MACRO MOLECULES 



119 



6. Cytochromes 



These pigments are haemoproteins and are found in all cells which 

 respire. The form of the haem-protein linkage, in this case, does not allow 

 oxygen to be involved in the complex, but there is a reversible oxidation 

 and reduction of iron. The cytochromes show the typical two-banded 

 absorption spectrum of the haemochromogens when their iron is in the 

 ferrous state. Since each haemochromogen is characterized by a particular 

 position of these bands in the visible region, Keilin has been able to detect 



(imidazole) N 



N (imidazole; 



HN— CH — C HN-CH — C 



H3C 



HC 



HaC- 



CH 



-CH» 



Reduced cytochrome-c 



the presence in cells of three cytochromes which he called a, b and c, and 

 which are characterized by the positions of the absorption bands of their 

 reduced forms. Cytochrome-^ has been isolated. It has been shown to be 

 present in all aerobic cells. The protein part of cytochrome-c is rich in 

 basic amino acids, particularly in lysine. 



The haem of cytochrome-c is derived from protophorphyrin, two vinyl 

 groups being reduced and bound as thioethers to cysteine residues on the 

 rest of the molecule. The side-chains of the protein are believed to be 

 attached to the /3 carbon of the vinyl groups, but this has not yet been 

 completely proved. But, certainly, the a carbon is the point of binding to 



