PROSTHETIC GROUPS, COENZYMES AND ENZYMES 



Nobel Prize, he kindly presented us his new model of an a-hclix. 

 We then attached a model of a heme-molecule to the a-helix, 

 using the side chains of the cysteine residues of the hemepeptide 

 (25). The heme was found to coincide easily with the cysteine 

 residues to form the thioether bonds. In both left- and right- 

 hand a-helices histidine imidazole in position 8 could be ar- 

 ranged sterically so that it could form a covalent bond with iron 

 perpendicular to the plane of the heme. There are two theo- 

 retical ways in which the heme could be attached : either the 

 heme side chain in position 2 to either cysteine residue number 4 

 or 7. With a right-hand screw both structures seemed to fit 

 equally well, but this kind of screw has so far not been found to 

 exist in Nature. The left-hand screw, which probably is pres- 

 ent, for example, in keratin, fits well only when the cysteine 

 residue number 7 is attached to the heme side chain in the 2- 

 position, as indicated in the formula. In this case the axis 

 of the peptide helix was practically parallel with the plane of the 

 heme. See Figure 1. 



Helices with 4.4 ("tt") and 5.1 ("7") amino acids per turn 

 were also constructed, and it was found that in the 7r-helix the 

 imidazole could with some strain form a covalent bond with the 

 iron atom. In the 7-helix the imidazole was not able to form 

 a bond with the iron atom. In none of the helices could any of 

 the other side chains form a bond with the iron opposite to the 

 bond with imidazole. On the other hand, if the Fe-imidazole 

 bond was broken, the free amino group of the lysine residue in 

 position 3 could form a covalent bond with the iron. 



Even if the hydrogen bonds in the a-helix were broken, it 

 was hardly possible for any two of the side chains to forin cova- 

 lent bonds at both sides of the heme-disc. The question then 

 arises how the hemopeptide can give typical hemochromogen ab- 

 sorption bands, indicating covalent bonds to the iron on both 

 sides of the heme. When the hemopeptide is titrated with acid 

 the two hemochromogen-forming groups are split off" in two 

 steps with /?K's 3.5 and 5.8 (12). This is of course to be ex- 

 pected since in this case the iron-linked groups must be chem- 



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