252 2. ANALOGS OF ENZYME REACTION COMPONENTS 



potent inhibitor of Eg, the rate may first rise as the inhibition on E^ is 

 released, and then fall later when the concentration of B' rises sufficiently. 

 On the other hand, if A' is not readily depleted and B' inhibits well, the in- 

 hibition will steadily increase. It is clear that the kinetics may not indicate 

 competitive inhibition when the rate of formation of C is measured. The 

 concentration of the intermediate B may rise and fall in a complex manner, 

 as discussed in Chapter 1-9 (page 438). This is actually the simplest case 

 of lethal synthesis complicated by competition in the initial reaction. 



MEANS OF EXPRESSING RESULTS 



The results in the study of analogs have usually been given in terms of 

 inhibitions at different concentrations of substrate and analog.* Such 

 results are often valuable but rather difficult to interpret, especially when 

 different analogs at different concentrations must be compared. It would 

 be more valuable if inhibitor constants were calculated and presented, 

 along with Michaelis or substrate constants, and fortunately this practice 

 is becoming more common. With values of K,„ and K^ it is possible to de- 

 termine the inhibition expected at any combination of substrate and analog 

 concentrations. Furthermore, it is possible to calculate relative interaction 

 energies from a series of K-q obtained for a group of analogs, and thereby 

 put the inhibition on a more molecularly interpretable basis. 



There are fundamentally two types of /i",. The actual dissociation constant 

 for the EI complex may be called the true inJiihitor constant and refers to 

 a particular free and active form of the inhibitor. The experimentally 

 determined K^, on the other hand, often differs from the true Ki and may 

 be termed the apparent inhibitor constant. This apparent K^ may depend 

 on a number of factors, the most important of which is usually the pH 

 since many inhibitors are weak acids or bases. This problem has been 

 discussed in some detail in Chapter 1-14. If the inhibitor is a weak acid 

 only one form may be the inhibitor, say the ionized I~ form, in which case 

 the apparent K^ will vary with pH in the range around the p^^ of the 

 inhibitor. The true K^, which refers to the equilibrium 



E + I- ^ EI- 



will not vary with the pH because of changes in the ionization of the inhi- 

 bitor, although it may for other reasons. The apparent K^, in fact, will 

 depend on any type of equilibrium between active and inactive forms of 

 the inhibitor, for example an enol-keto isomerism, or on the binding of 

 the inhibitor to nonenzyme components of the preparation. The Kj's given 



* The substrate concentration has been omitted in some reports and this vitiates 

 the results and makes them quantitatively meaningless. 



