802 7. MERCURIALS 



If we assume that the mercurial is so tightly bound that fiee mercurial in 

 the medium may be neglected, the conservation equations take the form: 



(M,) = (MSJ + (MS,) (7-8) 



(Sr). = (Si-) + (MSJ (7-9) 



(Sr)e = (S,-) + (MS,) (7-10) 



The fractions of each S~ group combined with mercurial will be represented 

 by fy and f^, and it is easy to show that these are related by: 



l-/i 1-/2 



The ionization of the SH groups has purposefully been assumed to be com- 

 plete in order to simplify the expressions. If S^" is assumed to be necessary 

 for enzyme activity, i = fi = (MSi)/Si-)^, while the reaction of Sg" is with- 

 out effect on the activity. Finally, we shall designate by r the fraction of 

 the total SH groups reacted with the mercurial, this being determined by 

 titration. In a specific case where (S^-)^ = (82")^ = 10"^ M, if we vary the 

 ratio K2IK1 — i.e., the relative affinities of the S~ groups for the mercurial — 

 the curves in Fig. 7-20 are obtained, r always being linear to complete reac- 

 tion while i can follow any of the curves between KJE^ = and 00. When 

 K2IK1 < 1, the situation corresponds to case C in Fig. 7-19, and when 

 KJKy > 1, it corresponds to case D. One thing we immediately note is 

 that the affinities of the mercurial for the different S" groups must be quite 

 different if the i curves are to deviate from the r line appreciably; i.e., 

 unless K2IK1 is much greater or much less than 1, it will be difficult to 

 demonstrate that only one of the S~ groups is necessary for the enzyme 

 activity. This treatment can be readily extended to enzymes with more 

 than 2 S~ groups, in which case: 



fiKi fiKz f„K„ (7-12) 



1-/1 1-A ■■■ l-/n 



and to situations in which more than one S" group are involved in the 

 catalysis. One can also plot i against r to obtain curves characteristic of 

 the various situations described above. 



A few examples of the different types of behavior are summarized here, 

 as far as it is possible to evaluate the data published. 



Type A 



Lactate dehydrogenase — beef heart (Millar and Schwert, 1963) 



Malate dehydrogenase — pig heart (Wolfe and Neilands, 1956; Pfleiderer et al., 1962) 



3-Phoshoglyceraldehyde dehydrogenase — yeast (Velick, 1953) 



Pyrophosphatase — pig brain (Seal and Binkley, 1957) 



Succinate dehydrogenase — rat liver (Hirade and Hayaishi, 1953) 



