HENRY R. MAHLER 



manner entirely analogous to that described for DPNH oxidase 

 (16). These fragments we might call A, AB, ABC to describe 

 their increasing order of complexity and multiplicity of pros- 

 thetic groups. They show different specificity toward electron 

 acceptors: A interacts well only with phenazine, AB with 

 phenazine and ferricyanide, ABC with phenazine, ferricyanide, 

 and methylene blue, etc. It can be shown that iron atoms are 

 associated with each of the fragments described, in such a manner 

 that AB contains a higher number than A, ABC a higher number 

 than AB, and so forth. Thus in both the cases mentioned 

 protein-bound iron may be conceived as linking together several 

 prosthetic groups of an enzyme molecule. 



It is obvious that this is a crude picture, a possible lead 

 toward a first approximation at best. There is no reason to 

 assume that this is the most probable means of electron transport 

 along a polypeptide chain. It is merely a possible one. Such 

 a partially metallic model of electron transport would lead to 

 transfers with rate constants of the order of 10^^ M~^ sec.~^ 

 These values are several orders of magnitude more rapid than 

 Chance's observed constants of 10^ to 10^ M~^ sec.~^ This alone 

 does not constitute a valid objection to the theory, however. 

 The constants were calculated assuming a truly metallic 

 conductor. It is obvious that there will be difTerences between 

 a series of metal ions inside a protein helix and a true metal. 

 These differences will lead to a slowing down of the speed of 

 transmission. Nor is there any reason to assume that coupling 

 between the electronic system inherent in a prosthetic group, say 

 an iron porphyrin, and the protein-bound chain of metals will 

 be perfect, nor even coupling from iron atom to iron atom, nor 

 will the distances between individual metal atoms be invariant. 

 There may be areas within the helix without metal atoms. All 

 these effects and many more due to the imperfection of both 

 the conductor and the insulator, i.e., the protein, will lead to 

 retardation. Part of the circuit may consist of yet another type, 

 neither resonating electrons in a prosthetic group 7r-system, nor 

 metal ions but instead it may be part of a thin conducting 



272 



