SPECIFICITY AND INHIBITION OF FUMARASE 



163 



Fig. 4. Theoretical curves for the pH variation of the competitive Inhibition con- 

 stant for two hypothetical cases. The lower curves give the number of equivalents of 

 acid produced per mole of enzymatic sites in the dissociation reaction. 



for the differential titration curve of the enzyme in the presence and absence of 

 the competitive inhibitor. When the number of enzymatic sites per molecule is 

 unknown, such a differential titration offers the means for determining the 

 number of sites. Differential titrations would also provide a check upon the 

 interpretation of the pH dependence of the kinetically-determined inhibition 

 constants. 



In comparing the plot of log {Kj/Kj) versus pH with the corresponding one 

 for Ax, it will be seen that when acid is produced the dissociation of the EI 

 complex increases with pH, and that when acid is consumed the dissociation 

 decreases with increasing pH. 



At extreme pH values (pH 8.5 at 0.01 ionic strength) there are deviations 

 from the simple equations described above. These deviations are in the direction 

 to be expected for electrostatic effects from the rest of the protein molecule 

 (Alberly, 1956). It is to be expected that they will be less important at higher 

 ionic strength values. 



Conclusion 



This discussion has been designed to bring out some of the complications 

 which may be encountered in the interpretation of enzyme kinetic data and 



