94 A SYMPOSIUM ON RESPIRATORY ENZYMES 



( 8 ) HO2- = Ha + electron 



Both these functions are related by the dissociation equihbrium 



(9) H2O2 ^11"^ + HOr (dissociation constant, K) 



It follows that there is no fundamental difference between the 

 homogeneous and heterogeneous reactions with regard to the ele- 

 mentary process. Weiss (79) has recently discussed the mechanism 

 of catalase and peroxidase action from the same point of view. In 

 his presentation of the central reaction, step 2 is replaced by the 

 equation 



(2a ) O2- + U2O2 = OH- + OH + O2 



where O2- has been substituted for HO^ on the basis of new evidence. 

 These two radicals are related to each other by the electrolytic dis- 

 sociation equilibrium 



(2b) O2- + H* <^ HO2 (dissociation constant, KHO2). 



Weiss treats the catalase-hydrogen peroxide reaction as a hetero- 

 geneous catalysis and as an analogon to the catalytic decomposition 

 of the same substrate by colloidal platinum, which has been so 

 carefully studied by Bredig. The similarity is accentuated by the 

 fact that neither catalase nor colloidal platinum shows peroxidase 

 activity toward acceptors of the type of iodide ion or pyrogallol. In 

 both instances the primary process is formulated as a surface reac- 

 tion involving the radicals OH and HO,. The author assumes that 

 the iron atom of the enzyme is alternatively reduced to the ferrous 

 and reoxidized to the ferric form by the hydrogen peroxide during 

 the course of the catalysis. The role of the metal atom in the 

 porphyrin skeleton is regarded as that of facilitating rapid electron 

 transfers, since the valency change of the iron in the porphyrin ring 

 system takes place without appreciable dislocation of heavy par- 

 ticles, in contrast to the situation in the instance of free ferrous and 

 ferric ions, where the water dipoles in the hydration shell must 

 undergo rearrangements upon a change in the charge of the central 

 atom. Furthermore, he believes that the system of conjugated double 

 bonds surrounding the iron in the heme group makes for a rapid 

 "conduction" of the inner electron by virtue of their loosely held 

 n-electrons. 



In analogy to the action of colloidal platinum on hydrogen per- 

 oxide, as formulated by the same author, the mechanism of catalase 

 action is depicted as follows: 



