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 O 2 for life processes i.e. respiration – a process by which cells derive energy with a controlled reaction between H + and O 2 ; the end product being water.

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

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.

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 E o ’ ( for biological systems) A reaction with a + ve  E o ’ has a – ve  G o ’ (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 Fe 2+ / Fe 3+ to Cu + / Cu 2+ (Fe 2+ + Cu 2+ = Cu + +Fe 3+ ) 2. As H – atom : AH 2   A + 2e - + 2H + ; where AH 2 & A make a conjugate redox pair and posses the tendency to reduce a next compd. B ( B/BH 2 = redox pair) AH 2 + B   A + BH 2 3. As a hydride ion (:H - which has 2 electrons) : AH + H +   A + + :H - + H +

4. Direct combination with Molecular oxygen A – H +  ½O 2 = A – OH A + O 2 = AO 2

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

Oxidases Catalyze the removal of hydrogen from a substrate with the involvement of oxygen as a H – acceptor Catalyze the removal of hydrogen from a substrate with the involvement of oxygen as a H – acceptor Exist in two different forms : 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 O 2. some of them are copper containing as, Cytochrome oxidase - the terminal component of ETC which transfer the e - finally to O 2. Other are flavoproteins as, L – aminoacid oxidase, xanthine oxidase Other are flavoproteins as, L – aminoacid oxidase, xanthine oxidase

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

Hydroperoxidases Includes 2 sets of enzymes : catalase and peroxidases Includes 2 sets of enzymes : catalase and peroxidases Peroxidases reduce H 2 O 2 at the expense of several other substances Peroxidases reduce H 2 O 2 at the expense of several other substances H 2 O 2 + AH 2  2H 2 O + A H 2 O 2 + AH 2  2H 2 O + A o Catalase uses H 2 O 2 as electron acceptor & electron donor 2H 2 O 2  2H 2 O 2H 2 O 2  2H 2 O Peroxisomes are rich in oxidases and catalases

Oxygenases Catalyse the incorporation of O 2 into subtrates in 2 steps Catalyse the incorporation of O 2 into subtrates in 2 steps - Oxygen is dound to the active site of the enzyme - bound O 2 is reduced or transferred to the substrate Consists of two sets of enzymes 1. Dioxygenases : incorporate both atoms of oxygen into the substrate ; A + O 2  AO 2 2. Monooxygenases : incorporates one atom of oxygen into the substrate & the other is reduced to water A – H + O 2 + ZH 2  A – OH + H 2 O + Z