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.

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.

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

agentReducing agent = e- donating moleculeOxidizing agent = e- accepting molecule

They together make a conjugate redox pair.

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

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+

4. Direct combination with Molecular oxygenA – H + ½O2 = A – OH

A + O2 = AO2

Enzymes involved in O/R reactionsAre k/as Oxidoreductases which includes : oxidases,

dehydrogenases, hydroperoxidaes and oxygenases.Oxidases use oxygen as an electron acceptorDehydrogenases can’t use as an electron

acceptorHydroperoxidases 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

OxidasesCatalyze the removal of hydrogen from a substrate

with the involvement of oxygen as a H – acceptorExist 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

Dehydrogenases Perform 2 main functions: 1. Transfer hydrogen from one substrate to

another in a coupled O/R reaction2. As components of Electron transport chain o Dehydrogenases use coenzymes –

nicotinamides & riboflavin - as hydrogen carriers

HydroperoxidasesIncludes 2 sets of enzymes : catalase and

peroxidasesPeroxidases 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

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