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ELECTRON TRANSPORT CHAIN
Energy-rich molecules, such as glucose, are
metabolized by a series of oxidation reactions
ultimately yielding CO2 and water (Figure 6.6). The
metabolic intermediates of these reactions donate
electrons to specific coenzymes—nicotinamide
adenine dinucleotide (NAD+) and flavin adenine
dinucleotide (FAD)—to form the energy-rich
reduced coenzymes, NADH and FADH2. These
reduced coenzymes can, in turn, each donate a pair
of electrons to a specialized set of electron carriers,
collectively called the electron transport chain.
• As electrons are passed down the electron
transport chain, they lose much of their free
energy. Part of this energy can be captured
and stored by the production of ATP from ADP
and inorganic phosphate (Pi). This process is
called oxidative phosphorylation
A. Mitochondrion
The electron transport chain is present in the
inner mitochondrial membrane and is the final
common pathway by which electrons derived
from different fuels of the body flow to oxygen.
Electron transport and ATP synthesis by
oxidative phosphorylation proceed
continuously in all tissues that contain
mitochondria.
1. Structure of the mitochondrion: The
components of the electron transport chain are
located in the inner membrane. Although the
outer membrane contains special pores, making
it freely permeable to most ions and small
molecules, the inner mitochondrial membrane
is a specialized structure that is impermeable to
most small ions, including H+, Na+, and K+,
small molecules such as ATP, ADP, pyruvate,
and other metabolites important to
mitochondrial function (Figure 6.7).
Specialized carriers or transport systems are
required to move ions or molecules across this
membrane. The inner mitochondrial
membrane is unusually rich in protein, half of
which is directly involved in electron transport
and oxidative phosphorylation. The inner
mitochondrial membrane is highly convoluted.
The convolutions, called cristae, serve to
greatly increase the surface area of the
membrane.
• Aerobic organisms are able to capture a far
greater proportion of the available free energy of
respiratory substrates than anaerobic organisms.
Most of this takes place inside mitochondria,
which have been termed the "powerhouses" of
the cell. Respiration is coupled to the generation
of the high-energy intermediate, ATP, by
oxidative phosphorylation.
2. ATP synthase complexes:
These complexes of proteins are referred
to as inner membrane particles and are
attached to the inner surface of the inner
mitochondrial membrane. They appear as
spheres that protrude into the
mitochondrial matrix.
3. Matrix of the mitochondrion: This gel-like solution
in the interior of mitochondria is fifty percent
protein. These molecules include the enzymes
responsible for the oxidation of pyruvate, amino
acids, fatty acids (by β-oxidation), and those of the
tricarboxylic acid (TCA) cycle. The synthesis of urea
and heme occur partially in the matrix of
mitochondria.
In addition, the matrix contains NAD+
and FAD (the oxidized forms of the two
coenzymes that are required as hydrogen
acceptors)and ADP and Pi, which are
used to produce ATP. [Note: The matrix
also contains mitochondrial RNA and
DNA (mtRNA and mtDNA) and
mitochondrial ribosomes.]
B. Organization of the chain
The inner mitochondrial membrane can be
disrupted into five separate enzyme complexes,
called complexes I,II,III, IV, and V.
Complexes I to IV each contain part of the
electron transport chain (Figure 6.8), whereas
complex V catalyzes ATP synthesis .
Each complex accepts or donates electrons to
relatively mobile electron carriers, such as
coenzyme Q and cytochrome c
Each carrier in the electron transport chain
can receive electrons from an electron donor,
and can subsequently donate electrons to the
next carrier in the chain. The electrons
ultimately combine with oxygen and protons to
form water. This requirement for oxygen makes
the electron transport process the respiratory
chain, which accounts for the greatest portion
of the body's use of oxygen.
C. Reactions of the electron transport
chain
With the exception of coenzyme Q, all
members of this chain are proteins. These may
function as enzymes as is the case with the
dehydrogenases, they may contain iron as part
of an iron-sulfur center, they may be
coordinated with a porphyrin ring as in the
cytochromes, or they may contain copper, as
does the cytochrome a + a3 complex.
1. Formation of NADH:
NAD+ is reduced to NADH by
dehydrogenases that remove two hydrogen
atoms from their substrate. (For examples of
these reactions, see the discussion of the
dehydrogenases found in the TCA cycle. Both
electrons but only one proton (that is a hydride
ion, :H ) are transferred to the NAD+, forming
NADH plus a free proton, H+.
2. NADH dehydrogenase:
The free proton plus the hydride ion carried by
NADH are next transferred to NADH
dehydrogenase, an enzyme complex (complex
I)embedded in the inner mitochondrial membrane.
This complex has a tightly bound molecule of flavin
mononucleotide (FMN, a coenzyme structurally
related to FAD, that accepts the two hydrogen
atoms 2_e~ + 2H+) becoming FMNH2.
