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ELECTRON TRANSPORT CHAINS &
OXIDATIVE PHOSPHORYLATION
CONTENTS:
 DEFINING ETC/ETP (ELECTRON TRANSPORT
CHAIN/PATHWAY)
 DEFINING REDOX (OXIDATION-REDUCTION REACTIONS)
 COMPONENTS OF ETC
 BIOENERGETICS & UNDERSTANDING ETC
 ROLE OF MITOCHONDRIA IN ETC
 CHEMIOSMOTIC THEORY
 OXIDATIVE STRESS
 MITOCHONDRIAL PATHWAYS OF ETC
 WHAT IS OXIDATIVE PHOSPHORYLATION?
 ASSOCIATION WITH KREB’S CYCLE OR CITRIC ACID CYCLE
 CHEMIOSMOTIC COUPLING
 SUMMARY
ELECTRON TRANSPORT CHAIN
1.
2.
Electron transport chains (also called
electron transfer pathways- ETP) are
biochemical reactions that produce ATP,
which is the energy currency of life.
An ETP is a series of linked membraneembedded electron carrier molecules that
transfer electrons, during a regulated
process of redox reactions, from one
electron carrier molecule to another
releasing and capturing energy for cellular
work.
ETC contd.
3. Only two sources of energy are available
to living organisms:
oxidation-reduction (redox) reactions
to produce ATP in chemotrophs
and using sunlight (photosynthesis) for
energy production in phototrophs.
4. Both chemotrophs and phototrophs utilize
electron transport chains to convert
energy into ATP.
REDOX REACTIONS
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Cells generate energy by using redox reactions
Redox reactions are chemical reactions in which
electrons are transferred from a oxidised donor
molecule (reducing agent) e.g. CH3 COOH (acetic acid)
to an electron-deficient acceptor molecule (oxidising
agent) e.g. CH3 CH2OH (ethyl alcohol).
The two processes always occur simultaneously.
The reducing agent that donates their electrons becomes
oxidised and the oxidising agent that that accept
electrons become reduced.
REDOX contd.
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When electrons are transferred energy is lost and
cells can capture this energy to do cellular work
such as synthesis of ATP – the energy carrier
molecule that directly supplies energy used to
maintain highly organised cellular structures and
function.
The underlying force driving these reactions is the
Gibbs free energy of the reactants and products.
The Gibbs free energy is the energy available
(“free”) to do work. Any reaction that decreases the
overall Gibbs free energy of a system will proceed
spontaneously
SOME BASIC DEFINITIONS
In an electron transfer reaction, an
element undergoing oxidation loses
electrons, whereas an element gaining
Electrons undergoes reduction.
Remember the formula: OIL RIG where
OIL refers to Oxidation Is Loss and RIG
Reduction Is Gain in electrons.
Basic concepts
When you take hydrogen ions or electrons
away from a molecule, you “oxidize” that
molecule.
When you give hydrogen ions or electrons to a
molecule, you “reduce” that molecule.
When you give phosphate molecules to a
molecule, you “phosphorylate” that
molecule.
Oxidation-reactions
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Reactions of metals or any other
organic compounds with oxygen to
give oxides are labelled as oxidation.
In other words, oxidation is addition of
oxygen to a compound or removal of
hydrogen from a compound
Oxidation increases C-O bonds.
Reduction reactions
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The removal of oxygen from metal oxides to give
the metals in their elemental forms is labeled as
reduction.
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Whereas, reduction is addition of hydrogen or
removal of oxygen; it increases C-H bonds.
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The more reduced is a molecule, the more H+
atoms and more energy it contains.
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Fatty acids have more hydrogen atoms than sugars;
hence yield more energy when oxidised.
