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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

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biological oxidation
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.
slide2
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
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

  • Fe 2+ = Fe 3+ + e- (oxidized); Fe 2+ = reducing agent
  • 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.

redox potential
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
Transfer of electrons
  • Can take place by any of the 4 different ways:
  • Directly as e – s : Transfer of an e – from Fe2+ / Fe3+ to Cu+/ Cu2+(Fe2+ + Cu2+ = Cu+ +Fe3+ )
  • 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
  • As a hydride ion (:H- which has 2 electrons) : AH + H+ A+ + :H - + H+
slide6
Direct combination with Molecular oxygen

A – H + ½O2 = A – OH

A + O2 = AO2

enzymes involved in o r reactions
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
  • Hydroperoxidasesuse H2O2 as a substrate
  • Oxygenasescatalyse the direct transfer of O2 into the substrate
  • Oxidases & dehydrogenases involved in respiration; hydroperoxidases neutralize free radicals & oxygenases are involved in biotransformation
oxidases
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
dehydrogenases
Dehydrogenases
  • 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
hydroperoxidases
Hydroperoxidases
  • Includes 2 sets of enzymes : catalase and peroxidases
  • Peroxidases reduce H2O2 at the expense of several other substances

H2O2 + AH2 2H2O + A

  • Catalase uses H2O2 as electron acceptor & electron donor

2H2O2  2H2O

Peroxisomes are rich in oxidases and catalases

oxygenases
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

  • Dioxygenases : incorporate both atoms of oxygen into the substrate ; A + O2  AO2
  • Monooxygenases : incorporates one atom of oxygen into the substrate & the other is reduced to water

A – H + O2 + ZH2  A – OH + H2O + Z