Biochemistry

1. 3/27/2024 1
FKBIWOTT/BIO121/2023-2024
MBM 110: MEDICAL
BIOCHEMISTRY
Lecture 3;
-Oxidative phosphorylation
Lecturer: Dr. Biwott Felix K.
Contacts: Email; fekiott@gmail.com
Mobile No: 0697-529-032
KIUT
1st Semester, 2023/2024

2. Oxidative phosphorylation
• The transport of electrons through the ETC is linked with
the release of free energy.
• The process of synthesizing ATP from ADP and Pi
coupled with the electron transport chain is known as
oxidative phosphorylation.
• The complex V of the inner mitochondrial membrane is the
site of oxidative phosphorylation.
• The number of inorganic phosphate molecules utilized
for ATP generation for every atom of oxygen consumed is
referred as P : O ratio (the number of molecules of ATP
synthesized per pair of electrons carried through ETC).
3/27/2024 2
FKBIWOTT/BIO121/2023-2024

3. 3/27/2024 3
FKBIWOTT/BIO121/2023-2024
• There are three sites in the ETC that are exergonic to result
in the synthesis of 3 ATP molecules.
1. Oxidation of FMNH2 by coenzyme
2. Oxidation of cytochrome b by cytochrome c1.
3. Cytochrome oxidase reaction.
• Each one of the above reactions represents a coupling site
for ATP production.
• There are only two coupling sites for the oxidation of
FADH2 (P : O ratio 2), since the first site is bypassed.
Sites of oxidative phosphorylation in ETC

4. 3/27/2024 4
FKBIWOTT/BIO121/2023-2024
• Several hypotheses have been put forth to explain the
process of oxidative phosphorylation.
• The most important among them—namely, chemical
coupling, and chemiosmotic.
Mechanism of Oxidative Phosphorylation

5. 3/27/2024 5
FKBIWOTT/BIO121/2023-2024
• It hypothesises that during the course of electron transfer
in respiratory chain, a series of phosphorylated high-
energy intermediates are first produced which are utilized
for the synthesis of ATP.
• These reactions are believed to be analogous to the
substrate level phosphorylation that occurs in glycolysis or
citric acid cycle.
Chemical coupling hypothesis

6. 3/27/2024 6
FKBIWOTT/BIO121/2023-2024
• This is the most accepted view of oxidative
phosphorylation.
• It 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.
• The concept of chemiosmotic hypothesis is comparable
with energy stored in a battery separated by positive and
negative charges.
• 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.
Chemiosmotic hypothesis

7. 3/27/2024 7
FKBIWOTT/BIO121/2023-2024
• It is an enzyme complex present in the inner mitochondrial
membrane. It is a conformational change in the
mitochondrial membrane proteins leading to the synthesis
of ATP.
• It is considered as rotary motor/engine driving model or
binding change model.
• It is now referred as COMPLEX V the enzyme complex
has two subunits – F0 and F1.
ATP SYNTHASE

8. 3/27/2024 FKBIWOTT/BIO121/2023-2024 8

9. 3/27/2024 9
FKBIWOTT/BIO121/2023-2024
F0 Unit or Subcomplex
• It spans inner mitochondrial membrane and serves as a
proton channel through which protons enter into
mitochondria.
• It is a disk of C-subunits. Attached to it is a γ-subunit in
the form of a bent axle. The γ-subunit fits inside the F1
subcomplex.

10. 3/27/2024 10
FKBIWOTT/BIO121/2023-2024
F1 Unit or Subcomplex
• This projects into the mitochondrial matrix. It catalyses
the ATP synthesis. F1 subcomplex consists of 3β chains
(β3) and 3α chains (α3). γ-subunit fits inside the F1
subcomplex of 3α and 3β
• subunits which are fixed to the membrane.
In addition, the complex has sigma and epsilon
• subunits, the function of which are not known.

11. 3/27/2024 11
FKBIWOTT/BIO121/2023-2024

12. 3/27/2024 12
FKBIWOTT/BIO121/2023-2024
Mechanism of ATP Synthesis (Boyer’s hypothesis)
• According to this hypothesis 3β subunits (catalytic sites)
though structurally similar, but functionally not same at a
particular time.
• It is envisaged that β-subunits occur in 3 forms:
• ‘O’ form (Open form): It has low affinity for
substrates ADP + Pi.
• ‘L’ form (Loose form): Can bind substrates ADP and
Pi with more affinity but catalytically it is inactive.
• ‘T’ form (Tight form): Binds substrates ADP and Pi
tightly and catalyses ATP synthesis.
• Protons entering the system cause conformational changes
in the β -subunits.

13. 3/27/2024 13
FKBIWOTT/BIO121/2023-2024