48 2. THE KINETICS OF ENZYME REACTIONS 



for which the rate equation may be obtained by setting (A) = (B): 



(A) + Z,„ 



where F,,, is the same as in Eq. 2-76 and K,,, — K j^ + K^^. It is worth noting 

 that whereas from the over-all equation one might expect (A)^ in the rate 

 equation, this is not so if the two molecules of substrate react sequentially. 



Enzymic Reactions of Greater Complexity 



The few reaction mechanisms discussed above do not by any means ex- 

 haust the possibilities of intricate patterns in enzyme catalysis. Many en- 

 zymes involve three or more reacting components in their mechanisms (e.g., 

 a substrate, a coenzyme, and ATP) or catalyze several individual reactions 

 on their surfaces before the product of interest is produced. The success of 

 a simple kinetic analysis in some of these complex situations resides in the 

 fact that occasionally only one of the component reactions controls the 

 over-all rate, being relatively so slow that it limits the entire process. In 

 such cases the kinetic data will apply only to this limiting reaction; however, 

 it is not always easy to determine exactly which of the multiple steps is 

 limiting. Furthermore, different steps may be rate-limiting under different 

 conditions and there is no guarantee that the same step is limiting in the 

 cell as in the isolated preparation. From the practical point of view, it 

 often minimizes the chance of misinteri:)retation in enzyme studies, with 

 or without inhibitors present, if each known component of the system is 

 varied independently and in relation to every other component, and to cover 

 a sufficient range of concentrations to allow valid kinetic analysis. 



