Biological oxidation involves the transfer of electrons, with oxidation being the removal of electrons and reduction being the gain of electrons. Higher life forms rely completely on oxygen for life processes like respiration, where cells derive energy from the reaction of hydrogen and oxygen to produce water. However, many reactions in living systems occur without oxygen involvement, catalyzed by dehydrogenases. Oxygen is also required to treat respiratory and cardiac failure. Redox reactions can be expressed as half reactions with a reducing agent donating electrons and an oxidizing agent accepting electrons. The redox potential measures a substance's affinity for electrons. Enzymes involved in redox reactions include oxidases, dehydrogenases, hydroperoxidases, and oxygenases.

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) :
AH + H+
↔ A+
+ :H-
+ H+

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

7. Enzymes involved in O/R reactionsEnzymes involved in O/R reactions
 Are k/as Oxidoreductases which includes : oxidases,Are k/as Oxidoreductases which includes : oxidases,
dehydrogenases, hydroperoxidaes and oxygenases.dehydrogenases, hydroperoxidaes and oxygenases.
 OxidasesOxidases use oxygen as an electron acceptoruse oxygen as an electron acceptor
 DehydrogenasesDehydrogenases can’t use as an electron acceptorcan’t use as an electron acceptor
 HydroperoxidasesHydroperoxidases use Huse H22OO22 as a substrateas a substrate
 OxygenasesOxygenases catalyse the direct transfer of Ocatalyse the direct transfer of O22 into theinto the
substratesubstrate
 Oxidases & dehydrogenases involved in respiration;Oxidases & dehydrogenases involved in respiration;
hydroperoxidases neutralize free radicals & oxygenases arehydroperoxidases neutralize free radicals & oxygenases are
involved in biotransformationinvolved in biotransformation

8. OxidasesOxidases
 Catalyze the removal of hydrogen from a substrate withCatalyze the removal of hydrogen from a substrate with
the involvement of oxygen as a H – acceptorthe involvement of oxygen as a H – acceptor
 Exist in two different forms :Exist in two different forms :
 some of them are copper containing as, Cytochromesome of them are copper containing as, Cytochrome
oxidase - the terminal component of ETC which transferoxidase - the terminal component of ETC which transfer
the ethe e --
finally to Ofinally to O22..
 Other are flavoproteins as , L – aminoacid oxidase,Other are flavoproteins as , L – aminoacid oxidase,
xanthine oxidasexanthine oxidase

9. DehydrogenasesDehydrogenases
 Perform 2 main functions:Perform 2 main functions:
1.1. Transfer hydrogen from one substrate to another in aTransfer hydrogen from one substrate to another in a
coupled O/R reactioncoupled O/R reaction
2.2. As components of Electron transport chainAs components of Electron transport chain
o Dehydrogenases use coenzymes – nicotinamides &Dehydrogenases use coenzymes – nicotinamides &
riboflavin - as hydrogen carriersriboflavin - as hydrogen carriers

10. HydroperoxidasesHydroperoxidases
 Includes 2 sets of enzymes : catalase and peroxidasesIncludes 2 sets of enzymes : catalase and peroxidases
 Peroxidases reduce HPeroxidases reduce H22OO22 at the expense of several otherat the expense of several other
substancessubstances
HH22OO22 + AH+ AH22 →→ 2H2H22O + AO + A
o Catalase uses HCatalase uses H22OO22 as electron acceptor & electronas electron acceptor & electron
donordonor
2H2H22OO22 →→ 2H2H22OO
Peroxisomes are rich in oxidases and catalasesPeroxisomes are rich in oxidases and catalases

11. OxygenasesOxygenases
 Catalyse the incorporation of OCatalyse the incorporation of O22 into subtrates in 2 stepsinto subtrates in 2 steps
- Oxygen is dound to the active site of the enzyme- Oxygen is dound to the active site of the enzyme
- bound O- bound O22 is reduced or transferred to the substrateis reduced or transferred to the substrate
Consists of two sets of enzymesConsists of two sets of enzymes
1.1. Dioxygenases : incorporate both atoms of oxygen into theDioxygenases : incorporate both atoms of oxygen into the
substrate ; A + Osubstrate ; A + O22 →→ AOAO22
2.2. Monooxygenases : incorporates one atom of oxygen intoMonooxygenases : incorporates one atom of oxygen into
the substrate & the other is reduced to waterthe substrate & the other is reduced to water
A – H + OA – H + O22 + ZH+ ZH22 →→ A – OH + HA – OH + H22O + ZO + Z