MEDICINAL BIOCHEMISTRY CHAPTER-III

BIOLOGICAL OXIDATION
Oxidation is a reaction with oxygen directly or indirectly / removal of hydrogen or

electron . This reaction carried out by enzymes . The electron released by this reaction
accepted by Electron acceptors (NAD,FAD) and then formation of ATP occurs , this
process takes place in living tissues and necessary for survival so it is called
Biological Oxidation .
COENZYME SYSTEM INVOLVED IN BIOLOGICAL OXIDATION:

All the Enzymes participating in β-Oxidation belongs to the class

Oxidoreductases. These are further grouped into four categories.
A) Oxidases
B) Dehydrogenases
C) Hydroperioxidases
D) Oxygenases
A) Oxidases: These enzymes catalyse the elimination of H+ from the substrate which
is accepted by oxygen to form mostly water.
Ex: Cytochrome oxidase, Tyrosinase, Monoamino oxidase (H202 formed instead of
H20).Herecytochrome oxidase,the terminal Component of Electron Transport Chain
B) Dehydrogenases: These enzymes cannot utilize oxygen as hydrogen
acceptor.They catalyses the reversible transfer of Hydrogen from one substrate to
another & thus bring about oxidation-reduction reactions.There are a large number of
Enzymes belonging to this group.NAD+ dependent dehydrogenase.
Ex: Alcohol Dehydrogenase, Glycerol-3-phosphate dehydrogenase.
NADP+ Dependent dehydrogenases:
Ex: HMC CoA Reductase, Enoyl reductase, FMN dependent dehydrogenases ex:
NADH Dehydrogenase.
FAD Dependent dehydrogenase:
Ex: Succinate dehydrogenase, AcylCoA dehydrogenase
The Cytochromes: All the Cytochromes of ETC (b,c1 & c) except the terminal
cytochrome oxidase (a & a1).
C) Hydroperoxidases: Hydrogen peroxide is the substrate for these enzyme .There is
a constract production of H202 in the reactions catalysed by the aerobic
dehydrogenases.The Harmful effects of H202 are prevented by Hydroperoxidases.

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Ex: Peroxidases & Catalase

2H2O2→ 2H2O + O2
D) Oxygenases: This group of enzymes catalyses the direct incorporation of oxygen
into the substrate molecule.
Dioxygenases (true oxygenase): They are responsible for the incorporation of both the
atoms of oxygen (O2) into the substrate.
Ex:Homogentiate oxidase,1-trytophan pyrrolase.
Monooxygenase (Mixed Fuction oxidases): They catalyse the incorporation of one
atom of oxygen, while the other oxygen is reduced to water.
BIOLOGICAL OXIDATION:
Oxidation is a reaction with oxygen directly or indirectly / removal of
hydrogen or electron . This reaction carried out by enzymes.The electron released by
this reaction accepted by Electron acceptors ( NAD , FAD ) , and then formation of
ATP occurs , this process takes place in living tissues , and necessary for survival so it
is called Biological Oxidation .
Electron Transport Chain and Oxidative Phosphorylation:
Electron Transport Chain is a series of protein complex and other molecules that
accept and transfers electron from NADH and FADH2 to Oxygen,when they combine
with oxygen the synthesis of ATP occurs. In formation of ATP Phosphorus is used
and Oxidation-Reduction reaction involved that is why it called Oxidative
phosphorylation.
ETC is formed of a series of electron carriers which catalyses the transfer of electrons
from reduced coenzymes to oxygen to form water.

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Biological role of the oxidative phosphorylation:

Bioenergetics: It describes the transfer and utilization of energy in biological system.

All the catabolic products of the food components (CHO, fats and proteins)
are metabolized into principle sources of reducing equivalents (such as NAD &
FAD). These NAD and FAD have a high transfer [redox] potentials.
RH2 + NAD+ → NADH+H+ + R
Electron Transport: Electrons carried by reduced coenzymes (NADH & FADH)
are passed through a chain of proteins and coenzymes to drive the generation of a
electrochemical or proton gradient across the inner mitochondrial membrane.
Redox potential (Electron affinity ) : Oxygen has the highest electron affinity (↑↑↑
highest
redox-potential), electrophilic. Hydrogen has the lowest electron affinity (↓↓↓ lowest
redox potential), nucleophilic.
Oxidative phosphorylation is the process of converting this high redox
potential into
energy-rich ATP molecules.
RH2→NADH & FADH→ETC Proteins (electron transport + H+ pumping→
Electro chemical Gradient)→O2→ H20 + ATP

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Components of ETC are arranged in order of increasing redox potential.

Electron pass on from electronegative NADH to electropositive O2 .

