences the oxidation state of the two iron ions and helps determine
their energy levels.
The most complex the iron-sulfur cluster is a cubic structure which contains
four iron atoms, four cysteine sulfur atoms, and four inorganic sulfur atoms.
Again the protein sulfur atoms and the inorganic sulfur atoms influence the
oxidation-reduction profile of the four iron ions.
Slide 9. Ubiquinone (Coenzyme Q). The lipid soluble cofactor ubiquinone
(aka coenzyme Q) can accept two electrons and two protons. Its reduction
occurs in a sequential manner. The addition of one electron and one proton
produces a semiquinone (a free radical with an unpaired electron). The
semiquinone is a very reactive and unstable intermediate which has to be
closely sequestered within the active sites of enzymes. The addition of a
second electron and proton produces the stable reduced intermediate
ubiquinol.
The coenzyme Q molecule carries electrons within the inner mitochondrial
membrane between complex I and complex III. It also functions to carry
electrons between complex II and complex III, and also within the
cytochrome c reductase complex.
Slide 10. Characteristics of Cytochromes. There are a number of
cytochrome proteins that function in the electron transport chain
(Cytochromes b, c1, c, a, a3). The soluble protein cytochrome c shuttles
electrons between the coenzyme Q cytochrome c reductase and cytochrome
c oxidase complexes. In addition, cytochromes b and c1 are components of
cytochrome c reductase, and cytochromes a and a3 are components of
cytochrome c oxidase. The cytochrome proteins all contain heme prosthetic
groups with a tetrapyrrole organic structure and a central iron ion. They are
covalently attached to the cytochrome protein molecules by thioether bonds
to protein cysteine residues. The cytochromes differ from one another in
protein sequence, the structure of the attached heme group, the absorption
spectrum, reduction potential and their role in the electron transport scheme.
Slide 11. Prosthetic Group of Cytochrome c. The slide shows the
structure of the heme group of cytochrome c. The prosthetic group of
cytochrome c is very similar to the heme found in hemoglobin except that it
is covalently attached to the protein through two thioether linkages. The
heme side chains that are connected to the protein were unsaturated vinyl
groups prior to attachment to the protein. The double bonds of these vinyl
groups were reduced when the heme was attached to the protein cysteine
residues.
Slide 12. Absorption Spectra of Cytochrome c. Similar to many of the
molecules in the electron transport chain, cytochrome c has an absorption
spectrum. It absorbs light in the visible wavelength giving it a color which
is visible to the human eye. That spectrum changes depending on whether
the molecule is oxidized or reduced. Such changes in absorption are useful
to biochemists because they provide a window through which investigators
can follow the reaction progress as these molecules are oxidized and reduced.
Slide 13. Sequence of Human Cytochrome c. The cytochrome c molecule
is a comparatively small protein with only 104 amino acids. That makes it
relatively easy to purify and sequence. Because of its importance in biology,
cytochrome c has been sequenced from many different species. The results
shown here indicate that of the 104 amino acids in the protein about 27
(highlighted in yellow) are invariant in 60 widely divergent species of
organism. Another 14 residues (highlighted in blue) are highly conserved
meaning that amino acid substitutions at those positions generally replace
one amino acid with a similar amino acid—for example leucine with
isoleucine. The amino acids that are invariant are probably critical to
cytochrome c function. Mutations causing changes in those amino acids
would inactivate cytochrome c, and so such mutations are lethal and are not
retained in the gene pool.
The amino acids that are not highlighted appear to be less critical to
cytochrome c function, and so a variety of changes have occurred in these
amino acids. Geneticists have a field day comparing these amino acids from
different species. It is possible to construct an evolutionary tree based on the
number of amino acid changes from one species to another.
Slide 14. Phylogenetic Tree for Cytochrome c. From amino acid
composition data biologists are able to construct a phylogenetic tree. The
number of amino acid changes between species is shown on the branches of
the tree. The assumption is that all living organism originated from a
common ancestor and that various species diverged from each other at
different times in evolutionary history. The number of amino acid
differences correspond to the passage of time since divergence occurred.
This particular phylogenetic tree relates to cytochrome c, but similar trees
have been constructed using other protein molecules. The fact that there is
good agreement between different proteins lends strong support to
evolutionary theory.
