Download Aerobic Metabolism ii: electron transport chain

AEROBIC METABOLISM II:
ELECTRON TRANSPORT
CHAIN
Khadijah Hanim Abdul Rahman
School of Bioprocess Eng, UniMAP
Week 15: 17/12/2012
Introduction
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The electron transport chain (ETC) is a series of
electron carriers in the inner membrane of the
mitochondria of eukaryotes and the plasma
membrane of aerobic prokaryotes.
To increase electron affinity that transfer the
electrons derived from reduced coenzymes to O2.
During this transfer, a decrease in oxidationreduction potential occurs.
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The process of which O2 is used to generate energy
from food molecules is referred as aerobic
respiration.
Energy released during electron transfer is coupled
to several endergonic processes- ATP synthesis.
Reduced coenzymes from glycolysis, the citric acid
cycle and fatty acid oxidation- principle sources of
electrons.
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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.
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 highenergy state with the potential to do work
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Electron transport and its components
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The components of the ETC in eukaryotes are
located in the inner mitochondrial membrane.
Most ETC components are organized into 4
complexes.
The 2 other molecules, coenzyme Q (ubiquinone,
UQ) and cytochrome C (cyt c).
OUTLINE OF ETC
NADH → Complex I → UQ → Complex III → cytochrome c → Complex IV → O2
Complex II
FADH2
Complex I
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Referred as NADH dehydrogenase complex- catalyzes
the transfer of 2 electrons from NADH to UQ.
The reduced product, ubiquinol (QH2) freely diffuses
within the membrane, and Complex I translocates four
protons (H+) across the membrane, thus producing a
proton gradient.
Major sources of NADH include several reactions of
TCA cycle and fatty acid oxidation.
Composed of at least 25 different polypeptides,
Complex 1 is the largest protein component in the inner
membrane.
Transfer of electrons thru Complex I of
the mitochondrial ETC
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NADH reduces FMN to FMNH2
Electrons are then transferred
from FMNH2 to an iron-sulfur
centre (Fe-S), 1 electron at a
time.
After transfer from 1 Fe-S
centre to another, the electrons
are eventually donated to UQ.
Electron transport accompanied
by the movement of protons
from the matrix across the innermembrane and into the
intermembrane space.
Complex II
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Complex II- succinate
dehydrogenase complex consists
of the TCA cycle enzyme succinate
dehydrogenase and 2 Fe-S
proteins.
Complex II mediates the transfer
of electrons from succinate to UQ.
In Complex II additional electrons
are delivered into the quinone
pool (UQ) originating from
succinate and transferred (via
FAD) to UQ.
Complex III
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Complex III transfers electrons from reduced
coenzyme Q (UQH2) to cytochrome c.
Complex III contains 2 b-type cytochromes, 1
cytochrome c1 (cyt c1) and 1 Fe-S center- referred
as cytochrome bc1 complex.
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Electron transfer begins with
the oxidation of UQH2 by
the iron-sulfur protein in
complex III, which generates
ubisemiquinone (UQH.)
The reduced Fe-S protein
transfers an electron to cyt
c1, which transfers it to cyt c.
4 protons are released on
the cytoplasmic side of the
inner membrane.
Complex IV
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In Complex IV (cytochrome c oxidase) sometimes
called cytochrome A3, four electrons are removed
from four molecules of cytochrome c (protein that
loosely attached to the inner membrane, transfers
electrons 1 at a time to cyt a)- and transferred to
molecular oxygen (O2), producing two molecules of
water.
At the same time, four protons are translocated
across the membrane, contributing to the proton
gradient.
O2 + 4H+ + 4e-  2H2O
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During each sequential redox reaction in the ETC, an
electron loses energy.
During the oxidation of NADH there are 3 steps in
which the change in reduction potential is sufficient for
ATP synthesis.
This steps occurs in complexes I, III and IV.
The resulting transmembrane proton gradient is used
to make ATP via ATP synthase.
Recent experimental evidence indicates that
approximately 2.5 molecules of ATP are synthesized for
each pair of electrons transferred between NADH and
O2 in the ETC.
~1.5 molecules of ATP result from transfer of each pair
donated by FADH2 produced by succinate oxidation.
Oxidative Phosphorylation
The process whereby the energy generated by the
ETC is conserved by the phosphorylation of ADP to
yield ATP.
The 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
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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
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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.
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.