HENRY R. MAHLER 



play at all; thus their effective concentration is equal to their 

 total concentration. With the same enzymes placed together 

 in true solution all the enzymic sites must be occupied for 

 maximal rates ; thus the effective concentration of coenzyme or 

 intermediate will be only a fraction of the total that has to be 

 added. Control by the action of some metabolic regulator 

 similarly can be effected in a more direct and efficient manner 

 intramolecularly : the selection of one of two or several possible 

 pathways, say of electron transport, is conditioned by placing 

 a block at, or by the addition of an alternate carrier to, a critical 

 junction point in the array, e.g., a metal atom. We might 

 postulate a scheme such as that shown in Figure 1. In this 



SUBSTRATE ■ 



ENZYME COMPLEX 



♦CARRIER r — Me — CARRIER I- 



-CARRIER m- 



le" 



♦ CYTOCHROMES 



•CARRIER DT- 



Figure 1. 



scheme carrier I might be one flavin prosthetic group linked to 

 one protein, say a dehydrogenase, carrier II a cytochrome 

 component linked directly to this metalloflavoprotein, and 

 carrier III an electron transport enzyme also contained within 

 the same complex, capable of linking the flavin to the cytochrome 

 either by a different metal or by another unspecified means. 

 Blocking at the metal site between I and II will divert electrons 

 through carrier III. If the internal cytochrome component were 

 to be blocked, electron transport to the cytochrome system as a 

 whole or to some other electron acceptor (s) can be made possible 

 by the addition of a carrier added externally (IV), say a quinone 

 of appropriate potential. Schemes very similar to that postu- 

 lated here actually do seem to play a role in electron transport 

 as shown in subsequent sections. 



Thus it becomes evident that there are inherent weaknesses 

 in both approaches: a study of unmodified enzyme complexes 



256 



