MODE OF ACTION OF RESPIRATORY FERMENT 385 



autoxidizable hematin compound of relatively high oxidation-reduc- 

 tion potential, the respiratory ferment or cytochrome oxidase, is 

 oxidized by atmospheric oxygen to its ferric form via an unstable 

 intermediate compound of oxygen with its ferrous form. Its ferric 

 form then oxidizes a ferrous cytochrome and similar reactions between 

 various cytochromes are repeated until finally a ferricytochrome is 

 reduced by a hydrogen donor. The way in which the primary oxygen 

 compound of the ferrous respiratory enzyme is transformed to its 

 ferric form has not received any special consideration. It is well 

 known of course, that ferrous heme compounds are autoxidizable, 

 but, from the fact that the reduction of a molecule of oxygen to 

 two molecules of water involves a tetravalent change, it can be 

 expected that the reaction is complicated. 



The theory of the reaction of ferrous compounds with molecular 

 oxygen has been developed by Haber and Weiss (1078,1079,3016- 

 3018,3020). The following scheme is suggested by Weiss: 



Fe^^ + 02^ Fe3+ + O^ 



[21 



/ 

 Oi + ¥t ^ HOo ;=± I O3 + I H2O 



/" (31 



Fe'"^ + HO2 -^ Fe'"^ + HO, 

 IIO^ + H^ ;=± H2O2 



14] / 



Fo*^ + H.Oo -^ Fe'" + OH- + OH 



/ 

 Fe'^ + OH -» Fe'" + OH" 



Both the velocity of reaction 3 and the oxidation-reduction poten- 

 tial will depend on the ratio of the velocity constants of reactions 

 1 and 2. This is in harmony with the observations of Barron {179) 

 on the relation of rates of autoxidation and oxidation-reduction 

 potentials of hemochromes. The speed of reaction 4 will determine 

 whether hydrogen peroxide formation can be observed in such systems 

 or not. 



This theory gives an excellent account of the autoxidation of 

 inorganic iron compounds and the simple hemochromes but fails to 

 explain satisfactorily the mode of action of the respiratory enzyme. 