• NADH dehydrogenase also contains several
iron atoms paired with sulfur atoms to make
iron-sulfur centers (Figure 6.9). These are
necessary for the transfer of the hydrogen
atoms to the next member of the chain,
ubiquinone (known as coenzyme Q).
3. Coenzyme Q: Coenzyme Q is a quinone
derivative with a long isoprenoid tail. It
is also called ubiquinone because it is
ubiquitous in biologic systems. Coenzyme Q
can accept hydrogen atoms both from FMNH2,
produced by NADH dehydrogenase, and from
FADH2 (Complex II), which is produced by
succinate dehydrogenase and
acyl CoA dehydrogenase .
4. Cytochromes: The remaining members of the
electron transport chain are cytochromes.
Each contains a heme group made of a
porphyrin ring containing an atom of iron (see
p. 277). Unlike the heme groups of
hemoglobin, the cytochrome iron atom is
reversibly converted from its ferric (Fe+ 3) to
its ferrous( Fe+2) form as a normal part of its
function as a reversible carrier of electrons.
Electrons are passed along the chain from
coenzyme Q to cytochromes b and c (Complex
III) and a +a3 (Complex IV, see Figure 6.8)
5. Cytochrome a +a3 This cytochrome
complexes the only electron
carrier in which the heme iron has a free
ligand that can react directly with molecular
oxygen. At this site, the transported electrons,
molecular oxygen, and free protons are
brought together to produce water (see Figure
6.8). Cytochrome a + a3(also called
cytochrome oxidase) contains bound copper
atoms that are required for this complex
reaction to occur.
• Q Accepts Electrons Via Complex I and
Complex II.
• The Q Cycle Couples Electron Transfer to
Proton Transport in Complex III.
• Molecular Oxygen Is Reduced to Water Via
Complex IV.
6. Site-specific inhibitors: Site-specific
inhibitors of electron transport have been
identified and are illustrated in Figure (6.10)
These compounds prevent the passage of
electrons by binding to a component of the
chain, blocking the oxidation/reduction
reaction. Therefore, all electron carriers
before the block are fully reduced, whereas
those located after the block are oxidized.
[Note: Because electron transport and
oxidative phosphorylation are tightly coupled,
site-specific inhibition of the electron transport
chain also inhibits ATP synthesis.]
Sequence of redox system in
respiratory chain
• The arrangement of component enzyme and
coenzyme in the respiratory chain depend on
the redox potential, the most negative
potential is that of NAD so it ś the first member
of the respiratory chain , which can receive H
from substrate and become reduced.
C. Release of free energy during electron
transport
Free energy is released as electrons are
transferred along the electron transport chain
from an electron donor (reducing agent or
reductant) to an electron acceptor (oxidizing
agent or oxidant). The (electrons can be
transferred in different forms, for example, as
hydride ions (:H-) to NAD+, as hydrogen
atoms (-H) to FMN, coenzyme Q, and FAD, or
as electrons –e-) to cytochromes.
Site of ATP Production
Each pair of electrons when it enter the chain
from the beginning till reach with oxygen then
one high energy product (ATP) is produced in 3
sites between NADH and FP, CoQ or ubiquinone
and cytochrome b , cytochrome a a3 &O2 .
Therefore 3molecule of ATP are produced from
the cycle when started from the beginning (NAD)
to the end. While when the electrons enter the
respiratory chain from CoQ then 2ATP are
formed as it miss or by pass the first sites of
synthesis of ATP.
site1
ADP+Pi
NAD
site3
ATP
O2
ATP
FP
ADP+Pi
Cyto aa3
site2
ADP+Pi
ATP
CoQ
Cyto b
Cyto c
Cyto c1
OXIDATIVE PHOSPHORYLATION
• The transfer of electrons down the electron
transport chain is energetically favored
because NADH is a strong electron donor and
molecular oxygen is an avid electron acceptor.
Mechanism of oxidative
phosphorylation
A. Chemiosmotic hypothesis
• The chemiosmotic hypothesis (also known as
the Mitchell hypothesis) explains how the free
energy generated by the transport of electrons
by the electron transport chain is used to
produce ATP from ADP +Pi.
1. Proton pump:
Electron transport is coupled to the
phosphorylation of ADP by the transport of
protons (H+) across the inner mitochondrial
membrane from the matrix to the
intermembrane space. This process creates
across the inner mitochondrial membrane an
electrical gradient (with more positive charges
on the outside of the membrane than on the
inside) and a pH gradient (the outside of the
membrane is at a lower pH than the inside;
Figure 6.13).
• The energy generated by this proton
gradient is sufficient to drive ATP
synthesis. Thus, the proton gradient
serves as the common intermediate that
couples oxidation to phosphorylation.
2. ATP synthase:
The enzyme complex ATP synthase (complex V,
see Figure )6.13(synthesizes ATP, using the
energy of the proton gradient generated by the
electron transport chain. [Note: It is also
called ATPase, because the isolated enzyme
also catalyzes the hydrolysis of ATP to ADP and
inorganic phosphate.]
