94 OXIDATION-REDUCTION POTENTIALS 



But the hydrogen peroxide may itself effect oxidations. In the presence of a 

 peroxidase the following type of reaction occurs : — 



-\- peroxidase 

 H2O2 + B > H2O + BO. 



Many bacteria possess peroxidase functions and these have been found to be 

 associated with the presence of complex iron compounds such as cytochrome. 



Warburg's (1923) theory is that oxygen activation is the crucial factor in 

 biological oxidations and he attributes to iron compounds the function of catalysing 

 oxidations by virtue of the change of valency from the ferrous to the ferric state. 



This view derives its main support from analogies with iron systems, regarded as 

 " models," and their inhibition by means of HCN and CO. The chief line of argument 

 against the universal applicability of the oxygen-activation h3rpothesis is the difficulty 

 of explaining on such a basis the high degree of specificity met with in biological 

 oxidation-reduction reactions. Fusion of the two mutually helpful views would 

 appear to promise the most reasonable hopes for progress. 



Clark (1926) prefers to regard biological oxidation-reduction reactions in terms 

 of electron transfer. Since oxidations and reductions may best be defined in terms 

 of electrons it would seem logical to consider such reactions as electronic rearrange- 

 ments, and to view the migration of atoms such as hydrogen as merely incidental 

 to the maintenance of electrical neutrality. In the oxidation-reduction of methylene 

 blue, for example, the reaction occurs, in accordance with electrode data, as follows : — 



Ox. ® + H® + 2e ^ HR. 



Methylene blue (cation) -f hydrion + 2 electrons ^ methylene white. 



Methylene blue in this reaction is just as much an " electron acceptor " as a 

 " hydrogen acceptor " and if the same consideration is applied to the " hydrogen 

 donator " in the experiment it will become apparent that the reaction may well be 

 regarded as an electron transfer accompanied secondarily by an exchange of hydrogen 

 ions. This point of view unifies and simplifies theories of oxidation-reduction pro- 

 cesses and leaves open the question of how electron transfers are effected in the 

 presence of the appropriate enzymic activity. 



Wieland's (1931) views on biological oxidations may pertinently be quoted 

 here : — 



" It follows, therefore, that in the metabolism of the cell reduction products 

 will be formed by the intervention of various hydrogen acceptors in the process 

 of dehydrogenation. Conversely we may conclude that, whenever vital 

 reduction takes place, it will be possible to demonstrate the process of aerobic 

 dehydrogenation. The pre-eminence of oxygen is based not only on its high 

 hydrogenation potential, but also on the chemical and physical inertness of its 

 hydrogenation product — water." 



Summary of Chapter V 

 Some of the chains and cycles of closely linked and delicately integrated enzyme 

 reactions essential to the metabolic systems of living organisms are discussed and 

 consideration is given to the significance of oxidation-reductions, free energy changes, 

 phosphate bond energies and energy storage mechanisms. 