Slide 15. Components of the oxidative phosphorylation system. We are
now ready to look at some details of the complexes that are involved in
electron transport. Remember that when NADH is the donor, the electrons
flow from complex I (NADH coenzyme Q reductase) to complex III
(coenzyme Q cytochrome c reductase) and then to complex IIII (cytochrome
c oxidase).
Slide 16. Complex I (NADH coenzyme Q reductase). In complex I two
electrons flow from NADH to coenzyme Q. The pathway of electron flow is
NADH to FMN through a series of iron sulfur proteins to coenzyme Q. The
final product of this process is reduced coenzyme Q (QH2). The flow of
electrons through complex I is coupled to the pumping of four protons out of
the matrix of the mitochondrion into the space between the inner and outer
mitochondrial membrane. Two additional protons are taken up from the
matrix to convert coenzyme Q to QH2.
Slide 17. Complex III (cytochrome c reductase). The second complex in
electron transport is cytochrome c reductase. The QH2 that is produced in
complex I diffuses through the hydrophobic membrane to complex III.
There is an interesting transition in this complex. That is, QH2 feeds two
electrons into the complex, but cytochrome c only accepts one electron. As
a result it takes two cycles of reduction involving two cytochrome c
molecules to effectively convert coenzyme Q from the reduced to oxidized
form.
In the first cycle (part A) the reduced coenzyme Q donates two electrons.
One electron goes through an iron-sulfur protein and a protein bound
cytochrome c1 to cytochrome c. The other electron goes to a protein bound
coenzyme Q molecule to form a free radical, then through two variants of
cytochrome b and finally ends up in a second coenzyme Q as a free radical
with an unpaired electron. This free radical intermediate is extremely
reactive and could damage the cell if it were set free from the surface of the
enzyme. Fortunately, the enzyme complex holds the intermediate closely
until further reaction can occur.
In the next cycle (part B) a second molecule of reduced coenzyme Q donates
two electrons. One electron follows the same pathway as in the first cycle-through an iron-sulfur protein and a protein bound cytochrome c1 to
cytochrome c. The second electron follows the pathway used by the second
electron in the first cycle and ends up converting the free radical back to
reduced coenzyme Q.
The result of this elaborate electronic dance is that two molecules of reduced
coenzyme Q donate four electrons. Two of those electrons produce two
molecules of reduced cytochrome c and the other two electrons end up
regenerating a molecule of QH2.
Slide 18. Cytochrome c oxidase. If you thought that the last reaction
sequence was complicated, here is something even more baroque. The
figure summarizes the last set of reactions in the electron transport chain
which is catalyzed by cytochrome c oxidase. Keep in mind that all four
electrons that enter this reaction scheme are provided by reduced
cytochrome c. The end result of this series of reactions is the reduction of
molecular oxygen (O2) to two molecules of water. It takes two electrons to
reduce each oxygen atom to water, so four electrons are required.
We will start at the upper left with the complex having an iron ion in the +3
state and a copper ion in the +2 state. The addition of one electron from
cytochrome c reduces the copper ion to the +1 state. The addition of a
second electron reduces the iron ion to the +2 state. At that point molecular
oxygen binds to one of the ligand binding sites on the iron ion. Then an
internal oxidation-reduction reaction takes place. Both the iron and copper
ions lose an electron and the molecular oxygen is converted to a double
anion. In the next step an electron and two protons are added and molecular
oxygen is split into two unconnected oxygen atoms. One of the oxygen
atoms, which is attached to copper +2, gains two protons and two electrons
to form water. The other oxygen atom, which is connected to a really weird
iron +4, ends up carrying a minus 2 charge. The addition of one more
electron and two protons produces a second water molecule and returns the
iron ion to the +3 state. At this point the cycle is complete.
Slide 19. Cytochrome c oxidase II. This slide summarizes the fate of
protons in the cytochrome c oxidase reaction. During the process of
converting molecular oxygen to water, four electrons from reduced
cytochrome c and four protons from the matrix are used to reduce the two
oxygen atoms to water. In addition to the four “chemical protons” absorbed
from the matrix and incorporated into water, four additional protons are
pumped from the matrix to the intermembrane space.
Slide 20. Electron Transport Participants. Here is a more detailed
diagram showing not only the four complexes that participate in electron
transport, but also some of the key participants in those complexes. We will
now begin a discussion of the standard reduction potentials of these
component reactions. Remember that the electron transport process starts
with NADH donating electrons to the system and finishes with oxygen
receiving electrons to form water. The critical point is that as the electrons
pass through the various carriers, there is a gradual loss of reducing potential
and a corresponding release of energy. Some of that energy is used to pump
protons across the inner mitochondrial membrane to create a proton gradient.
