Catalase Oxidation Mechanisms 251 



which is virtually a donation of H~ by the peroxide molecule to the FeO 

 ion. A donation of H~ by an alcohol is also conceivable, as described by 

 King and Winfield( 1959a). 

 In the presence of excess H2O2 there is a slow reaction: 



\ / \ / 



C Fe^^=0 + HOOH >C Fei^— OH + OOH (8) 



P- P- 



Cat. HoGj I Cat. H2O2 II 



Little HoOo is normally decomposed by the pathway (8) because catalase and 

 Cat. H2O2 1 are predominantly outer orbital while Cat. H2O2 II is assumed to 

 be appreciably inner orbital. The small activation energy introduced by the 

 change in bond type involved in (8), compared with no change in (7), is 

 sufficient to ensure that most of the peroxide molecules give up two electrons 

 simultaneously, provided that the thermodynamics of the reaction are 

 favourable. 



In the type II complexes the 77 bond from oxygen to metal must be of 

 fractional order, since the reduction of catalase or peroxidase to the ferrous 

 state is known to be difficult. The Fe^^ — O bond is therefore much weaker 



\ / , \/ 



than that in C Fe^^O or -C Fe^^=0, and the energy of the complex is 



I ^1 I l_ 



comparable with that of Fe^^ — O (inner orbital). Resonance hybrids are 

 therefore possible (see, for example, Williams, 1956), and we may expect to 

 find among those complexes of catalase and peroxidase which have one 

 oxidizing equivalent, some with predominantly inner orbital characteristics, 

 some largely outer orbital, and some which fit neither category. The latter 

 are apt to have an absorption spectrum which is not obviously related to 

 their magnetic moment. It is possible that they are sensitive to pH. 



\ / 

 If we assume that Per. HgOo II is predominantly C Fe^^=0 (outer 



I ^1 



orbital) with a fractional -n bond from oxygen to metal, while Cat. H2O2 II 



contains about an equal contribution from the two forms C Fe^^=0 (outer 



orbital) and C Fe^^ — O (inner orbital), the comparative sluggishness of 



Cat. H2O2 II as an oxidant, while having a spectrum little different from that 

 of Per. H2O2 II, is understandable (see Theorell and Ehrenberg, 1952). 



Peroxidase is a feeble catalyst for H2O2 decomposition for the very reason 

 that it has high 'peroxidatic' activity, namely that Per. H2O2 1 can be reduced 

 very rapidly by a 1 -electron mechanism. In the presence of H2O2 alone, or of 

 H2O2 and a 2-electron donor, reaction will mostly follow the pathway via 



H.E. — VOL. I— s 