COMPONENTS OF ETC/ETP
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MITOCHONDRIA
ELECTRONS AND PROTONS
OXYGEN
NAD+/NADH (redox coenzymes)
FLAVIN NUCLEOTIDES (redox
coenzymes)
COENZYME Q (UBIQUINONE)
IRON-SULFUR PROTEINS
BIOENERGETICS
The study of bioenergetics involves the processes
which reduce nicitinamides and flavin nucleotides,
generated from oxidation of carbohydrates and
lipids by molecular oxygen via mitochondrial
electron-transport chain (ETC) and the mechanism
(oxidative phosphorylation) where oxidation is
coupled to ATP synthesis.
Oxidative phosphorylation is central to
metabolism because the free energy of hydrolysis
of the ATP generated is used in the synthesis of
biomolecules and biological activities such as
muscle contraction and transmission of nerve
impulses.
ATP synthase & ETC
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ATP is made by an enzyme called ATP synthase.
The structure of this enzyme and its underlying
genetic code is remarkably similar in all known
forms of life.
ATP synthase is powered by a transmembrane
electrochemical potential gradient, usually in the
form of a proton gradient.
The function of the electron transport chain is to
produce this gradient.
In all living organisms, a series of redox
reactions is used to produce a transmembrane
electrochemical potential gradient.
UNDERSTANDING THE ELECTRON TRANSFER CHAIN
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The transfer of electrons from a high-energy molecule (the donor) to
a lower-energy molecule (the acceptor) can be spatially separated
into a series of intermediate redox reactions. This is an electron
transport chain.
Electron transport chains produce energy in the form of a
transmembrane electrochemical potential gradient. This energy is
used to do useful work. The gradient can be used to transport
molecules across membranes. It can be used to produce ATP and
NADH, high-energy molecules that are necessary for growth.
A small amount of ATP is available from substrate-level
phosphorylation (for example, in glycolysis).
Some organisms can obtain ATP exclusively by fermentation. In most
organisms, however, the majority of ATP is generated by electron
transport chains.
ETC IN MITOCHONDRIA
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The cells of all eukaryotes (all animals, plants, fungi,
algae – in other words, all living things except bacteria
and archaea) contain intracellular organelles called
mitochondria that produce ATP.
Energy sources such as glucose are initially metabolized
in the cytoplasm. The products are imported into
mitochondria.
Mitochondria continue the process of catabolism using
metabolic pathways including the Krebs cycle, fatty acid
oxidation and amino acid oxidation.
The end result of these pathways is the production of
two energy-rich electron donors, NADH and FADH2.
ETC IN MITOCHONDRIA- contd.
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Electrons from these donors are passed
through an electron transport chain to
oxygen, which is reduced to water. This is a
multi-step redox process that occurs on the
mitochondrial inner membrane.
The enzymes that catalyze these reactions
have the remarkable ability to
simultaneously create a proton gradient
across the membrane, producing a
thermodynamically unlikely high-energy
state with the potential to do work
CHEMIOSMOTIC THEORY
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Peter Mitchell, a Britiah biochemist, in 1961,
proposed a mechanism by which the free energy
generated during electron transport drives ATP
synthesis
As electrons pass through the ETC, protons ae
transported from the matrix and released into the
inter membrane space
As a result, an electrical potential and proton
gradient (pH) arise across the inner membrane and
this elecrochemical proton gradient is often referred
as protonmotive force
CHEMIOSMO THEORY contd.
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Protons, present in the intermembrane in
excess can pass through the inner
membrane and back into the matrix down
their concentration gradient only through
special channels as the inner membrane is
impermeable to ions (protons)
As the themodynamically favorable flow of
protons occur through a channel, each of
which contains an ATP synthase activity,
an ATP synthesis occurs.
Chemiosmo theory contd.
Uncouplers:
A variety of molecules such as, dinitrophenol
(DNP) and gramicidin can collapse the
proton gradient by equalising the proton
concentration on both sides of the
membrane and according to the
chemiosmotic theory, a disrupted proton
gradient dissipates the energy (derived from
food) as heat
OXIDATIVE STRESS
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Oxygen can accept single electrons to form
unstable derivatives, referred as reactive
oxygen species (ROS) e.g. Hydrogen
peroxide
Because ROS are so reactive, they can
seriously damage living cells if formed in
significant ammounts
They can be kept to the minimum by
antioxidants e.g. Alpha-tocopherol
(Vitamin E), beta-Carotene (Vitamin A) that
inhibit the reaction of molecules with
oxygen radicals.
