356 



VIII. HEMATIN ENZYMES, I. CYTOCHROME SYSTEM 



The dissociations III — » II and II -+ 1 are not comparable with dissocia- 

 tions of imidazoHnium ions. They probably involve changes in the protein 

 molecule comparable to those occurring when hemoglobin is transformed to 

 "acid hematin"; these changes have not yet been studied potentiometrically. 

 They are reversible even in hem?globin; for cytochrome c, with its thioether 

 linkages remaining intact, this reversibility is to be expected. The scheme 

 does not explain why on reduction of forms I, II, and V the neutral hemo- 

 chrome spectrum reappears, although changes in autoxidizability and 

 reaction with carbon monoxide are found. 



On heating, ferricytochrome c is reversibly dissociated. The hemt- 

 chrome spectrum disappears, but returns on cooling. 



In alkaline solution one molecule of cytochrome c is still able to 

 combine with four additional molecules of protoheme giving mixed 

 cytochrome c - protohemochromes (3165). Since the affinity between 

 the thioether-linked heme and the hemochrome-forming histidines is 

 too great to allow other hemes to compete, and since anyhow only 

 three histidine imidazoles are present, it is evident that the additional 

 protohemes must be bound by groups other than the histidine 

 imidazoles. 



Theorell (2776) has pointed out that the reaction of the ferric iron 

 of ferricytochrome with hydrogen donors may be facilitated by the 

 formation of seraiquinoid structures in the covalently linked imidazole 

 groups. Theorell formulates this: 



H + III + H h/+ III H + II 



r=N — Fe — N=C. +H .C — NH— Fe— /C-=N Fe- 



HN I I >H -HN; f --''_♦ HN I ...+ 



+H 



This can perhaps more clearly be put in the following way: 



+H 



3.3.3. Protein of Cytochrome c. The amino acid composition of cyto- 

 chrome c has been studied by Theorell and Akesson {2787) and is given in 

 Table IV. Cytochrome c contains six atoms of sulfur. The table shown 



