248 2. ANALOGS OF ENZYME REACTION COMPONENTS 



(J) The analog may act not on the enzyme attacking the normal sub- 

 strate but on an enzyme involved in the formation of this substrate, since 

 the precursors of the substrate will usually be structurally similar to it. 

 In the linear sequence: 



E, Ej 

 X -^S-> P 



an analog of S may have been designed to inhibit Eg but actually in cellular 

 metabolism acts primarily on Ej to reduce the rate of formation of P. 



KINETICS OF ANALOG INHIBITION 



The kinetics of competitive inhibition have been presented in Chapter 

 1-3, and the graphical analyses for the proof of competition and the deter- 

 mination of the constants discussed in Chapter 1-5. Type A plots of l/v 

 against 1/(S) have almost invariably been used to demonstrate the types 

 of inhibition produced by analogs, but other types of plotting may be more 

 satisfactory in certain situations, especially when the inhibition is not 

 clearly and completely competitive. It is my opinion that many kinetic 

 analyses of inhibition would be improved if several types of plotting proce- 

 dure were used, allowing comparison of the results and more accurate cal- 

 culations of the constants. 



It is quite often the case that the inhibition by an analog is not, by the 

 usual methods of analysis, competitive with the substrate or coenzyme 

 to which the analog is structurally related, and such results have puzzled 

 many workers. The plotting may indicate noncompetitive, coupling, mixed, 

 or indeterminable inhibition mechanisms. It has been pointed out (Chap- 

 ter 1-3) that true noncompetitive inhibition must be rather rare and this 

 is particularly true for inhibition by analogs. Coupling or uncompetitive 

 inhibition is perhaps more common among analogs than with other in- 

 hibitors, especially with regard to coenzymes or cofactors, inasmuch as 

 the substrate combines with enzyme-coenzyme or enzyme-cofactor com- 

 plexes in many reactions. It is important to determine in any case if the 

 inhibition is really competitive and the kinetics modified to obscure this, 

 or whether the mechanism is actually other than competitive, in those 

 instances in which the graphical analysis does not demonstrate the typical 

 and expected competitive picture. 



There are several reasons why an analog inhibition may not turn out to 

 be competitive by the usual plotting procedures, and it may be useful to 

 list them at this point. 



(A) The analog may be acting by some mechanism other than specific 

 attachment to an active site on the enzyme, that is, by one of the mecha- 

 nisms listed in the previous section. 



