The document discusses oxidation-reduction (redox) reactions in biological systems. It begins by defining oxidation as the removal of electrons and reduction as the gain of electrons. It states that redox reactions involve the transfer of electrons from substances of higher electrochemical potential to those of lower potential. The document outlines several types of redox enzymes, including oxidases, dehydrogenases, hydroperoxidases, and oxygenases. It provides examples of important redox reactions and enzymes in biological systems, such as cytochrome oxidase and alcohol dehydrogenase. The role of redox reactions in energy production through electron transport chains is also briefly mentioned.

1. Department of Biochemistry, Nepalgunj Medical College, Chisapani, Nepal
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2016
Rajesh Chaudhary
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2. Biological oxidation-reduction
 Biological oxidation in photosynthetic Vs non-
photosynthetic organisms.
 Work can be accomplished by Electromotive Force (EMF).
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So, what is Electromotive force (EMF)?

3. Biological oxidation-reduction
 Oxidation: removal of electron
 Reduction: gain of electron
Fe2+ Fe3+
 LEO GER: Loss of electron (Oxidation) & Gain of electron
(Reduction)
 Flow of electron is DOWNHILL. (From higher to lower
electrochemical potential)
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e-

4. Oxidation-reduction can be described as
Half-Reactions
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5. Biological oxidations often
involve dehydrogenation
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6. Biological oxidations often involve
dehydrogenation
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 The order of increasing electronegativity is
H<C<S<N<O.
Alkane Alkene
NOTE: With each formal loss of “owned” electron, a carbon
atom goes into oxidation though there is no oxygen involved.
to

7. Electron transfer occurs in 4 different
ways
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 1. Directly as electrons.
 2. As hydrogen atoms.
 3. As hydride ions.
 4. Through direct combination with oxygen.
The neutral term “reducing equivalent” is commonly used to designate a
single electron equivalent participating in an oxidation-reduction reaction.

8. Redox potential and Free energy
 Is a measure of its affinity for electrons
 In O/R reactions the free energy change is proportional to the
tendency of reactants to donate / accept e-s known as
Standard Reduction Potential denoted by Eo
’ (for biological
systems)
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9. Enzymes
Used as a catalyst
Classification of enzymes: 6 classes
1. Oxido-reductase (E.g Alcohol dehydrogenase)
2. Transferase (E.g. Aminotransferase / Transaminase)
3. Hydrolase
4. Lyases (E.g. Decarboxylase)
5. Isomerase (E.g. Epimerase, Racemases, Mutases)
6. Ligases (e.g Pyruvate carboxylase)
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10. Oxidation-reduction enzymes
 Enzyme responsible: OXIDO-REDUCTASE
 In fact, there are seven different types of OXIDO-
REDUCTASE.
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Oxidases Dehydrogenases Hydroperoxidases Oxygenases

11. 1. Oxidase
 Use oxygen as hydrogen acceptor
 Reaction product: H2O, H2O2
 Some contains copper
 Example: Cytochrome oxidase / Cytochrome a3
 Other oxidases are FLAVOPROTEIN
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FAD FMN
L-amino acid oxidase, Xanthin oxidase, aldehyde dehydrogenase

12. 1. Oxidase
1. L-amino oxidase: FMN-linked (kidney)
2. Xanthin oxidase: milk, small intestine, kidney,
and liver.
3. Aldehyde dehydrogenase: FAD-linked
(mammalian liver)
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13. 2. Dehydrogenase
 Doesn’t use oxygen as hydrogen acceptor
 Contains large no. of enzymes in this family
 Nicotinamide-dependent
 NAD-linked dehydrogenase
 NADP-linked dehydrogenase
 2 major functions:
1. Transfer of hydrogen from one substrate to another
2. Transfer of electron in respiratory chain from substrate to oxygen.
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14. Differences between Aerobic dehydrogenase
and Anaerobic dehydrogenase
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Aerobic dehydrogenase Anaerobic dehydrogenase
1. Can react directly with O2 1. Cannot react directly with molecular O2
2. Transfers hydrogen / electrons to Fp
which is autooxidisable.
2. Transfers hydrogen / electron to NAD+
or Fp which is oxidized in ETC.
3. H2O2 is produced which is catabolized
by catalase.
3. H2O2 is never produced. NADH + H+ or
Fb.H2 are produced.
4. ATP is never produced. 4. ATP is produced.

15. Mechanism of oxidation and reduction
of nicotinamide coenzyme
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16. Other dehydrogenases
Depends on Riboflavin.
Similar to FMN and FAD.
Examples:
NADH-dehydrogenase.
Succinate dehydrogenase
Acyl-CoA dehydrogenase
Mitochondrial glycerol-3-phosphate dehydrogenase.
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Except cytochrome oxidase, other types of cytochromes are considered as dehydrogenases.

17. 3. Hydroperoxidase
Use hydrogen peroxide / organic peroxide as
substrate
2 types of enzymes
Peroxidase
Catalase
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Hydrogen peroxide protects our body from various harmful
peroxidases !

18. Hydroperoxidase
Accumulation of peroxidases  generation of free
radicals  leads to atherosclerosis, cancer.
Peroxidases
Originally considered plant enzyme
(Milk, Leukocytes, platelets and in tissues involved in Eicosanoid
metabolism)
Reduces peroxides using various electron acceptor.
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19. Peroxides and Catalase
H2O2 + AH2
peroxidase
2H2O + A
2H202
catalase
2H20 + 02
 Catalase
H202 as electron donor and acceptor
Sites: blood, bone marrow, mucus membrane, kidney, liver.
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20. Oxygenase
 Catalyze the incorporation of O2 into substrates in 2 steps
1st. Oxygen is bound to the active site of the enzyme
2nd. Bound O2 is reduced or transferred to the substrate
Consists of two sets of enzymes
1. Dioxygenases : incorporate both atoms of oxygen into the
substrate ; A + O2  AO2
2. Monooxygenases : incorporates one atom of oxygen into the
substrate & the other is reduced to water
A – H + O2 + ZH2  A – OH + H2O + Z
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