664 



14. EFFECTS OF pH ON ENZYME INHIBITION 



separation between the two dissociation constants. In Fig. 14-4 the i>K^'s 

 are only one unit apart and the transitions are not clearly evident, the 

 curves being smooth and scarcely distinguishable from those of a monobasic 

 ionization (Fig. 14-3). The farther the pK^^s are apart, the easier it will 

 be to locate the inflection points and determine the values of the constants. 

 In Fig. 14-5 the difference between p-K",/ and piiC^" is four units and here 

 the inflections in the curves are evident. 



+1 





-2 



■3- 



pf 



-4 



B + HS^HB 



Fig. 14-3. Logarithmic plots of the pH functions for a monobasic acid with pK^ = 7 

 (as in the dashed curves in Fig. 14-2). The intersections of the hnear portions of the 



curves occurs at the TpK^. 



Let us assume the simple case where p^^' = pK, + p/j". AVhen p^/ is 

 plotted against pH, a curve of the type shown for p/j" will be obtained, 

 but shifted on the vertical axis by the constant amount, p-fiT ,. If we examine 

 the variation of p/j" = - log {1 -j- [(H)/^,"] + [{Jl)^IK,'KJ']} with pH, the 

 following may be noted: (1) at low pH's, the term {Hy-IKJK^" will domi- 

 nate and the logarithm of this is given by p/j" = ]iKJ + V^a" — 2 pH; 

 (2) at intermediate values of the pH, the (H)IKg" will dominate and vfb" — 

 pKJ' — pH; and (3) at high pH, the second and third terms will be insignif- 

 icant and p/j" = 0. Thus, as one goes from low to high pH's, the slope of the 

 curve will initially be 2, then 1, and finally 0; the transitions between these 



