HENRY R. MAHLER 



component, and then to such complex and complete electron 

 transport enzymes as the DPNH oxidase. Here the cytochrome 

 component is presumably linked to the conductive system of the 

 protein moiety by means of the histidine residues bonded to the 

 iron-porphyrin (40) . 



Let us reflect for a moment and consider whether such a 

 radical and highly speculative step as the suggestion that electron 

 transport between individual cytochromes may be metal- 

 mediated is at all necessary or whether more classical explana- 

 tions may not be equally satisfactory. Now in the picture 

 presented here, i.e., the sequential arrangement of individual 

 electron transfer enzymes as part of a larger self-contained 

 supramolecular unit, some method of connecting the individual 

 parts both structurally and electronically must obviously be 

 made. But this picture is certainly not the only possible one. 

 Chance (7), for instance, has suggested that simple collisions of 

 individual cytochromes, each anchored to a matrix but able to 

 interact freely with its neighbor, can account for the kinetic data 

 obtained by him. Although simple and kinetically unobjection- 

 able, this picture may also require a possible role for metal ions. 

 For if we are to take cytochrome c as our model it is apparent 

 that all the coordination valencies of iron are permanently 

 occupied (4 by the porphyrin ring and 2 by histidine residues 

 of the protein) . Thus the primary interaction of the cytochrome 

 with electron donor or acceptor must of necessity take place at 

 a locus removed in space from the iron atom. Therefore we 

 must here too look for a possible mechanism for the transmission 

 of electrons from one part of the protein to another, i.e., from that 

 spot where collision with donor has taken place to the iron 

 porphyrin and thence to the spot where collision with acceptor 

 is to take place. This problem appears to be only quantitatively 

 and not qualitatively different from that to be considered in our 

 model. In all cases it reduces to the question of possible trans- 

 mission of electronic interactions through proteins. This is a 

 problem which arises in connection with a wide variety of ob- 

 servations in biochemistry. Various theories, most of which 



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