342 



7. INHIBITION IN MULTIENZYME SYSTEMS 



The concentration of B is given by: 



{(A)Fi -(F, + V,)[{A) +AM}(Br- 



+ {{A)VdK, + /Q - {V,K, + V,K,)[(A) + K,)]}{^) 

 + V,K,K,{A) = 



(7-27) 



If we assume that the formation of C is being measured, the addition of 

 reaction 3 will reduce this rate by reduction in (B). There will not be a 

 constant ratio of d{C)ldt to d{D)ldt but this will vary with (B) and hence 

 with the rate of reaction 1. For this reason, inhibition of E^ will reduce 

 i'2 + Vg to the same degree as v^, but need not reduce ■Ug ^^^ v^ equally. 

 In fact, it is possible that only d{C)ldt will be reduced and d(D)jdt remain 

 unaffected if the steady-state level of (B) is such as to saturate Eg, as shown 



graphically in Fig. 

 inhibited. 



7-13. Conversely, only the formation of D may be 



•2" '3 





(BHA) 



(B) 



(S)- 



FiG. 7-13. Rate curves for a divergent chain. The curves labeled v-^, Vj, and V3 

 show the variation of the rates with substrate concentration for each of the reac- 

 tions alone; curve v^ + V3 shows the summed rates of v^ and v-^. Neither reaction 2 

 nor reaction 3 could maintain a steady state alone when the concentration of A 

 is (A), but together they can (Vjt). If E^ is inhibited the rate curve is decreased 

 to Vij, the steady-state rate is depressed to Vi, and the concentration of the inter- 

 mediate falls from (B) to (B)j. However, v, is still greater than F3 so that the fall 

 in (B) has not changed the rate of reaction 3; thus only d{Q)idl is depressed. 



Inhibition of E.^ may or may not reduce d{C)ldt depending on the state 

 of the system before inhibition. If (B) is high enough to saturate Eg, inhi- 

 bition of E2 will cause (B) to rise and diQjdt will increase to maintain the 

 steady state as in the monolinear chain A -^ B -^ C. However, if Eg is 



