ENZYME COMPLEXES 



genase preparations which contain flavin, iron, and cytochrome 

 b. In the Hght of these observations it would be of great interest 

 to determine whether the yeast enzyme contains a metal instru- 

 mental in linking flavin to cytochrome ^2-* The potentialities 

 for alternate schemes of electron transport and for their control 

 by external factors with an enzyme such as aldehyde oxidase are 

 manifold. Electrons may funnel into the cytochrome chain 

 "directly" through the "built-in" cytochrome. They may be 

 diverted to cytochrome c and into alternate channels in the 

 cytochrome system through interaction with external cyto- 

 chrome c, or they may be removed by oxygen or by an auto- 

 oxidizable quinone, in both cases bypassing the metal and the 

 cytochrome system entirely. Finally, the quinone may act as 

 carrier between the enzyme and some cytochrome. The analogy 

 to the hypothetical case exemplified by Figure 1 is thus almost 

 perfect. As far as selective inhibitors are concerned, the — SH 

 groups of the enzyme may be blocked by the usual inhibitors 

 and the enzyme protected from their action by substrate; the 

 flavin may be blocked by atabrine, this inhibition being re- 

 versed by FAD ; the molybdenum may be blocked by chelat- 

 ing agents, this inhibition being reversed by excess Mo; whereas 

 the iron-porphyrin is irreversibly blocked by cyanide, azide, etc. 

 Finally, soluble succinic dehydrogenase preparations, ca- 

 pable of interacting with one-electron acceptors (phenazine, 

 ferricyanide) and having the properties of ferroflavoproteins, 

 have been isolated (38). 



The Electron-Transferring Flavoprotein (ETF) 



During their studies of tlie three enzymes concerned with 

 the oxidation of saturated fatty acyl CoA derivatives to the 

 corresponding a-^ unsaturated compounds. Crane and Beinert 

 (9) discovered a new flavoprotein concerned in the electron 



* This has been verified experimentally. Yeast lactic dehydrogenase con- 

 tains 8 atoms Fe per mole of flavin or cytochrome &2- (E. Boeri, private com- 

 munication.) 



261 



