700 



14. EFFECTS OF pH OX ENZYME INHIBITION 



where e is the electronic charge, b is the radius of the enzyme protein sphere, 

 a is the distance from the surface of the si)here in which small ions are ex- 

 cluded, and K is the Debye-Hiickel constant representing the reciprocal of 

 the thickness of the ionic atmosphere. The value of w will depend on the 

 temperature, the ionic strength, and numerous other factors. For a given 

 total net protein charge, the larger the protein molecule and the smaller 

 the surface charge density, the smaller w will be. Deviations of the protein 

 from sphericity also will modify w. Methods for experimentally determining 

 w have been given by Tanford (1955) and he has reported values of 0.04 

 for /?-lactoglobulin and 0.021 for bovine serum albumin at an ionic strength 

 of 0.15 (the value for /?-lactoglobulin was calculated from a curve based on 

 Tanford's values at other ionic strengths). Many enzyme test media have 

 much lower ionic strengths (see Chapter 2), in which case iv will be greater 

 (for /5-lactoglobulin, iv = 0.06 at an ionic strength of 0.019). 



The variations of the inhibition and the pii^, due to changes in the net 

 protein charge are shown in Fig. 14-15 as calculated from Eq. 14-125 with 

 IV = 0.05. A total of 50 net charges on the enzyme will shift the piiC, by 



+ 8 +16 +24 +32 +40 +48 



Fig. 14-1.5. Variation of p/iTj and the inhibition with the net protein charge on the 

 enzyme (Eq. 14-125). Ki = 1 mM, (I) = 1 mM, and w = 0.05. Curve A: pA',; curve B: 



inhibition. 