Electron transfer to O2 is highly exergonic. Called respiratory chain because

of the reduction of O2 from respiration into H2O. 95% of oxygen consumed by
humans is reduced to H2O by cytochrome oxidase (300 ml H2O/day) and called
metabolic water.
Correlation between ElectronTransport system & oxidative phosphorylation
Electron Transport: Electrons carried by reduced coenzymes are passed through a
chain of proteins and coenzymes (in ETC) to drive the generation of a proton gradient
across the inner mitochondrial membrane.
Oxidative Phosphorylation: The proton gradient runs downhill to drive the synthesis
of ATP.
In biologic systems: Cells use electron transport chain to transfer electrons stepwise
from substrates to oxygen. Thus producing energy gradually. This process is stepwise,
efficient and controlled.
 During hydrogen (H+ and electron) transfer through different components of the
redox chain, energy is released gradually in small utilizable amounts instead of a

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massive energy production in the form of heat , which if happens may destroy the
living cells.
COMPONENTS OF ETC:
 Four protein complexes (I to IV) in the inner mitochondrial membrane and one
ATP synthase complex.
 A lipid soluble coenzyme (UQ, CoQ) and a water soluble protein (cytc) shuttle
between protein complexes.
 Electrons generally fall or flow in energy through the chain - from complexes I
and II to complex III IV (RH2→H+ e → O2 ) .
 Complex I = NADH-CoQ10 oxidoreductase (Electron transfer from NADH to
CoQ10) = 4H+ pumped.
 This complex accept H+ and Hydride ion from reduced NAD.
 Complex II = succinate dehydrogenase (succinate CoQ10 oxidoreductase) This
complex accept H+ and Hydride ion from reduced FAD and no H+ pumped.
 Co Q: Lipid soluble Ubiquinone called Coenzyme Q that accept H atoms from
complex I and II to transfer it into complex III.
 Complex III= CoQ10-Cytochrome c oxidoreductase CoQ10 (contains
cytochromes, b and c) passes electrons to Cyt c (and pumps H+ ) in a unique
redox cycle known as the Q cycle. 4H+ pumped.
 Cyt c: is a water-soluble electron carrier, transfer electrons from complex III to
complex IV.
 Complex IV = Cytochrome oxidases (a+a3 and copper center). Electrons from cyt
c are used in a four-electron reduction of O2 to produce 2H2O. O2 is the final
electron acceptor. 2H+ pumped.
 Complex V = ATP Synthase It is H+ channel responsible for the Coupling of the
energy from e - Transport and H+ flow with oxidative phosphorylation to
produce energy as ATP.
 The enzyme use the proton gradient across the inner membrane to drive the
synthesis of ATP
Mitchell’s hypothesis (chemiosmosis model):
 Complex I, III and IV act as proton pumps.
 The translocation of protons H+ from the mitochondrial matrix into the inter-
mitochondrial space is called (proton pumping)
 H+ pumping & electron transport results in an electrochemical gradient .

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Chemiosmotic Hypothesis
 Proton motive force: energy released by flow of H+ down its gradient is used for
ATP synthesis.
 The energy obtained from electron transport is coupled to the proton motive force
in what’s called Chemiosmosis.
 Mitchell proposed that a proton gradient across the inner membrane could be
used to drive ATP synthesis.
 More +ve on the outside of the membrane than on the inside Electrochemical
gradient.
 Energy generated by Electrochemical gradient is sufficient to drive ATP
synthesis i.e. couples oxidation to phosphorylation.
Findings to support chemiosmosis model
1. Addition of protons (acid) to the external medium of the mitochondria stimulates
ATP production.
2. Oxidative phosphorylation does not occur in case of solubilising mitochondrial
membranes.
3. Uncouplers.

Summary of ETC and oxidative phosphorylation :

 If substrate enter ETC through NADH+ H+ → 3ATPs .
 If substrate enter ETC through FADH2 (flavoprotein) → 2ATPs.

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Respiratory mechanism control:

 There is no storage form for ATP.
 So all ATP formed is only present to cover the needs of the cell at the moment as
a source of Energy.
 This is why there should be a controlled way for the production of ATP under
electron transport chain (ETC).
1. Availability of ADP (ADP/ATP transporter is a rate limiting step in ETC).
2. Availability of electrons: NADH/NAD ratio or FADH2/FAD ratio.
3.Availability of O2 .
UNCOUPLERS

These compounds abolished the coupling between oxidation and

phosphorylation through increasing the permeability of the intra-membrane
space→Failure of the electrochemical gradient formation→ATP formation stops
while oxidation proceeds →Energy is released as heat rather than ATP.
Physiological Uncoupling
 An uncoupling protein (thermogenin) is produced physiologically in brown
adipose tissue of newborn mammals including human→this protein is in inner
mitochondrial membrane →This protein is H+ carrier →blocks development of a
H+ electrochemical gradient → energy of respiration is dissipated as heat rather
than ATP.
Toxic Uncoupling (DNP)

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 N.B: DNP, thyroid hormones (hyperthyroidism), high doses of aspirin and

arsenate are toxic uncouples→ feeling of increased body temperature (hotness)
and weight lose.
Inhibitors and uncouplers of oxidative phosphorylation

Inhibitors:
 Atractyloside: inhibits ADP/ATP antiporter.
 Oligomycin: inhibits ATP synthase (Uncoupler).
Toxic Uncoupler: DNP shuttles H+ across inner membrane, remove potential
gradient.
CaCl2 :It stimulates oxidative phosphorylation and ATP production (++ F0-F1, ++
dehydrogenases).
Oxidation of Extra-Mitochondrial NAD
 Some NADH molecules are reduced in the cytosol and must be transported into
the mitochondria for electrons to enter the electron transport pathway.
 Two different “shuttles” are commonly encountered:
Glycerol 3-phosphate shuttle (transfers electrons to FADH2 ) .
Malate-aspartate shuttle (transfers electrons to NADH) .

Malate-aspartate shuttle (In eukaryotes→38 ATP)

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