368 S. Paleus and H. Tuppy 



original Rhodospirillwn rubrum cytochrome c and in the haemopeptide 

 prepared from it by tryptic hydrolysis. 



The specificity of trypsin already mentioned suggests that the amino-acid 

 residue preceding the above sequence must be either lysine or arginine. 



The cleavage by chymotrypsin of a bond between histidine and threonine 

 (residues 5 and 6) may appear unexpected in view of the general belief that 

 chymotrypsin will split bonds involving the carboxyl groups of the aromatic 

 residues phenylalanine and tyrosine. The finding is in agreement with recent 

 observations of Davis (1956), who, however, has reported that bonds in- 

 volving the carboxyl groups of histidine residues are somewhat susceptible 

 to chymotryptic hydrolysis. 



The comparison of the amino-acid sequences that have been shown to 

 occur in the vicinity of the haem moiety in different cytochromes demonstrates 

 that they have various structural features in common (Table 2). The presence 

 of two cysteine residues linked to side chains 2 and 4 of the porphyrin of 

 the prosthetic group, first indicated by Theorell (1939) for ox cytochrome c, 

 has been confirmed and found to be characteristic of all c-type cytochromes. 

 The distance between the two cysteine residues along the polypeptide chain 

 is always the same, two other amino acids being situated in between. In- 

 variably a histidine residue has been found to adjoin one of the haem-bound 

 cysteine residues. Stereochemical considerations (Ehrenberg and Theorell, 

 1955) have indicated that the chain of amino acids in which the two haem- 

 bound cysteine residues and the histidine residue are included is likely to be 

 in the form of an a-helix. This coiling will bring the histidine residue into a 

 steric position favourable for covalent attachment of an imidazole nitrogen 

 to the iron of the haem disc. 



There are still other structural features encountered in all cytochrome c 

 samples examined so far, such as, for example, the presence of a basic 

 residue (lysine or arginine) close to cysteine and of a threonine residue close 

 to histidine. The significance of these similarities is at present not known. 



It is remarkable that in so many structural respects the cytochrome c of 

 Rhodospirillwn ruhrum appears to resemble the c-type cytochromes of other 

 organisms, in spite of an overall amino-acid composition, oxidation-reduction 

 potential, and a specificity of action quite different from those of vertebrate 

 and yeast cytochromes (Kamen and Takeda, 1956). 



REFERENCES 

 Akabori, S., Ohno, K. & Narita, K. (1952). Bull. chem. Soc, Japan 25, 214. 

 Bartsch, R. G. & Kamen, M. D. (1958). J. biol. Chem. 230, 41. 

 Davis, N. C. (1956). /. biol. Chem. 223, 935. 

 Ehrenberg, A. & Theorell, H. (1955). Acta chem. Scand. 9, 1193. 

 Kamen, M. D. & Takeda, Y. (1956). Bioclnm. biophys. Acta 21, 518. 

 Kamen, M. D. & Vernon, L. P. (1955). Bioclnm. biophys. Acta 17, 10. 

 Keilin, D. & Hartree, E. F. (1937). Proc. ray. Soc. 8122, 298. 

 Paul, K. G. (1949). Acta chem. Scand. 3, 1178. 



