VARIATION OF ENZYME INHIBITION WITH pH 



691 



(number 11 in Fig. 14-10). Actually the inflection will occur at a pH = 

 pK^ only if (I) is close to K,; if (I) = lOK^, the inflection will be shifted 

 one pH unit higher and if (I) is much less than K^, the inflection will be 

 shifted to lower pH's. 



Each method of graphical analysis to determine K,' has certain advan- 

 tages but for the present purpose, the determination of a K/ that is simply 

 related to the true constant, K^, by the usual pH functions, it would ap- 

 pear that the single curve method of type F, where (I) (1 — O/i is plotted 

 against (S), is generally the most reliable. 



Variation of Fumarase Inhibition with pH 



There are very few reported data on enzyme inhibition to which the Dix- 

 on treatment can be applied. However, Massey (1953 b) conducted an 

 excellent study of the competitive inhibition of fumarase by various or- 

 ganic ions, the results, of which are summarized in Fig. 14-11. It is believed 

 that the inhibition is chiefly brought about by the binding of these organic 

 anions electrostatically to two properly spaced cationic groups at the active 

 center. This is indicated by the variations with pH and also by the lack of 



4.0 



Fig. 14-11. The pK/ — pH curves for the inhibition of fumarase by 

 various organic anions. (From Massey, 1953 6.) 



