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.

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 • 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 • 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: • 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+

Direct combination with Molecular oxygen A – H + ½O2 = A – OH A + O2 = AO2

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 • 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 • 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 • 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 • 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

Biological Oxidation