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MITOCHONDRIAL REDOX
CARRIERS
Four membrane-bound complexes have
been identified in mitochondria.
Each is an extremely complex
transmembrane structure that is
embedded in the inner membrane.
Three of them are proton pumps. The
structures are electrically connected by
lipid-soluble electron carriers and watersoluble electron carriers
PATHWAYS (COMPLEXES) OF ETC
The overall electron transport chain is:
NADH → Complex I → Q → Complex III →cytochrome c →
Complex IV → O2
↑
Complex II
Complex I (NADH dehydrogenase) removes
two electrons from NADH and transfers
them to a lipid-soluble carrier, ubiquinone
(Q). The reduced product, ubiquinol (QH2)
is free to diffuse within the membrane. At
the same time, Complex I moves four
protons (H+) across the membrane,
producing a proton gradient.
Complex I is also called NADH:ubiquinone
oxidoreductase.
COMPLEX I
1.
2.
3.
NADH (nicotinamide adenine dinucleotide
reduced form) is oxidized to NAD+, reducing
FMN (flavin mononucleotide) to FMNH2 in one
two-electron site
The next electron carrier is a Fe-S cluster, which
can only accept one electron at a time to reduce
the ferric ion into a ferrous ion.
Conveniently, FMNH2 can only be oxidized in two
one-electron steps, through a semiquinone
intermediate.
COMPEX I contd.
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The electron thus travels from the FMNH2 to the
Fe-S cluster, then from the Fe-S cluster to the
oxidized Q to give the free-radical (semiquinone)
form of Q.
This happens again to reduce the semiquinone
form to the ubiquinol form, QH2. During this
process, four protons are translocated across the
inner mitochondrial membrane, from the matrix
to the intermembrane space
This creates a proton gradient that will be later
used to generate ATP through oxidative
phosphorylation.
COMPLEX II
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Complex II (succinate dehydrogenase) is
not a proton pump. It serves to funnel
additional electrons into the quinone pool
(Q) by removing electrons from succinate
and transferring them (via FAD) to Q.
Other electron donors (e.g. fatty acids and
glycerol 3-phosphate) also funnel electrons
into Q (via FAD), again without producing a
proton gradient.
COMPLEX III
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Complex III (cytochrome bc1 complex) removes in
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At the same time, it moves four protons across the
membrane, producing a proton gradient.
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a stepwise fashion two electrons from QH2 and
transfers them to two molecules of cytochrome c, a
water-soluble electron carrier located on the outer
surface of the membrane.
When electron transfer is hindered (by a high
membrane potential, point mutations or respiratory
inhibitors such as antimycin A), Complex III may
leak electrons to oxygen resulting in the formation
of a superoxide.
COMPLEX IV
Complex IV (cytochrome c oxidase)
removes four electrons from four
molecules of cytochrome c and
transfers them to molecular oxygen
(O2), producing two molecules of
water (H2O). At the same time, it
moves four protons across the
membrane, producing a proton
gradient.
PROTON PUMP MECHNISMS
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There are three proton pumps: I, III and IV.
The resulting transmembrane proton
gradient is used to make ATP via ATP
synthase.
The trans membrane electrochemical
gradient acts as the intermediate in the
transfer of energy to ATP.
Oxidation of NADH results in the production
of about 3 molecules per O reduced to
water.
Oxidation of FADH2 yields 2 ATP’s.
WHAT IS OXIDATIVE
PHOSPHORYLATION?
When you give phosphate molecules to a
molecule, you “phosphorylate” that
molecule.
So, oxidative phosphorylation (very
simply) means the process that couples
the removal of hydrogen ions from
one molecule and giving phosphate
molecules to another molecule.
MITOCHONDRIA IN OXIDATIVE
PHOSPHORYLATION
Why do we need mitochondria?
