VAKIATION OF EXZYME ACTIVITY WITH pH 



665 



slopes will provide the values of pZ^' and pZ^", these being given by the 

 intersections of the linear slope lines. If pJ^/ = piC,, + vfhb' ^ tlie slope will 

 change from 1 through to — 1 as the pH rises, and for pE^/ = piiC^. + 

 p//j ft" the slope changes from through — 1 to — 2. However, it must 

 be remembered that in Eq. 14-45, pf,, is opposite in sign to p/, and p/, 

 so that the curves relating to the former will be inverted with respect to 

 the latter and the inflections will be concave upwards. 



+ 1 



-I - 



-2- 



-3- 



-4 



10 



pH- 



FiG. 14-4. Logarithmic plots of the pH functions for a dibasic acid with ^K^ 

 and P-K^o" — 7 (as in the soHd curves in Fig. 14-2). 



The rules for interpreting such curves as formulated by Dixon may now 

 be summarized. 



I. A curve obtained by plotting pE"/ against pH will consist of linear 

 sections (if the pjS^/s are sufficiently separated) and the slopes of these 

 sections will have integral values of 0. 1. 2, 3. .... either positive or negative. 



II. The slope of any linear section is numerically equal to the change 

 of charge upon dissociation of the ES complex in that pH region. 



III. Each inflection of the curve indicates the p/i , of an ionizing group 

 on a component of the enzyme system; continuations of the linear sections 

 intersect at a pH value corresponding to this p^^. 