Slide 21. Nernst Equation. This is the Nernst Equation which has been
beloved by physiologists for a number of generations. The Nernst Equation
is similar to the free energy equation in that it allows us to calculate the
potential energy of chemical reactions. However, the Nernst Equation is
used for oxidation-reduction reactions, and the energy values are expressed
in volts not in joules or calories. Take note that this is a biologist’s
formulation of the Nernst Equation with the hydrogen ion (proton)
concentration set at pH 7 to reflect the normal condition that prevails in
living cells. The Nernst Equation tells us that the reduction potential for any
reaction is equal to the standard state reduction potential plus RT/nFln
(electron acceptor)/(electron donor).
Slide 22 . Energetics of redox reactions. This slide gives a generalized
equation for an oxidation-reduction reaction. The standard state voltage
change (E0’ ) for the combined reaction equals the standard reduction
potential for the electron acceptor (the substance being reduced) minus the
standard reduction potential for the electron donor (the substance being
oxidized).
The slide also gives a formula for determining the standard state free energy
change for a reaction if you know the value for the voltage change. That
formula is:
G0’ = -nFE0’
This formula allows us to compare standard state voltage change in a
reaction with the standard state free energy change. In other words we can
convert voltage changes to Joules or Calories. Note that there is a minus sign
on the right side of the equation. That means that an energy releasing
reaction which has a negative G0’ will have a positive E0’.
Slide 23. Table of Standard Reduction Potentials. This is a table
showing the standard reduction potentials for a number of the participants in
the electron transport pathway. These are half-reaction potentials which are
measured against an arbitrary standard (a hydrogen electrode). The
reactions are shown in the direction of reduction. In order to correspond to
the real world there must be another half reaction involved which would be
going in the direction of oxidation and which would serve as a source of
electrons for the reduction half reaction. This table was constructed using a
hydrogen electrode as the other half reaction, but one can combine any two
half reactions to determine the voltage change for a physiologically relevant
oxidation-reduction pair. If you combine two half reactions from a table
such as this, one of the reactions runs in the direction of reduction and uses
the E0’ value shown in the table. The other half reaction runs in the
direction of oxidation (reversed from that shown) and the E0’ value is given
the opposite sign from that shown in the table. (For example, a positive
value becomes a negative value of the same magnitude.)
Slide 24. Standard State Reduction Potentials for NADH to H20. Here
is a relevant example of how to calculate the change in standard state
reduction potential for an oxidation-reduction reaction. This particular
example corresponds to the change in standard state reduction potential for
the overall electron transport process starting with NADH donating electrons
and ending with molecular oxygen being converted to water.
The upper two equations show the half reactions and give the standard
reduction potentials for the reduction of NAD+ to NADH and the reduction
of oxygen to water. However, because NADH is being oxidized in the
electron transport process, we have to reverse the direction of that half
reaction and change the sign of its E0’.
The two reactions shown on the lower part of the slide can then be added to
give the overall reaction and the net voltage change (E0’). The net voltage
change for NADH oxidation by the electron transport system is +1.14 V.
Slide 25. Reduction Potentials for Participants of Electron Transport.
This is a very revealing diagram. The reduction potentials of the electron
transport participants are plotted in the order that they occur in the overall
pathway. Notice that as the sequence proceeds there is a constant drop in
reduction potential (voltage)—there is a continuous loss of energy. In
addition, there is more than sufficient drop in voltage in each complex (I, III
and IV) to account for the synthesis of one molecule of ATP. In fact if the
process were 100% efficient there would be enough energy released by each
complex to synthesize at least two molecules of ATP. Complex I, which
accepts electrons from NADH, passes them through FMN and donates them
to coenzyme Q, has a voltage drop of about 0.4 volts. Complex III, which
starts with coenzyme Q and ends with cytochrome c, has a drop of about 0.3
volts. Complex IV, which accepts electrons from cytochrome c and donates
them to molecular oxygen, has a drop of about 0.45 volts.
Slide 26. Overview of Electron Transport and ATP synthesis We will
take a second look at this simplified scheme to reflect on how the eneregy
from electron transport is used to create a proton gradient across the inner
mitochondrial membrane, and then how the the ATP synthase captures the
energy from that proton gradient to drive the synthesis of ATP from ADP
and Pi.
