Catalase Oxidation Mechanisms 253 



In the physiological pH range these compounds have oxidation-reduction potentials 

 in the neighbourhood of 1 V and are relatively inert, both with regard to mutual 

 disproportionation reactions and to the spontaneous reduction that can occur with 

 oxidizable groups on the protein. This stabilization of higher oxidation states can well 

 be considered as remarkable a property as the reversible oxygenation of the 

 haemoglobins. 



Only in the case of myoglobin has the complete oxidation-reduction reaction for 

 one of the couples been established. For the single equivalent oxidation it takes the 

 form 



FcMb"' ^ FcMb"' + 2H+ -f e- 



On the basis of a hydrate structure for FcMb'" (or an isomer) the appearance of two 

 H+ ions as product is consistent with a ferryl ion type of structure (or an isomer) 

 for FcMb'^, i-e. 



FeMb+++ (HP) -^ FejibO++ -f 2H+ + e- 



acidic ferrimyoglobin ferrylmyoglobin 



(George and Irvine, Symposium on Co-ordination Compounds, Copenhagen, 1953: 

 Danish Chemical Society, p. 135, 1954; Biochem. J. 60, 596, 1955). The aquo ferryl 

 ion was suggested by Bray and Gorin in 1933 as an intermediate in the reactions of 

 iron salts; while higher oxidation states, with structures very similar to that of 

 'ferrylmyoglobin', are exemplified by the vanadyl porphyrins and the manganyl 

 phthalocyanine pyridine complex referred to by Orgel. 



As the above reaction and countless other examples in inorganic and organic 

 chemistry show, a knowledge of the way the H+ ion participates to balance the 

 oxidation-reduction equation, as reactant or product or not at all, is an essential piece 

 of evidence for eliminating some of the many structures that otherwise account for 

 the increments in oxidation equivalents, i.e. Fe" -^ Fe"' -^ Fe'^ -> Fe^. This 

 knowledge is clearly important too in deciding the type of reaction by which the 

 reduction of a higher oxidation state is brought about, namely by the net transfer of 

 electrons or hydrogen atoms. However, the structure for a higher oxidation state, 

 or more precisely a family of isomeric structures, can only be specified with certainty 

 if the structure of the lower oxidation state of the couple has already been established. 

 This point is well illustrated by the relationship between the ferric hydrate and ferryl 

 ion structures in the myoglobin reaction as written above. In this case a hydrate 

 structure for the acidic ferrimyoglobin accounts very satisfactorily for all its other 

 reactions, i.e. the reduction to ferromyoglobin, and the formation of cyanide and 

 fluoride complexes, etc. 



But in the case of ferriperoxidase and ferricatalase, neither the hydrate structure 

 nor the structure with a carboxylate group bonded to the iron in the sixth co-ordination 

 position will account for their combination with the familiar ligands, although such 

 structures are often accepted as being well established. The variation with pH of the 

 equilibrium constants for the formation of their complexes indicates that proton 

 addition accompanies the bonding of anionic ligands — in contrast to ferrimyoglobin 

 and ferrihaemoglobin, where the pH variation is consistent with the simple replacement 

 of the co-ordinated water molecule in the hydrate structure, or the reaction of some 

 equivalent structure. This difference in H+ ion dependence suggests that the parent 

 ferric oxidation states of peroxidase and catalase have another kind of structure 

 entirely, and, in view of the role of H+ ion in oxidation-reduction reactions, it may 

 also be an important clue to difl"erent active types of higher oxidation state. This 

 seems not unlikely, because, as is well known, ferriperoxidase and ferricatalase behave 

 differently with strong oxidizing agents giving two relatively stable higher oxidation 

 states (Fe'^ and Fe^) under conditions where ferrimyoglobin and ferrihaemiglobin 

 give only one (Fe'^ ). Moreover the absorption spectra of the Fe'^' derivatives are not 

 of the same form; the maxima, especially in the visible region, occur at different 

 wavelengths, which is a further indication of important structural differences. 



Labile crevice structures for ferriperoxidase and ferricatalase, in which the group 

 that is liberated when complex formation occurs has a high proton affinity, e.g. a 



