Cytochrome Cg 415 



On the Structural Significance of Two Haein Groupsj Molecule in Cytochrome C3 



HoRio. As I said in the previous session, one of the modified cytochrome c's exists in a 

 dimer form; the S..q_,o and Dao.w of the modified cytochrome c gave its mol. wt. to 

 be 24,400. On the oliier hand, Thcorell reported that tryptic cytochrome c (mol. wt. 

 approximately 2,000) can form a polymer. But the polymer can be changed into the 

 original monomer (maybe containing histidine) if histidine is added to the polymer 

 sample. 



Of course, I do not attempt to say that the modified cytochrome c^ is formed in the 



same way, because its iron content is very high compared with other cytochrome c's. 



But at least, if it is assumed that cytochrome c can occur in a state in which the 



imidazole radical (or radicals) is dissociated from haem iron, then two molecules in 



such a state could form a dimer. Of course, this is only one of the possibilities. 



Morton: Heart-muscle cytochrome c may be heated to about 100°C in dilute salt solutions 

 at pH 7 and, on cooling, shows the same absorption spectrum as before heating. If 

 cytochrome Cg can exist as a dimer it might be expected that the absorption spectrum 

 would change on boiling. Is there any evidence available on this point? 



Postgate: The material retains its enzymic activity and chromatographic properties 

 after boiling at pH 7 in dilute salt solutions. I do not know if the spectrum changes, 

 but the solution does not obviously decolorize. 



Henderson: Knowledge of the n value of cytochrome Cg would be useful. This would 

 indicate whether there is interaction between the haems, i.e. if so « = 2, if not n = 1, 

 and still in the latter case it could have two haems/molecule or aggregate. This would 

 help considerably in elucidation of electron transport problems (cf. Shack and Clark: 

 /. biol. Chem. Ill, 143, 1947). 



Amino Acid Composition and Absorption Spectrum of Cytochrome C3 

 Egami: Takahashi (Takahashi, Titani and Minakami, /. Biochem. Tokyo, 46, 1323, 1959) 

 in ray laboratory in Tokyo carried out the amino acid analysis of cytochrome Cg. He 

 found that it contains no tyrosine and just twice as many (6) residues of histidine as 

 mammaUan cytochrome c. The former finding is consistent with the observation that 

 it has no absorption at 280 m/t and the latter seems to be consistent with the fact that 

 it contains just twice the haem iron. 

 Drabkin: I believe that the mystery of the missing band at 280 m/z of cytochrome c^ is 

 now somewhat dispelled, since we have now seen the absorption spectrum of the 

 oxidized form. If you will turn to Fig. 5 of my paper (p. 150) you will find that the 

 spectrum of ferriprotohaemin dicyanide is very similar indeed to that of ferricyto- 

 chrome Cg. There is no distinct maximum, but certainly definite absorption in this 

 region. The analysis discloses the presence of my band No. 8 at approximately 280 m/A, 

 masked in this particular case, and perhaps also in cytochrome Cg. 



Functional Aspects of Cytochrome Cg 



On tlie Functional Significance of Two Haem Groups! Molecule in Cytochrome Cg 



Trudinger : Desulphovibrio desulphuricans is unusual in being one of the few known bacteria 

 containing a single haemoprotein. Moreover this protein contains two haem groups/ 

 molecule as distinct from the usual case of one haem group/protein molecule in 'c type' 

 cytochromes. Would any of the workers involved with cytochrome Cg care to comment 

 on a possible significance in this coincidence? 



Postgate: I cannot suggest any specific reason for this haematin being bifunctional, but 

 the haem residues appear to be identical in character and linkage, and the organism 

 seems to be the only recorded organism to have one, and not more than one, 

 cytochrome. 



Kameis/: An interesting situation exists in the energetics of the electron transport system of 

 Desulphovibrio. The potential difference between hydrogen and the terminal sulphur 