Notice that the role of the electron transport complexes is to pump protons
across the membrane. These complexes are not directly linked to the ATP
synthase, and they are not directly involved in ATP synthesis. The
connection between electron transport and oxidative phosphorylation is
through the proton gradient.
Slide 27. ATP Synthase Structure. The synthesis of ATP is catalyzed by
the ATP synthase. The ATP synthase complex is made up of an F0
component which is embedded in the inner mitochondrial membrane and the
F1 component which is a peripheral protein assembly. The F0 component
consists of two different protein subunits, and the F1 component contains
five different subunits.
Slide 28. Electron Micrograph of Mitochondrial Fractions. Here is some
experimental evidence for the involvement of the F1 component in ATP
synthesis. In this slide we see a series of four electron micrographs of
mitochondrial preparations. Panel A shows peripheral proteins (The F1 part
of the ATP synthase) lining the matrix surface of the inner mitochondrial
membrane. In panel B those proteins have been removed from the
mitochondrial preparations by treatment with urea. Panel C shows the
isolated F1 components of the ATP synthase. In panel D those F1
components have been added back to mitochondrial fractions.
It turns out that mitochondrial membrane preparations that contain the F 1
components can synthesize ATP from ADP and Pi as long as the membrane
structure remains intact and there is a gradient of hydrogen ions. When the
F1 component of the ATP synthase is removed the ability to synthesize ATP
is lost. When the F1 component is added back the reconstituted preparation
regains the ability to synthesize ATP.
Slide 29. Subunits of the F1 Complex. The F1 complex has 3  and 3 
subunits surrounding a central  subunit. The  subunit passes through the
middle of the  3 3 hexamer which consists of alternating  and 
subunits. At any given time each of the three  subunits exists in a different
nucleotide binding form designated O, L, and T. These three forms are
interconvertible. The  subunits bind ATP but do not participate in any
catalytic or transport reaction.
Slide 30. Rotation of  Subunit Affects Three Subunits. Proton driven
ATP synthesis involves a binding change mechanism in which three
sequential 120o rotations of the  subunit drives the subunits through the
three different forms, T (tight), O (open) and L (loose). The subunit in the T
form converts ADP and Pi to ATP but does not allow the ATP product to be
released. When the  subunit is rotated by 120 degrees in a counter
clockwise direction the T form is converted into the O form allowing ATP
release. Then ADP and Pi can bind to the O form. An additional 120 degree
rotation traps ADP and Pi in the L form.
Slide 31. Actin Filament Connected to  Subunit. The figure shows an
actin filament bound to the  subunit. This is an artificial hybrid system
constructed in a research lab to demonstrate that the  subunit actually
rotates. In the experiment the hydrolysis of ATP drives the rotation of the 
subunit. This could be directly observed by fluorescence microscopy.
Under physiological conditions there would be no actin present and the
rotation of the  subunit would drive the synthesis of ATP from ADP and Pi.
Slide 32. Subunits of F0 Component. The membrane spanning F0
component consists of a and c subunits. The c subunit consists of two helix structures with a negatively charged aspartate in the center. The a
subunit contains a cytoplasmic and a matrix half-channel.
Slide 33. Subunit Structure of the F0 component. Each of the c subunits
consists of two  helices. Between 10 and 14 of the c subunits form a
membrane spanning ring. An aspartic acid residue in one of the helices lies
at the center of the membrane. These aspartate residues are protonated and
deprotonated during the passage of protons around the ring. The a subunit
appears to include two half-channels that allow protons to enter and pass
partway but not completely through the membrane.
Slide 34. Rotation of the c Ring. Proton movement across the membrane
drives rotation of the c ring. A proton enters from the intermembrane space
into the cytoplasmic half channel of the a subunit to neutralize the charge on
an aspartate residue in a c subunit. With this charge neutralized the c ring
can rotate clockwise by one c subunit. This moves another protonated
aspartic acid residue out of the membrane and into contact with the matrix
half-channel. This proton can diffuse into the matrix resetting the system to
its initial state.
In summary, each proton enters the cytoplasmic half-channel, follows a
complete rotation of the c ring, and exits through the matrix half-channel.
The flux of protons through the F0 component drives the rotation of the c
ring which in turn drives the rotation of the  subunit of the F1 component.
The rotation of the  subunit in turn powers the synthesis of ATP from ADP
and Pi—This overall process constitutes the phosphorylation part of
oxidative phosphorylation.
