HENRY R. MAHLER 



ions imbedded in the polypeptide chain. Some or all of the 

 metal ions may not even be linked to the helix by classical bonds ; 

 instead they may be simply immersed inside the helix and held 

 there trapped by the geometry of the situation. 



Let us now consider whether the quantitative information 

 at our disposal is capable of demonstrating the consistency or 

 inconsistency of this model. This information is of two kinds: 

 first we must see whether the atoms of iron are of the proper 

 size to fit into the Pauling-Corey helix (34) as the most likely 

 protein model. Obviously if the radius of iron (which will be 

 of partially covalent and of partially metallic character) is too 

 large to be fitted into the dimensions of the helix, we would have 

 to look at the model with deep misgivings. Similarly if the radius 

 of the metal ion were too small, then tight bonding of the metal 

 could not take place and once again the model would be open to 

 question a priori. Fortunately the covalent diameters of iron 

 and copper are 2.46 and 2.7 A, respectively, while their metallic 

 diameters are about 2.5 A each (33), of just the right dimensions 

 to fit snugly into the circular cross-sectional area produced by 

 one turn of the helix (the distance between nitrogen and oxygen 

 being between 2.5 and 2.6 A), providing bonding to the metal 

 occurs. 



Next we must see whether the number of metal atoms 

 associated with one enzyme molecule is sufficient to account for 

 transmission along a significant proportion of the chain, assum- 

 ing one metal atom per turn of the helix. Here we must make 

 certain assumptions. The first concerns the molecular weight 

 of a "typical" cytochrome molecule. Clearly not all portions 

 of the enzyme molecule will be concerned with its primary 

 purpose, the transport of electrons, nor will the distance between 

 the two prosthetic groups to be linked be necessarily identical 

 with the over-all length of the molecule. Let us boldly assume 

 that the "effective minimum molecular weight" of a hemo- 

 protein, i.e., that which is directly related to its function, is that 

 of cytochrome c or of myoglobin, i.e., about 15,000. This then, 

 assuming but one polypeptide chain, leads to about 130 amino 



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