1. Biological Oxidation
 Involves the transfer of electrons:
oxidation being termed for the removal of electrons
& reduction for gain of electrons
 Oxidation is always accompanied by reduction of an e- acceptor
 Higher forms of lives – completely rely on O2 for life processes
i.e. respiration – a process by which cells derive energy with a
controlled reaction between H+ and O2; the end product being
water.

2.  However there do occur large no. of reactions in living system
without the involvement of molecular O2.
 The reactions are catalyzed by a set of enzymes called as
Dehydrogenases.
 Other reactions do incorporate molecular O2 for the completion
of reaction.
 O2 is also required during treatment for respiratory and cardiac
failure – for, the proper functioning of both require O2.

3. Expressing Redox reactions as half reactions

E.g. Fe 2+ + Cu 2+ = Fe 3+ + Cu +
which can be expressed in the form of 2 half reactions
1.
Fe 2+ = Fe 3+ + e- (oxidized); Fe 2+ = reducing agent
2.
Cu 2+ + e- = Cu + (reduced) ; Cu 2+ = oxidizing agent
Reducing agent = e- donating molecule
Oxidizing agent = e- accepting molecule
They together make a conjugate redox pair.

4. Redox Potential
 Also k/as oxidation reduction potential
 Redox potential of any substance 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 denoted by Eo’ ( for biological systems)
 A reaction with a + ve ∆ Eo’ has a – ve ∆Go’ (exergonic)
 The redox potential of a biological system is usually compared with the
potential of H electrode expressed at pH 7.0

5. Transfer of electrons

Can take place by any of the 4 different ways:
1.
Directly as e – s : Transfer of an e – from Fe2+ / Fe3+ to Cu+/ Cu2+
(Fe2+ + Cu2+ = Cu+ +Fe3+ )
2.
As H – atom : AH2 ↔ A + 2e - + 2H+ ; where AH2 & A make a
conjugate redox pair and posses the tendency to reduce a next
compd. B ( B/BH2 = redox pair)
AH2 + B ↔ A + BH2
3.
As a hydride ion (:H- which has 2 electrons) :
H+ ↔ A+ + :H - + H+
AH +

6. 4.
Direct combination with Molecular oxygen
A – H + ½O2 = A – OH
A + O2 = AO2

7. Enzymes involved in O/R reactions
 Are k/as Oxidoreductases which includes : oxidases, dehydrogenases,
hydroperoxidaes and oxygenases.
 Oxidases use oxygen as an electron acceptor
 Dehydrogenases can’t use as an electron acceptor
 Hydroperoxidases use H2O2 as a substrate
 Oxygenases catalyse the direct transfer of O2 into the substrate
 Oxidases & dehydrogenases involved in respiration; hydroperoxidases
neutralize free radicals & oxygenases are involved in biotransformation

8. Oxidases
 Catalyze the removal of hydrogen from a substrate with the
involvement of oxygen as a H – acceptor
 Exist in two different forms :
 some of them are copper containing as, Cytochrome oxidase - the
terminal component of ETC which transfer the e - finally to O2.
 Other are flavoproteins as , L – aminoacid oxidase, xanthine oxidase

9. Dehydrogenases

1.
2.
o
Perform 2 main functions:
Transfer hydrogen from one substrate to another in a coupled
O/R reaction
As components of Electron transport chain
Dehydrogenases use coenzymes – nicotinamides & riboflavin as hydrogen carriers

10. Hydroperoxidases
 Includes 2 sets of enzymes : catalase and peroxidases
 Peroxidases reduce H2O2 at the expense of several other substances
H2O2 + AH2 → 2H2O + A
o Catalase uses H2O2 as electron acceptor & electron donor
2H2O2 → 2H2O
Peroxisomes are rich in oxidases and catalases

11. Oxygenases

Catalyse the incorporation of O2 into subtrates in 2 steps
- Oxygen is dound to the active site of the enzyme
- 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