The whole idea behind this process is to
get as much ATP out of glucose (or other
food products) as possible. If we have no
oxygen, we get only 4 molecules of ATP
energy packets for each glucose molecule
(in glycolysis).
However, if we have oxygen, then we get
to run the Kreb’s cycle to produce many
more hydrogen ions that can run those ATP
pumps.
WHY MITOCHONDRIA?
From the Kreb’s cycle we get 24-28 ATP
molecules out of one molecule of glucose
converted to pyruvate (plus the 4
molecules we got out of glycolysis).
So, you can see how much more energy we
can get out of a molecule of glucose if our
mitochondria are working and if we have
oxygen.
ATP synthase complex
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This complex (sometimes termed as
COMPLEX V of ETC) is found in all energy
transducing membranes including that of
the mitochondria.
It contains a proton transport channel, the
only way for the protons to reenter the
mitochondrial matrix.
The energy proton potential gradient is used
in the synthesis of ATP from ADP and Pi
ETC ON CRISTAE
IMPORTANCE OF CRISTAE
You can now appreciate the
importance of the cristae....not only do
they contain and organize the electron
transport chain and the ATP pumps,
they also serve to separate the matrix
from the space that will contain the
hydrogen ions, allowing the gradient
needed to drive the pump
ASSOCIATION WITH KREB’S
CYCLE OR CITRIC ACID CYCLE
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As the Kreb’s cycle runs, hydrogen ions (or
electrons) are donated to the two carrier
molecules in 4 of the steps. They are
picked up by either NAD or FAD and these
carrier molecules become NADH and FADH
(because they now are carrying a hydrogen
ion).
The NADH and FADH essentially serve as a
ferry in the lateral plane of the membrane
diffusing from one complex to the next. At
each site is a hydrogen (or proton) pump
which transfers hydrogen from one side of
the membrane to the other.
KREB’S CYCLE contd.
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This creates a gradient across the
inner membrane with a higher
concentration of Hydrogen ions in the
inter cristae space (this is the space
between the inner and outer
membranes).
The electrons are carried from
complex to complex by ubiquinone
and cycochrome C.
OXIDATIVE PHOSPHORYLATION
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The chemiosmotic coupling hypothesis, as proposed
by Nobel Prize in Chemistry winner Peter D. Mitchell
explains that the electron transport chain and
oxidative phosphorylation are coupled by a proton
gradient across the inner mitochondrial
membrane. The efflux of protons creates both a
pH gradient and an electrochemical gradient
This proton gradient is used by the F0F1 ATP
synthase complex to make ATP via oxidative
phosphorylation. ATP synthase is sometimes
regarded as complex V of the electron transport
chain.
OXYDAPHOS-contd.
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The F0 component of ATP synthase acts as an ion
channel for return of protons back to mitochondrial
matrix. During their return, the free energy
produced during the generation of the oxidized
forms of the electron carriers (NAD+ and FAD) is
released.
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This energy is used to drive ATP synthesis,
catalyzed by the F1 component of the complex.
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Coupling with oxidaive phosphorylation is a key
step for ATP production.
SUMMARY
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The mitochondrial electron transport chain removes
electrons from an electron donor (NADH or FADH2)
and passes them to a terminal electron acceptor
(O2) via a series of redox reactions.
These reactions are coupled to the creation of a
proton gradient across the mitochondrial inner
membrane.
There are three proton pumps: I, III and IV. The
resulting transmembrane proton gradient is used to
make ATP via ATP synthase.
Summary contd.
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The reactions catalyzed by Complex I and
Complex III exist roughly at equilibrium.
The steady-state concentrations of the
reactants and products are approximately
equal.
This means that these reactions are readily
reversible, simply by increasing the
concentration of the products relative to the
concentration of the reactants (for example,
by increasing the proton gradient).
Summary contd.
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ATP synthase is also readily reversible.
Thus ATP can be used to make a
proton gradient, which in turn can be
used to make NADH.
This process of reverse electron
transport is important in many
prokaryotic electron transport chains
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