CYCLIC SYSTEMS 



351 



(B) rise so that reactions 1 and 2 keep pace with the entry of (X). Above a 

 certain value of (X), here about 1 mM, the system may be said to be sat- 

 urated and little further change is brought about by increasing (X). It 

 may be noted that this is far below the maximal rates for any of the reac- 

 tions. The rate of formation of Y is thus limited by the total concentration 

 of cycle intermediates. 



Effect of Total Concentration of Cycle Intermediates (M^) 



The steady-state rate is shown to be quite dependent on (M), in Fig. 7-17 

 and the maximal rate reached at high values of (M); is seen to be equal to 

 the lowest maximal rate of the individual reactions, in this case reaction 2 



10- 



8 



06 



04 



02 

 'st 



— 100 



%{B) 



01 



IOOttiM 



CM). 



Fig. 7-17. Effect of the total concentration of cycle intermediates on the steady- 

 state rate and concentrations of the individual intermediates in the cyclic system 

 whose characteristics are given in Fig. 7-16. (X) = 1 mM and v^^ = d(Y)/dt. 



It is also important to note that as (M), changes, the relative concentrations 

 of the cycle intermediates change, in the i? resent case (B) increasing as 

 reaction 2 becomes limiting. Any factors that change (M)^, such as reactions 

 of the intermediates outside the cycle or reaction of an inhibitor with one 

 of the intermediates, may alter the steady-state rate appreciably. 



Effects of Inhibition of the Cycle Enzymes 



The changes occurring upon noncompetitive inhibition of E^, Eg. and E3 

 are shown in Figs. 7-18 to 7-28. Curves are given for three levels of (M); 

 in each case, inasmuch as the total concentration of intermediates is of 

 importance in the quantitative changes occurring during inhibition. The 