• The chemiosmotic hypothesis proposes that
after protons have been transferred to the
cytosolic side of the inner mitochondrial
membrane, they reenter the mitochondrial
matrix by passing through a channel in the
ATP synthase complex, resulting in the
synthesis of ATP from ADP + Pi .
A.Oligomycin: This drug binds to the stalk of
ATP synthase, closing the H+ channel, and
preventing reentry of protons into the
mitochondrial matrix. Electron transport stops
because of the difficulty of pumping any more
protons against the steep gradients. Electron
transport and phosphorylation are, therefore,
again shown to be tightly coupled processes—
inhibition of phosphorylation inhibits
oxidation.
b. Uncoupling proteins (UCP):
UCPs occur in the inner mitochondrial membrane of
mammals, including humans. These proteins
create a "proton leak," that is, they allow protons
to reenter the mitochondrial matrix without energy
being captured as ATP (Figure 6.14). [Note:
Energy is released in the form of heat.]UCP1, also
called thermogenin, is responsible for the
activation of fatty acid oxidation and heat
production in the brown adipocytes of mammals.
Brown fat, unlike the more abundant white fat,
wastes a most ninety percent of its respiratory
energy for thermogensis in response to cold, at
birth, and during arousal in hibernating
animals. However humans have little brown fat
(except in the newborn), and UCP1 does not
appear to play a major role in energy balance.
Other uncoupling proteins (UCP2, UCP3)
have been found in humans, but their
significance remains controversial.
c. Synthetic uncouplers: Electron transport and
phosphorylation can be uncoupled by compounds
that increase the permeability of the inner
mitochondrial membrane to protons. The classic
example is a 2,4-dinitrophenol, lipophilic proton
carrier that readily diffuses through the
mitochondial membrane. This uncoupler causes
electron transport to proceed at a rapid rate without
establishing a proton gradient, much as do the
UCPs (see Figure 6.14). The energy produced by the
transport of electrons is released as heat rather than
being used to synthesize ATP.
P/O Ratio
The P/O ratio is the ratio of the number of moles
of ATP generated to the number of atoms of
oxygen reduced in the electron transport
chain. It is used to express the efficiency of
oxidative phosphorylation of the system.
There are 3 sites of generation of ATP in the
electron transport chain. Thus P/O ratio is 3 if
reducing equivalents enter electron transport
chain through NAD. On the other hand for the
FAD requiring reactions P/O ratio is 2.
Transport of reducing equivalents
The inner mitochondrial membrane lacks an
NADH transport protein, and NADH produced
in the cytosol cannot directly penetrate into
mitochondria. However, two electrons of
NADH (also called reducing equivalents) are
transported from the cytosol into the
mitochondria using shuttle mechanisms.
❖glycerophosphate shuttle
Through the glycerophosphate shuttle electrons are
transported to the mitochondrial electron
transport chain via FAD.
In the first step, 3-phosphoglycerol dehydrogenase
with the help of dihydroxyacetone phosphate,
catalyzes the oxidation of cytosolic NADH to
NAD+ which re-enters glycolysis.
• In the next step, electrons from 3-phosphoglycerol
(glycerophosphate ) are transferred to a
flavoprotein . This oxidation of 3-phosphoglycerol
results in the reduction of FAD to FADH2. Since
flavoprotein dehydrogenase is situated on the outer
surface of the inner mitochondrial membrane, it
supplies electrons directly to the electron transport
chain and results in the reoxidation of FADH2 to
FAD within the mitochondria. The overall process
thus results in the transport of the cytosolic
reducing equivalents directly into the
mitochondrial electron transport chain (Figure
13.12).
❖Malate- Aspartate Shuttle
In Malate Aspartate shuttle, cytosolic
oxaloacetate is converted either to aspartate
through the action of aspartate
aminotransferase or to malate by malate
dehydrogenase.
With the use of reducing equivalents (from the
cytosolic NADH), oxaloacetate is reduced (by
malate dehydrogenase) to malate and gets
transported into the mitochondria. After its
reoxidation in the matrix by the same enzyme,
malate releases its reducing equivalents and
reoxidized to oxaloacetate.
• Oxaloacetae since is impermeable through the
mitochondrial membrane, it reacts with
glutamate. Glutamate oxaloacetate
transaminase transminates oxaloacetate to
aspartate, and glutamate to α-ketoglutarate.
Aspartate and α-ketoglutarate are transported
through the mitochondrial membrane to the
cytosol by the α-ketoglutarate transporter and
the Glutamate- Aspartate transporter,
respectively. In the cytosol, Aspartate is
reconverted to oxaloacetate.
• As both, aspartate as well as malate are
freely permeable through the inner
mitochondrial membrane, reducing
equivalents which are accepted by NAD+
and originated in the cytosol, are thus
transported to the mitochondria for their
entry into the electron transport
chain(Figure 13.13).