PROSTHETIC GROUPS, COENZYMES AND ENZYMES 



SO important. It is especially important to learn about the 

 groups functioning as binding sites for cofactors and substrates, 

 and the groups that are close enough to influence the reactions. 

 The more remote parts of the protein molecule are probably less 

 important. For instance, the mode of action of hemoglobin has 

 at least partly been explained on the basis of the chemical nature 

 of its "heme-linked" groups. 



Cytochrome c, a Protein with Immobile Prosthetic 



Group 



Cytochrome c is from some points of view a suitable material 

 for investigating the chemistry of the coupling between hematin 

 and protein. It is comparatively readily available and is 

 stable over a very wide range of pW. As in all hemo-proteins 

 the porphyrin moiety puts light absorption bands at our dis- 

 posal for spectrophotometry. In addition, the iron atom under 

 different conditions has 0, 1, 3, or 5 unpaired electrons; there- 

 fore magnetometric determinations are useful for structure 

 studies. 



There have been some advances in our knowledge of the 

 structure of the cytochrome c molecule. The a-carbon atoms of 

 the side chains in the 2- and 4-positions of the porphyrin are 

 linked by thioether bonds to sulfur atoms in two cysteine residues 

 of the protein. This was demonstrated by Theorell in 1938-39 

 (19). Paul (13), in this institute, further corroborated the ex- 

 position of the thioether bonds to the porphyrin side chains by 

 isolating an optically active hematoporphyrin after splitting the 

 thioether bonds with silver acetate. In 1941 (22) the theory was 

 advanced that the groups forming a hemochromogen with the 

 iron atom were both imidazoles from histidine residues; this was 

 based upon titrimetric, spectrophotometric, and magnetic data. 

 Support to this theory was given by Paul's titration experiments 

 on the cytochrome apoprotein, liberated by the silver-splitting 

 method 



Reduced cytochrome c is not autoxidizable. This fact, 



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