CYCLIC SYSTEMS 



353 



(2) The buffer capacity at lower inhibitions depends on (M)^ and be- 

 comes greater as (M), increases (Figs. 7-21 and 7-29). Thus for inhibition 

 on El or Eg, dijdii has the following approximate values for inhibitions up 

 to 40%: when (M), = 0.1 mil/ it is 4, when (M), = 1 mM it is 5.3, and when 

 (M), =- 10 niM it is 16. The reason for this is that at high levels of (M)^, 

 the high concentration of B is approaching saturation of E, and it requires 

 a fair degree of inhibition to reduce (B) so that the rate falls. 



(3) Inhibition of either E^ or Eg affects the cycle rate identically but the 

 changes in the concentrations of A and C are reversed. Inhibition of Eg 

 alters the cycle rate readily and little buffer capacity is evident, since 



Fig. 7-19. Noncompetitive inhibition of Ej in the cydic system shown in Fig. 7-16. 



(M)( = 1 mM. 



this is the reaction that forms Y. Although inhibition of E.^ causes a rise 

 in (B), this in turn causes a fall in (A) and (C) so that the cycle slows down, 

 and this finally prevents the rise in (B) from being as marked as it would 

 be in a linear chain. 



(4) When the inhiliition on a component enzyme passes beyond a cer- 

 tain point (in the present case around 50% inhibition), the buffer capacity. 



