ENZYME COMPLEXES 



are butyryl CoA dehydrogenase (Cu), DPNH-cytochrome reduc- 

 tase (Fe), nitrate reductase (Mo), xantfiine oxidase (Mo, Fe), 

 aldehyde oxidase (Mo), hydrogenase (Mo). The metal appears 

 to be involved in linking not only flavin to protein but also the 

 acceptor to the otlier two components during the lifetime of the 

 enzyme acceptor complex (27). During this period there is 

 evidence that all four components become part of the same 

 resonating system with a free flow of 7r-electrons from reduced 

 enzyme (or substrate) to acceptor. Among the acceptors in this 

 class are nitrate and cytochrome c. Thus, for instance for the 

 oxidation of a reduced normal flavoprotein there is observed an 

 optimal reactivity with only one member of a series of closely 

 related electron acceptors (e.g., substituted 1,4-naphthoquinones) 

 (28). This acceptor has a definite redox potential (called the 

 optimum interaction potential e). If the logarithm of tlie 

 reaction rate for reduction of quinone is plotted against potential 

 of quinone, the rate of interaction falls oflT sharply and linearly 

 on either side of €. If instead the enzyme is in the metallo- 

 flavoprotein form, then not only is t shifted to a more positive 

 or more negative value, but also the drop-off" of rates at either 

 side of 6 is much less rapid. Let us briefly develop one of a 

 number of possible interpretations which may be consistent 

 with these observations. In accordance with current views it is 

 assumed that (7) reduced flavoprotein and acceptor must 

 interact witliin a complex and (2) that x-bonding or the inter- 

 action between two such mobile electronic systems can take 

 place only if both partners possess certain closely matched 

 energy lev^els (2,10,21). For the interaction of a series of quinones 

 with one given flavoprotein, matching might well be optimal 

 for only one member of tlie series and less eflicient in a constant 

 and predictable manner if the inherent electronic structure of 

 the quinone is modified by a variation of substituents. This 

 inherent difference in energy levels will manifest itself then ( 7) 

 in a variation of the £"0' of the various members of the series and 

 (2) in a variation of the ability to match energy levels with a 

 constant interacting system (the flavoprotein), as observed. 



259 



