CATALASE 439 



Steps 0-9. The ferroperoxidase now acts as oxidase. This reaction is 

 inhibited by carbon monoxide. The substrate is oxidized in an intracomplex 

 reaction without valency change and several such cycles are completed before 

 ferroperoxidase is oxidized back to ferriperoxidase in side reactions and the 

 cycle is thus broken. The intermediate formation of a ferroperoxidase- 

 hydrogen peroxide compound in steps 4 and 7 destroys part of the enzyme 

 irreversibly. 



4.2. Catalase 

 4.2.1. Radical Chain Theory. It is still impossible to explain the 

 action of catalase satisfactorily in spite of inorganic models and pains- 

 taking research on the enzyme itself. Research in this field began 

 early with Bredig's work on the catalatic action of colloidal metals, 

 which revealed a surprising similarity with phenomena observed with 

 the enzyme (e.g., cyanide inhibition). The radical chain theories of 

 Haber and Willstatter and of Weiss have continued this line of attack 

 on the problem. 



The catalatic activity of platinum metal is initiated by one of the reactions 

 {Weiss, 3017,3019): 



/ 

 H2O2 + e(metal) -* OH" + OH 



/ 

 or: HO2 -+ HO2 + e(metal) 



The destruction of hydrogen peroxide is then caused by the following 

 radical chain: 



/ / 



HO2 + H2O2 -» O2 + H2O + OH 1 



OH + H2O2 -* H2O + HO2 



Similarly the action of catalase was explained by Haber and Willstatter 

 {1080) by the initiation of the same radical chain by the reaction: 



/ 

 (Fe3+) + H2O2 -> (Fe2+) + HO2 + H+ 



Haldane raised the objection that this theory would demand the reaction 

 velocity of hydrogen peroxide decomposition to be proportional to a higher 

 power of enzyme concentration. 



The theory was modified by Weiss {3020) in order to meet this objection. 

 It is now assumed that the following reactions are involved: 



/ 

 Fe'+ + HO;^ Fe2+ + HO2 (1) 



Fe2+ + HjOj;^^ Fe'+ + OH" + OH (2) 



/ 

 Fe2+ + OH — Fe^+ + OH" (3) 



