Oxidative Phosphorylation 31.05.2022.ppt

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1. Oxidative Phosphorylation
31.05.2022

2. oWhat is phosphorylation?
oWhat is Oxidative phosphorylation?
Formation of ATP from coupling of ADP and Pi is
known as phosphorylation.
Three types-
(1) Substrate level phosphorylation
(2) Photophosphorylation
(3) Oxidative phosphorylation

3. (1) Substrate level phosphorylation
•Substrate-level phosphorylation 
is a metabolic
reaction
 that results in the formation of ATP
or
 GTP by the direct transfer of a phosphoryl(PO
3)
group to ADP or
 GDP from another phosphorylated
compound.
•Enzymatic Transfer of phosphate from
substrate to ADP to form ATP.
•ATP made in glycolysis and the TCA cycle is
the result of substrate-level phosphorylation

4. (2) Photophosphorylation
•Formation of ATP in light reaction of photosynthesis.
•In which photosynthetic organisms capture the energy of
sunlight—the ultimate source of energy in the biosphere
and harness it to make ATP
•Photophosphorylation involves the oxidation of H
2O to
O
2, with NADP as ultimate electron acceptor; it is
absolutely dependent on the energy of light.

5. Oxidative phosphorylation
How cells convert the stored metabolic energy of NADH
and [FADH2] into ATP?
NADH or FADH
2-dependent ATP synthesis is the
result of oxidative phosphorylation, i.e. Formation of
ATP from the oxidation of NADH or FADH
2
in
presence of oxygen through electron transport chain
is known as oxidative phosphorylation.
Oxidative phosphorylation is the collection of energy
yielding metabolism in aerobic organisms.
All oxidative steps in the degradation of
carbohydrates, fats, and amino go through this final
stage of cellular respiration, in which the energy of
oxidation drives the synthesis of ATP.

6. Electrons stored in the form of the reduced coenzymes,
NADH or [FADH2], are passed through an elaborate and
highly organized chain of proteins and coenzymes, therefore
called electron transport chain, finally reaching O
2
(molecular
oxygen) is the terminal electron acceptor.
Each component of the chain can exist in (at least) two
oxidation states, and each component is successively
reduced and re-oxidized as electrons move through the chain
from NADH (or [FADH2]) to O
2
.

7.  In the course of electron transport, a proton gradient is
established across the inner mitochondrial membrane.
It is the energy of this proton gradient that drives ATP
synthesis.
In eukaryotes, oxidative phosphorylation occurs in inner
mitochondrial membrane, and in prokaryote occurs in
plasma membrane.
Oxidative phosphorylation involves the reduction of O
2 to
H
2O with electrons donated by NADH and FADH
2.
It occurs equally well in light or darkness.

8. Oxidative phosphorylation and photophosphorylation are
mechanistically similar in three respects.
(1) Both processes involve the flow of electrons through a
chain of membrane-bound carriers.
(2) The free energy made available by this “downhill”
(exergonic) electron flow is coupled to the “uphill”
transport of protons across a proton-impermeable
membrane, conserving the free energy of fuel oxidation
as a transmembrane electrochemical potential.
(3) The transmembrane flow of protons down their
concentration gradient through specific protein channels
provides the free energy for synthesis of ATP, catalyzed
by a membrane protein complex (ATP synthase) that
couples proton flow to phosphorylation of ADP.

9. The process of oxidative phosphorylation is closely
associated with the functioning of the electron transport
chain.
This was studied by fragmentation of mitochondria.
In the first fragmentation step, the outer membrane is
removed by treatment with various detergents such as
saponin, digitonin.
The two particulate fractions that result are:
1.The outer membrane, either in the form of vesicles or
completely solubilized.
2. The inner membrane and the mitochondrial matrix
enzymes.
This fraction is found to contain the enzymes of:
• The electron transport chain
• Oxidative phosphorylation
• The TCA cycle.

10. In oxidative phosphorylation ATP is produced
by combining ADP and Pi with the energy
generated by the flow of electrons from
NADH to molecular oxygen in the electron
transport chain.
There are three sites in the respiratory chain
where ATP is formed by oxidative
phosphorylation.

11. These sites have been proved by the free energy
changes of the various redox couples.
Since hydrolysis of ATP to ADP + Pi releases
around 7.3 K. Cal/mole, the formation ATP from
ADP + Pi requires a minimum of around 8
KCal/mole.
The formation of ATP is therefore not possible at
the sites where free energy released is less than
8KCal/mole.

12. Sites of ATP formation:
There are three sites in the respiratory chain where
ATP can be formed.
Site I: This involves the transfer of electrons from
NADH–CoQ. Obviously this step is omitted by succinic
dehydrogenase whose FADH2 prosthetic group
transfers its electrons directly to CoQ by passing
NADH. This step is blocked by piericidin,
rotenone,amobarbital,certain drugs
like,chlorpromazine, guanethidine.

13. Site II: This involves the transfer of electrons from
Cyt.b–Cyt-c1. This step as well as the previous one is
by passed in oxidation of L-ascorbate whose electrons
are directly transferred to Cyt-c.
This step is blocked by, Antimycin A, Hypoglycemic
drug like phenformin.
Site III: Transfer of electrons from Cyt-a3 to
molecular oxygen which is blocked by CO, CN,
H2S, and azide.

14. Mechanism of Oxidative Phosphorylation
Three major proposals for the mechanism of oxidative phosphorylation
have been considered.
The synthesis of ATP is carried out by a molecular assembly in the inner
mitochondrial membrane.
This enzyme complex is called Mitochondrial ATPase or H+-ATPase.
It is also called ATP synthase.
Theories
The three hypothesis do make use of the information
available on ATP synthase.
A. The chemical coupling hypothesis: This is developed
from the concept of a high energy intermediate common
to both electron transport and phosphorylation of ADP.
However, such intermediate has not been identified so
far.
B. The conformational coupling hypothesis: According to
this hypothesis the mitochondrial cristae undergo conformational changes
and these changes in architecture of the mitochondrial cristae reflect the
changes in the different components of the electron chain to one
another. It is believed that these conformational change represents the
formation of high energy state.

15. C. Chemiosmotic theory: This is the most accepted view of oxidative
phosphorylation postulated by Peter Mitchell in 1961. Mitchell’s
chemiosmotic theory postulates that the energy from oxidation of
components in the respiration chain is coupled to the translocation of
hydrogen ions (Protons, H+) from the inside to the outside of the inner
mitochondrial membrane. Each of the respiratory chain complexes I,
III and IV acts as a proton pump.
The inner membrane is impermeable to ions in general but particularly to
protons, which accumulate outside the membrane,creating an
electrochemical potential difference across the membrane (ΔμH+).
This consists of a chemical potential (difference in pH) and an electrical
potential. The electrochemical potential resulting from the asymmetric
distribution of the hydrogen ion is used to drive the mechanism
responsible for the formation of ATP

16. Thank You