An electron transport chain (ETC) is a series of protein complexes and other molecules that transfer electrons from electron donors to electron acceptors via redox reactions (both reduction and oxidation occurring simultaneously) and couples this electron transfer with the transfer of protons (H+ ions) across a membrane. The electrons that are transferred from NADH and FADH2 to the ETC involves four multi-subunit large enzymes complexes and two mobile electron carriers. Many of the enzymes in the electron transport chain are embedded within the membrane. In an electron transport chain, the redox reactions are driven by the difference in the Gibbs free energy of reactants and products. The free energy released when a higher-energy electron donor and acceptor convert to lower-energy products, while electrons are transferred from a lower to a higher redox potential, is used by the complexes in the electron transport chain to create an electrochemical gradient of ions. It is this electrochemical gradient that drives the synthesis of ATP via coupling with oxidative phosphorylation with ATP synthase.

1. CHM-903
BIOCHEMISTRY
ELECTRON TRANSPORT
CHAIN
Presented by
DEEKSHA RANI
2103345
M.Sc(FINAL YEAR)

2. In eukaryotes => Electron transport and oxidative
phosphorylation => inner mitochondrial membrane.
These processes => re-oxidize NADH and FADH2 <=
from the citric acid cycle (mitochondrial matrix ),
glycolysis (cytoplasm ) and fatty acid oxidation (
mitochondrial matrix) and => trap the energy released
as ATP.
Oxidative phosphorylation => major source of ATP in
the cell.
In prokaryotes => electron transport and oxidative
phosphorylation components => in the plasma
membrane.

3. Oxidation => loss of electrons.
 Reduction => gain of electrons.
 In chemical reaction :
 if one molecule is oxidized => another must
be reduced
 i.e. oxidation-reduction reaction => transfer of
electrons.

4.  when NADH => oxidized to NAD+ => it
loses electrons.
 When molecular oxygen => reduced to water
=> it gains electrons :

5. ATP is the energy currency of the cell and is used to
power a variety of cellular reactions
Biosynthesis of macromolecules
(e.g. polymer assembly )
Emission of light (e.g.
bioluminescence
Movement (e.g. muscle
contraction)
Active transport (e.g.
endocytosis / exocytosis)
Nerve transmission (e.g.
propagation of action potentials)
Growth and repair (e.g. mitotic
division)
Biochemical
processes that
utilize ATP include

7. Step 1: Generating a Proton Motive Force
 The hydrogen carriers (NADH and FADH2) are oxidised and release high
energy electrons and protons
 The electrons are transferred to the electron transport chain, which
consists of several transmembrane carrier proteins
 As electrons pass through the chain, they lose energy – which is used
by the chain to pump protons (H+ ions) from the matrix
 The accumulation of H+ ions within the intermembrane space creates
an electrochemical gradient (or a proton motive force)

8. Step Two: ATP Synthesis
 The proton motive force will cause H+ ions to move down
their electrochemical gradient and diffuse back into matrix.
 This diffusion of protons is called chemiosmosis and is
facilitated by the transmembrane enzyme ATP synthase
 As the H+ ions move through ATP synthase they trigger the
molecular rotation of the enzyme, synthesising ATP

9. Step Three: Reduction of Oxygen
 In order for the electron transport chain to continue
functioning, the de-energised electrons must be removed
 Oxygen acts as the final electron acceptor, removing the de-
energised electrons to prevent the chain from becoming
blocked
 Oxygen also binds with free protons in the matrix to form
water – removing matrix protons maintains the hydrogen
gradient
 In the absence of oxygen, hydrogen carriers cannot transfer
energised electrons to the chain and ATP production is halted

10. The electron
transport
chain is a set
of four
protein compl
exes
Complex 1-
NADH-Q
oxidoreduc
tase
Q and
Complex 2-
Succinate-
Q
reductase
Complex 3-
Cytochrome
c reductase
Complex 4-
Cytochrom
e c oxidase

11. Complex 1- NADH-Q oxidoreductase: It
comprises enzymes consisting of iron-sulfur and
FMN. Here two electrons are carried out to the
first complex aboard NADH. FMN is derived from
vitamin B2.

12. • Q and Complex 2- Succinate-Q reductase: FADH2
that is not passed through complex 1 is received
directly from complex 2. The first and the second
complexes are connected to a third complex
through compound ubiquinone (Q). The Q
molecule is soluble in water and moves freely in
the hydrophobic core of the membrane. In this
phase, an electron is delivered directly to the
electron protein chain. The number of ATP
obtained at this stage is directly proportional to
the number of protons that are pumped across
the inner membrane of the mitochondria.

13. • Complex 3- Cytochrome c reductase: The
third complex is comprised of Fe-S protein,
Cytochrome b, and Cytochrome c proteins.
Cytochrome proteins consist of the heme
group. Complex 3 is responsible for pumping
protons across the membrane. It also passes
electrons to the cytochrome c where it is
transported to the 4th complex of enzymes
and proteins. Here, Q is the electron donor
and Cytochrome C is the electron acceptor.

14. • Complex 4- Cytochrome c oxidase: The 4th
complex is comprised of cytochrome c, a and
a3. There are two heme groups where each of
them is present in cytochromes c and a3. The
cytochromes are responsible for holding
oxygen molecule between copper and iron
until the oxygen content is reduced
completely. In this phase, the reduced oxygen
picks two hydrogen ions from the surrounding
environment to make water.

15. Summary
 Hydrogen carriers donate high energy electrons to the electron transport
chain (located on the cristae)
 As the electrons move through the chain they lose energy, which is
transferred to the electron carriers within the chain
 The electron carriers use this energy to pump hydrogen ions from the
matrix and into the intermembrane space
 The accumulation of H+ ions in the intermembrane space creates an
electrochemical gradient (or a proton motive force)
 H+ ions return to the matrix via the transmembrane enzyme ATP synthase
(this diffusion of ions is called chemiosmosis)
 As the ions pass through ATP synthase they trigger a phosphorylation
reaction which produces ATP (from ADP + Pi)
 The de-energised electrons are removed from the chain by oxygen,
allowing new high energy electrons to enter the chain
 Oxygen also binds matrix protons to form water – this maintains the
hydrogen gradient by removing H+ ions from the matrix