ON THE NATURE OF HEMOPROTEIN REACTIONS 



The "ferryl ion" structure is the simplest that fits all the 

 evidence, 



Fei+ (H2O) ^=^ FeMb02+ + 2H+ - e~ (8) 



and provisionally the higher oxidation state of ferrimyoglobin 

 can be named "ferrylmyoglobin" (23). Bonded oxygen atom 

 complexes are already well known in certain higher oxidation 

 states of simple transition element ions, e.g., manganates, 

 permanganates, vanadyl, and uranyl ions, etc., and also in certain 

 complex ions, e.g., Os(NH3)402'^, U(N-base)202'^, the vanadyl 

 porphyrins and phthalocyanine. 



If compound II of peroxidase has this structure too, in 

 spite of its absorption bands being different, which may be a 

 reflection of the heme being joined to a different amino acid 

 residue, then the formation of compound I from compound II 

 can be envisaged as simple electron removal, and the catalytic 

 cycle in peroxidase action represented as follows 



Fe^P+ (H2O) + H2O2 -^^l^lL^ Fep3,0^+ + m,0 



^®' oxidation '^^^ 



Fep^p3+ + AH, --^>- --^- -'^-^'^ Fep,p^+ + AH- + H + 



(electron transfer only) 



Fe,„0- + AH, + H. '^^^, FeS,(H.O) + AH- 



The diff'erent character of the two single equivalent reduction 

 steps is to be noted, and although these structures are purely 

 speculative in the case of peroxidase, they have the merit of 

 explaining why in the peroxide system compound I is apparently 

 more stable with respect to compound II formation in acid 

 solution (3), whereas the reverse holds in more alkaline solutions. 

 The oxidation-reduction potential of Fepgj./Fep^j.(H20) would 

 vary with /)H in a very similar manner to FeMb/Fe^(H20), 

 heme-linked ionizations and the bound water molecule ionization 

 only modifying the slope in different pW ranges. The potential 

 of the Fepej./Fep^j. couple, apart from the effect of linked ioniza- 

 tions, would be independent of /?H. As a consequence, the plots 

 of E'q against />H could cross, as shown in Figure 4, and the free- 



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