TRANSITION BETWEEN STEADY STATES 



383 



appreciably from that in single enzyme systems. It is even impossible in 

 many cases to establish the type of inhibition — competititive or noncom- 

 petitive — in multienzyme systems, because the special kinetics of these 

 systems tend to obscure the behavior of the single enzyme. A valid de- 

 termination of K^ in a multienzyme system depends on the assumption 

 that the step inhibited is truly limiting the rate measured under the con- 

 ditions of the experiment, and in the previous section the dangers of such 

 assumptions have been pointed out. Even the simple equating of K^ to 

 (I), =0 5 for noncompetitive inhibition is generally unjustified in multi- 

 enzyme systems. 



TRANSITION BETWEEN STEADY STATES 



Inhibition of an enzyme in a reaction sequence may induce the system to 

 change into a new steady state. The transition from one steady state to 

 another does not occur instantaneously because a certain interval of time 

 is required for the concentrations of the intermediates to reach their new 

 values. This interval will be called the- transition time. Since this depends 



TIME " 



Fig. 7-39. Changes in the rate and concentration of intermediate, B, with time follow- 

 ing inhibition of Eg in a monohnear chain, A-^B^C. The interval. At required 

 for the steady state to be resumed is the transition time. The arrow at I indicates 

 the addition of the inhibitor. 



on the rate of change of intermediate concentration, it will be determined 

 by the relative rates of the reactions, the degree of inhibition, and the phys- 

 ical state of the system. The situation for the simple monolinear chain, 

 A ^ B -^ C, is shown in Fig. 7-39, where inhibition of Eg requires a rise 

 in (B) for the steady state to be maintained. The importance of this in 

 multienzyme systems was first pointed out by Burton (1939). If the tran- 



