LEONOR MICHAELIS 



Fe3+ + 2RS- + O2- (g) 



Fe + + 4- RS- + RS + O2- (h) 



Fe++ + RS + RS + O2— (i) 



step (f) being identical in both cases. Now, the complex containing 

 the constituents as in (f) is unstable and disintegrates to form three 

 separate molecular species: Fe++; RSSR, cystine; and hydrogen 

 peroxide. The latter may be used to oxidize more cysteine (stoichio- 

 metrically, not catalytically), or to oxidize Fe++ to Fe^+j and this 

 Fe3+ may oxidize (stoichiometrically) more cysteine. Finally, all the 

 iron is again in the ferrous state and the whole cycle is repeated with 

 iron thus acting as a catalyst. 



It is an essential prerequisite of this cycle that the change from 

 the ferrous to the ferric state occurs readily and reversibly. In using 

 cobalt instead of iron, the first stages are similar; but, once the cobaltic 

 complex has been established, it shares with all cobalt complexes of 

 the Werner type the property that the cobaltic state cannot be readily 

 reduced to the cobaltous state, even by means of rather strong reducing 

 agents. The final result is, therefore, the formation of the cobaltic 

 complex, stoichiometrically, without starting a catalytic cycle. Cop- 

 per, but not cobalt, can replace iron as a catalyst. 



What is furthermore essential in this process is the fact that 

 each single step in this chain reaction consists of the transfer of a single 

 electron. This assertion is more than a mere hypothesis. Since the 

 change of ferrous to ferric state involves one electron only, the sub- 

 division of the over-ail process into one-electron transfers is obvious. 

 It is remarkable that, even for such a simple case of iron catalysis, the 

 whole chain is of such an intricate nature, allowing for different path- 

 ways leading to the same final result. 



Oxidation Catalysts and Enzymes 



, The physiologically occurring catalysts (or enzymes) for oxida- 

 tion or reduction are characterized by their specificity. All oxidation 

 enzymes have been recognized as compounds of reversible redox sys- 

 tems and a specific protein. The same redox system, when attached 

 to different specific proteins, may have a different specificity. The 

 present state of our knowledge is on what may be called a descriptive 



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