REGENERATIVE SYSTEMS 



355 



will be termed a " regenerative system" because these intermediate sub- 

 stances are not transformed into one another to move along the chain, 

 but shuttle back and forth as some group is transferred along the chain. 

 Oxidation-reduction chains and systems transferring phosphate or other 



(cycle) 



Fig. 7-21. Noncompetitive inhibition of either E^ or E3 in the cycHc 

 system shown in Fig. 7-16. The variation of the over-all cycle inhi- 

 bition with the individual inhibitions is shown. Curve 1: (M)^ = 0.1 

 ml/; curve 2: (M)^ = 1 nu¥; curve 3: (M)^ = 10 mM. 



groups would be regenerative systems. The simplest representation of 

 such reactions would be: 



A-X + B -» B-X + A 



E2 (7-39) 



B-X ^ C -> C-X + B 



A-X + C 



C-X + A 



where X is the group transferred from A to C, in which B functions as a 

 link between the two reactions, as pointed out by Dixon (1949, p. 8), 

 and is regenerated continuously as the reaction proceeds, being part of 

 the catalytic mechanism and often termed a cofactor or coenzyme. For 



