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Biological oxidation -1

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Biological oxidation -1

  1. 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. 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. 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. 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. 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. 6. 4. Direct combination with Molecular oxygen A – H + ½O2 = A – OH A + O2 = AO2
  7. 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. 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. 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. 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. 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

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