Catalase Oxidation Mechanisms 249 



and which we therefore write as C Fe^^ = O. We can think of no alterna- 



tive structure which will satisfy the titration evidence, the lack of free radical 

 character and the observation that oxidation of Mb^" in the presence of 

 excess H2O2 yields oxymyoglobin (Keilin and Hartree, 1951). A more 

 detailed investigation of the reaction is desirable. 



Oxidation o/Mb"^ 



In following the oxidation by H2O2 of a complex of Ru" to the Ru^^ state 

 it was noted that there was a tendency for the two forms to interact, giving 

 Ru^" complexes (Dwyer et al. 1959). It would be of value to study further 

 the peroxide decomposition in the presence of Mb^^^ particularly noting the 

 effect of Mb^^^ dilution on the concentration of the intermediate which 

 Keilin and Hartree (1951) remarked upon as having no distinct absorption 

 bands in the visible spectrum, and also to study the effect of dilution on 

 the free radical detected by Gibson and Ingram (1956), which may or may 

 not be identical with Keilin and Hartree's intermediate. It might well be 

 found that the mechanism of H2O2 decomposition by Mb^" is closer to that 

 of catalase and peroxidase than has generally been admitted, and that the 

 most significant difference in reaction path is due to interaction between two 

 different oxidation states of myoglobin, for example an electron transfer 

 reaction such as 



\/ \/ \/ 



• C Feiv=o + C Fe™— OH2->2C Feiv=o 



I I I I I I I ! I 



P- P- P— H (3) 



Significance of Bond Type 



The behaviour of the ruthenium complexes mentioned in the previous 

 section resembles in certain respects that of catalase and peroxidase, and is 

 indicative of the kind of kinetic barrier which could limit the oxidation- 

 reduction reactions of the enzymes. Dwyer et al. (1959) suggested that 

 electron transfer between Ru" and Ru^^ could be minimized by preparing a 

 catalyst in which the bond types were different in the two states. Although 

 both Per. H0O2 1 and peroxidase (and Cat. H2O2 1 and catalase) are probably 

 outer orbital complexes, it is possible that their interaction is limited by a 

 kinetic as well as a thermodynamic factor and that the kinetic factor is related 

 to changes in bond type. 



Hydrogen peroxide is able to carry out the full reduction of Ru^^ to Ru^^ 

 rapidly, probably without passing through an intermediate oxidation state, 

 while most donors yield the relatively inert green Ru^^^ complex. The simplest 

 explanation is that an appreciable activation energy is required for reduction 



