386 VIII. HEMATIN ENZYMES, I. CYTOCHROME SYSTEM 



First, it leaves unexplained the formation of an oxygen compound in 

 which carbon monoxide could compete with oxygen for the heme iron. 

 Second, it postulates the formation of free reactive radicals which, 

 for reasons discussed in the next subsection, we do not consider 

 likely to occur in biological reactions. 



While Smythe {2582) claimed that only iron complex salts were autoxi- 

 dizable, Weiss assumed first that on the contrary only iron compounds with 

 incompletely filled electron orbits were subject to autoxidation, and that 

 iron complexes could only be oxidized by atmospheric oxygen, if the ferric 

 complex were more stable than the ferrous. The autoxidizable hemochromes 

 have however a completely filled electron shell, and the complex is more 

 stable in its ferrous form. Later Weiss explained the ready autoxidizability 

 of hemochromes in the following way; first, the resonance system facilitates 

 rapid conduction of the inner electrons, and second, the valency change is 

 not connected, as it is in the oxidation of ionic iron, with changes in hydration, 

 requiring movement of water molecules. 



In the scheme of Haber and Weiss free radicals occur, but no 

 radical chain is postulated. Haber and Willstatter {1080} had pre- 

 viously postulated a more complicated reaction mechanism for the 

 autoxidation of the ferrous enzyme in which the substrate H2A was 

 assumed to be involved. This may be formulated as: 



/ 

 (Fe*^) + O2 + H2A -^ (Fe'+) + OH + OH" + A 



It will be seen below that this formulation, in a modified form, 

 describes the autoxidation of cytochrome oxidase probably more 

 correctly than the scheme of Haber and Weiss. 



6.2. Radical Chain Theory 



In 1931 Haber and Willstatter {1080} published a universal theory 

 of the reaction mechanism of enzymes and other catalysts, which has 

 been widely discussed and is still of importance, although it had to 

 be modified to a great extent. Actually the theory comprises two 

 different though connected ideas, namely, first, the monovalent 

 reaction of a catalyst with a hydrogen donor, with the formation of 

 a radical, e.g., Fe^+ + H2A^Fe^+ + HA, where H.A is the sub- 

 strate, Fe^"^ the enzyme, and Fe*"^ the "desoxyenzyme"; second is 

 the formation of radical chains, e.g.: 



/ / 



HA + H2A + 02-^ 2A ^- H2O + OH 



/ / 



OH + H2A -^ H2O -f HA 



