CYTOCHROME C 



353 



cytochrome c of two hematin-linked acid groups with a pK close to 

 9.6, which is in good agreement with the pK of the imidazole group 

 in imidazole hemoglobin, pK = 9.5 {2397). They cannot be a-amino 

 acid groups as assumed by Zeile and Meyer. Evidently the pK of 

 the same groups in ferrocytochrome must be far higher and outside 



+ 10 



+ £ 



a - 5 



J - 10 

 _> 

 §- - 15 



- 20 



- 25 



10 



12 



pH 



Fig. 3. Titration curves of ferrocytochrome c and ferricvtochrome c at 20°C. 

 (after' Theorell, 2118). 



the range of the titration, which is rather remarkable. In this range 

 spectrophotometry indicates only one pK of 9.35 for ferricytochrome 

 c. In Table III the results of the investigations of Theorell are 

 summarized together with his interpretation.* 



There are several points in this scheme which require discussion. The 

 magnetochemical resuHs indicate that form V has only one free electron. In 

 this it resembles hem/chrome hydroxides {cj. Chapter V) rather than hemi- 

 globih hydroxide, which has three free electrons. The absorption spectrum, 

 however, differs from tliose of both hemJchrome hydroxide and hem/globin 

 hydroxide. 



Spectroscopic evidence was found for only one of the two dissociations 

 III — > IVa and IVa -^ IVb, the two forms IVa and IVb having apparently 

 very similar spectra. The alterations in the spectra throughout the whole 

 system are small and, being in the red region, must lie close to the limits of 

 spectrophotometric procedure. The pK 9.35 is not considered that of the 

 reaction Fe"*" +0H ' — > FeOH, since forms IVa and IVTj do not react with 

 cyanide (c/., however, ISJ^l). 



* Recently, Theorell {2179a) found that measurements of the oxidation-reduction 

 potential indicate another pK value of 6.86. 



