IRREVERSIBLE MONOLINEAR CHAINS 335 



at least during the interval of measurement. An interesting situation 

 might arise if ^,j were very small and the amount of I not much greater 

 than B, since the rate would be decreased to near zero and would remain 

 there mitil enough B was formed from reaction 1 to combine with all the 

 I present, at which time (B) would rise and the formation of C would 

 recommence. 



Diffusion-Limited Monolinear Chain 



The diffusion of A into the multienzyme region may be so slow that the 

 concentration of A inside cannot be maintained equal to that outside and 

 in this sense the diffusion may be considered to limit the formation of C. 



This restriction of diffusion may be imposed by a membrane surrounding 

 the multienzyme system. The rate of inward diffusion will be r^^ = k[{A ) — 

 (Aj)], where k is the diffusion constant, and equating this to v^ in the 

 steady state, the value of (Aj) is found to be given by a quadratic equation: 



(A,)^ + 



-^ + A^ - (A,) 



(A,) - A,(Ao) =0 (7-11) 



From (Aj) may be determined the steady-state rate of the enzyme chain. 

 If Ej is now inhibited, (A;) will rise but it will not rise to a level where the 

 enzyme rates have returned to the uninhibited state because the inward 

 diffusion of A will decrease as (Aj) increases. The relationship of the inhibi- 

 tion on the formation of C to the inhibition on E^ is shown in Fig. 7-10. 

 At first, as inhibition on Ej^ is increased, there is a resistance to inhibition 

 of the over-all rate, but this is eventually lost and at high inhibitions of 

 EjL the system is quite sensitive. In terms of buffer capacity, dildi^ is large 

 at^rnall inhibitions of E^ and decreases with inhibition: in the case plotted, 

 dildit is 6.8 between i = to 0.25 and decreases to 0.24 between i = 0.9 

 to 1.0. The buffer capacity thus changes constantly and markedly in such 

 a system. It may be pointed out that working in the middle range of in- 

 hibition may obscure this effect; for example, in the present case if 

 inhibitions of 40% and 90% were tested, dildit would be very close to 

 one, although ?\ would be definitely less than i. Determination of buffer 

 capacity with many levels of inhibition over as wide a range as possible 

 may be useful in elucidation of mechanisms in multienzyme systems. The 

 occurrence of a diffusion-limited step, for example, might be detected, 

 because for the monolinear chain the i — ?', curves are quite different from 

 those in which diffusion is not a factor. 



The enzymic oxidation of ascorbate, catalyzed by peroxidase, was studied 



