250 M. E. WiNFIELD 



in two distinct 1 -electron steps, while little activation is required for the direct 

 addition of two electrons to Ru.^^ 



In the reactions of catalase and peroxidase the kinetic barriers, namely the 

 activation energies required to transform the complexes from one bond type 

 to another, are supplemented by steric barriers. Chance (1951) has suggested, 

 for example, that in catalase the reaction sites are well below the protein 

 surface. We may perhaps regard the iron-porphyrin as lying at the bottom of 

 a pore whose radius is only a few A units. Thus oxidizing or reducing agents 

 may reach the metal atom without much restriction and yet be hindered from 

 direct reaction with the porphyrin. By contrast the prosthetic group of 

 peroxidase is thought to be relatively unprotected (Chance, 1951). 



The following scheme is an example of how the barriers mentioned above 

 can lead to apparently unidirectional pathways in electron transfer, and to 

 preferences for 2-electron steps. Elsewhere we have indicated the way in 

 which the 2-electron steps can be thermodynamically favoured (King and 

 Winfield, 1959b). 



In reaction (4) it is assumed that Per. H.^O.^ I is in equilibrium with a small 

 concentration of a more reactive, free radical form. In (5) the isomer reacts 

 very rapidly with a hydrogen donor to give Per. H2O2 II. More slowly the 

 latter is reduced by a second donor to free peroxidase. 



\ / \ / 



C Fe^'=0 :^ ■ C Fei^=0 (4) 



I ^1 I I ^1 I 



p- p- 



Per. HaOa I Isomer of Per. H^Oj I 



"^ ^ TV +e + nA / ,, 

 .C Feiv=o >C Feiv=0 (5) 



I I I rapid | j i 



P- P— H 



Per. HjOj II 



\ / +e-fHA / 



C Feiv=0 >C Fe"i— OH (6) 



P— H P— H 



Peroxidase 



If in catalase the porphyrin molecule is sterically protected from direct 

 attack by H-donors, we have for the O2 liberation reaction : 



\ / \ / 



C FeV=0 -h HOOH — -> C Fe"i— OH + O, (7) 



I ^1 I '^^'^ I ^^ I ' 



P- P— H 



Cat. H,0, I Catalase 



