752 15. EFFECTS OF VARIOUS FACTORS ON INHIBITION 



all enzyme reactions occurring in a milieu of water, any modification of 

 this structure can have an influence on the reactions of molecules with the 

 enzyme. This is particularly apt to be expressed in terms of changes in the 

 entropy terms for the formation of the EI complex. 



(B) Changes in the dielectric constant. This factor is closely related to 

 the previous one because the variations in the dielectric constant with 

 temperature are mainly the result of changes in the water structure. The 

 dielectric constant is generally proportional to IjT. The variation of the 

 dielectric constant of water with temperature at constant pressure is given 

 by clDjclT — — 0.338 at physiological temperatures. The interaction ener- 

 gies between charged groups will vary with the temperature for this reason. 

 Since electrostatic interaction energy varies with 1/Z), it will be propor- 

 tional to the temperature with respect to this particular factor. 



(C) Changes in the hydration of the enzyme and inhibitor. Inasmuch as 

 the hydration of the interacting molecules is a major factor in the consid- 

 eration of the forces involved, one must take into account hydration 

 changes resulting from variation in the temperature. Unfortunately, very 

 little accurate information on such changes is available. From studies on 

 the viscosity of water and salt solutions, Kaminsky (1957) concluded that 

 certain ions such as Mg++ produce a long-range ordering of the water struc- 

 ture, significant orientation occurring beyond the primary hydration layer. 

 As the temperature rises, this secondary hydration is progressively disturbed 

 by thermal motion over a temperature range of 12° to 42.5°, although the 

 primary shell is not affected. The extent of secondary hydration around 

 singly charged ions or ionic groups is unknown but some ordering probably 

 does occur and would be susceptible to temperature variation. Hasted 

 and Roderick (1958) determined the dielectric constants and relaxation 

 times at different temperatures and frequencies and from such data obtained 

 some idea of the numlier of water molecules that are prevented from rotat- 

 ing when a solute is introduced into water. It might be predicted that this 

 number would decrease with rise in the temjjerature, but actually it did 

 not and in some cases actually rose slightly. Indeed, very little difference 

 was found between 3^ and 25° in any case. Other than ionic types of hy- 

 dration occur. Sometimes water molecules are attached to inhibitors or 

 enzymes through hydrogen bonding and this type of hydration is probably 

 quite sensitive to temperature. The interaction energy upon the approach 

 of an inhibitor molecule to an enzyme may thus be modified as the temper- 

 ature changes by differing degrees of hydration of the groups involved. 



{D) Changes in the ionization constants of enzyme grouj)s. Acidic and basic 

 ionization constants are sensitive to temperatures as are all dissociation 

 constants and consequently the ionization state of a protein will vary to 

 some extent with temperature. This is of significance not only with respect 



