666 



14. EFFECTS OF pH ON ENZYME INHIBITION 



IV. An inflection that is concave downwards indicates an ionization of 

 the enzyme or the substrate; an inflection that is concave upwards indicates 

 an ionization of the ES complex. 



From the curves alone it is not possible to determine if a particular down- 

 ward inflection relates to the ionization of the enzyme or the substrate; 

 however, the ionization constants of the substrate are generally known so 

 that experimentally determined p/^,/s that do not correspond to these may 



+ 2 



~i r 



"1 1 1 r 



Fig. 14-5. Logarithmic plots of tlie pH functions for a dibasic acid with 



TpKa' = 4 and pA'„" = 8. The intersections of the Hnear portions of the curves 



at the pX(j values are shown. 



be attributed to the enzyme. It is often the case tliat there are limitations 

 to the pH range that may be studied and in such cases only parts of the 

 total pii^/-pH curves may be obtained. Unless a pH range of at least 4 

 to 10 can be investigated, it is not safe to conclude that all the ionizing 

 groups have been detected. Extensions and criticisms of Dixon's procedure 

 have been made by Reiner (1959, p. 230) and Laidler (1955, c,d, 1958, 

 p. 140). 