Slide 35. Glycerol 3-phosphate shuttle. When we calculate the ATP yield
from various catabolic pathways there is one additional factor that we have
to take into account. That is the “energy cost” of transporting materials from
one site to another within the cell. An excellent example of this energy cost
is the metabolic fate of the NADH produced by glyceraldehyde-3-phosphate
dehydrogenase. That NADH is produced in the glycolysis pathway which is
located in the cytoplasm of the cell. However, the NADH is used by the
electron transport system in the mitochondrial inner membrane. There is a
metabolic system called the glycerol 3-phosphate shuttle that accomplishes
the translocation of NADH from the cytoplasm to the mitochondria.
Slide 36. Mechanism of the glycerol 3-phosphate shuttle. The reducing
equivalents from cytoplasmic NADH are carried into the matrix by the
glycerol 3-phosphate shuttle. The end product of this process is FADH2
rather than NADH. The net cost of this transport is that one ATP equivalent
is lost during the process, because FADH2 only yields 1.5 ATP’s in
oxidative phosphorylation (see below).
In the glycerol 3-phosphate shuttle electrons from NADH are used to reduce
dihydroxyacetone phosphate to glycerol-3-phosphate. The glycerol-3phosphate is reoxidized by electron transfer to an FAD prosthetic group of a
dehydrogenase that is located in the inner mitochondrial membrane.
Subsequently the electrons are transfered to coenzyme Q forming QH2
which enters the electron transport chain. This mechanism is predominate in
muscle cells.
Slide 37. Malate-aspartate shuttle. In heart and liver, electrons are carried
into the matrix by the malate-aspartate shuttle. This convoluted mechanism
is mediated by two membrane transport systems and four enzymes. The
major benefit of this elaborate process is that the electrons from cytoplasmic
NADH end up in mitochondrial NADH so no reducing power is lost in the
process.
Slide 38. Mechanism of the malate-aspartate shuttle. The malateaspartate shuttle begins with the reduction of oxaloacetate to malate. The
shuttle process involves two antiporters. One antiporter exchanges
cytoplasmic malate for -ketoglutarate and the other exchanges aspartate for
glutamate. The overall reaction scheme involves the sequential formation of
malate, oxaloacetate, a transamination reaction to form -ketoglutarate and
aspartate, and a second transamination reaction to form glutamate and
oxaloacetate.
Slide 39. Stoichiometry of ATP yield. Earlier studies suggested that 3
ATP's were generated from NADH, whereas 2 ATP's could be derived from
FADH2. Recently, this stoichiometry has been revised to 2.5 and 1.5
respectively
Slide 40. ATP Yield for the Complete Oxidation of Glucose. So here is
the latest up to the date scoop on the ATP production from the complete
oxidation of one glucose molecule. We take into account the latest estimate
of ATP yield per NADH and FADH2 and the energy cost of moving
reducing power from the cytoplasm into the mitochondria. Using these
values for ATP production, the complete oxidation of one glucose molecule
should yield about 30 ATP’s.
Slide 41. UCP-1 protein uncouples electron transport from oxidative
phosphorylation. Some organisms have a mechanism for using the electron
transport system to generate heat rather than ATP. An uncoupling protein
(UCP-1) acts as a proton channel. That allows protons to enter the matrix of
the mitochondria without passing through the ATP synthase. This
mechanism dissipates the proton gradient and generates heat, but eliminates
the synthesis of ATP.
Slide 42. Inhibitors of electron transport. In addition to natural systems
for uncoupling oxidative phosphorylation by dissipating the proton gradient,
there are also natural and chemical substances which work directly to inhibit
the flow of electrons through the electron transport system. There are a
variety of such inhibitors that exhibit specific inhibition at each of the
electron transport complexes In the presence of these inhibitors no NADH
is oxidized.
Slide 43. 2,4-dinitrophenol (DNP) uncouples oxidative phosphorylation.
The formation of ATP by oxidative phosphorylation can also be prevented
by chemical uncouplers. Reagents such as 2,4-dinitrophenol (DNP) act like
the protein UCP-1 in that they allow electron transport to proceed in a
normal manner but they dissipate the proton gradient by allowing protons to
leak back into the matrix.
Slide 44. Functions of the proton gradient. In addition to driving ATP
formation, the proton gradient is used for to energize a variety of other
cellular processes such as active transport and flagellar rotation.