ENZYME COMPLEXES 



acid residues or 35 turns of the o;-helix. Assuming one metal 

 atom for each turn we would therefore require 35 atoms to 

 account for complete and sole electronic transmission by our 

 model. The actual number of iron atoms per flavin in the 

 isolated antimycin-sensitive, DPNH oxidase equals 15; thus a 

 considerable portion of the over-all molecule could be involved 

 in the electron transfer of the postulated type. This model is 

 based on a catalytic unit consisting of one flavin and four hemin 

 prosthetic groups (similar to the conditions in hemoglobin or 

 catalase). Another estimate can be arrived at from the beautiful 

 work of Perutz and his collaborators on the structure of the 

 hemoglobin molecule. This molecule, of molecular weight 

 68,000, is believed to be 65 A long with the other two dimensions 

 being 55 and 55 A, respectively (13,18). On the basis of x-ray 

 diffraction evidence, the presence of rods, each having the 

 structure of cv-keratin. i.e., the a-helix of Pauling and Corey, 

 extending in the 65 A direction is indicated. These rods are 

 packed in a pseud ohexagonal array with the centers 10.5 A 

 apart. Now let us consider the make-up of one individual rod : 

 the a-helix has 48 amino acid residues per 13 turns of the helix, 

 corresponding to a distance of about 72 A. Thus a rod 65 A long 

 would consist of about 12 turns. Assuming only one of the rods 

 to be actually concerned with electron transport, "semimetallic" 

 transmission of electrons could therefore extend over most of the 

 distance occupied by a hemoprotein molecule constructed like 

 hemoglobin. Two further observations may have some bearing 

 on the proposed distribution of metal atoms throughout the 

 protein chain of an enzyme. (7) The molybdo-flavoprotein 

 xanthine oxidase contains more than one flavin nucleotide 

 prosthetic group per mole of enzyme. Thus the possibility of 

 interaction of these various prosthetic groups in an inter- 

 dependent fashion must receive some consideration. This 

 enzyme has been shown by Richert and Westerfeld (36) to 

 contain several iron atoms per mole. 



(2) The succinic dehydrogenase complex can be degraded 

 in a serial fashion by tryptic digestion and other means in a 



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