35^ VIII. HEMATIN ENZYMES, I. CYTOCHROME SYSTEM 



linkages to the iron. Lemberg (1683) pointed out that the herao- 

 chrome structure of cytochrome c can only be explained by assuming 

 that the heme is bound in a crevice of the protein molecule, in which 

 it is firmly anchored by the thioether linkages. Theorell assumes 

 that this rigid structure explains why cytochrome c does not react 

 with oxygen or carbon monoxide. 



Zeile and Meyer {3167) showed that the iron-linked groups of the proteins 

 might be the primary amino groups of the cysteine molecules whose sulfur 

 atoms are attached to the side chains; on reduction, the hematin of the 

 cysteine adduct of protoporphyrin yields a hemochrome without addition of 

 other nitrogenous substances. Sterically a linkage such as is shown in 

 Figure 2 is possible. This type of linkage does not appear to occur in cyto- 

 chrome c, however. 



CO2H 



S 



/ 

 H.N 



I CH — CH3 



Fe ^N 



Fig. 2. Hemochrome c. The bonds indicated by dotted lines 

 do not lie in the plane of the paper. 



By spectrophotometric and magnetochemical investigations of ferri- 

 cytochrome c and by differential titration of ferrocytochrome and 



o 



ferricytochrome c from horse and ox heart, Theorell and Akesson 

 {2783-2785) conclude that two imidazole rings of histidine bound 

 to the iron constitute the hemochrome linkage in cytochrome c. 

 Cytochrome c contains three molecules of histidine. Two equivalents 

 of ferricytochrome c are titrated between pH 5.5 and 8.5, the range in 

 which the histidine imidazole is usually titrated, but the fact that 

 the heat of dissociation is continually rising is interpreted by Theorell 

 to indicate that only one of these two groups is histidine imidazole 

 (c/. Chapter VI, 5.2.2.3.). In this range the relation between ferro- 

 cytochrome and ferricytochrome is normal, the solution becoming 

 one equivalent more acid on reduction: Fc'^"^ + H — >• Fe^"*" + H . 

 The titration curves of ferrocytochrome and ferricytochrome cross, 

 however, at /;H 9.6 (Fig. 3). Above this />H the solution becomes 

 more alkaline on reduction. This indicates the presence in ferri- 



