262 



6. INTERACTIONS OF INHIBITORS WITH ENZYMES 



is possible that d may be less than fg in the second case above if the inter- 

 acting ions lose their primary hydration. Within the cell, r^ for positive 

 ions is probably greater than given above due to the larger size of the 

 anions present, and the ionic strength and x may be less than assumed. 



When the ionic atmosphere is displaced by any type of interaction, a 

 rough estimate of the energy involved may be made from the value of the 

 ionic energy in the Debye-Hiickel treatment. The potential energy due 

 to the ionic atmosphere at 37.5° and at an ionic strength of 0.16 is about 

 — 0.3 kcal/mole. The energy is probably somewhat less than this for ionic 

 groups on molecules due to steric factors. When ionic groups interact in 

 enzyme inhibition it is likely that the entire ionic atmosphere of each is 

 displaced, so that approximately 0.4 kcal/mole must be expended on this. 

 Thus this represents only a minor correction factor in such interactions. 



The binding of ions to some proteins is more complex than indicated by 

 the treatment above, as studies on myosin have demonstrated (Lewis and 

 Saroff, 1957). Myosin A binds maximally about 210 K"*" or Na"^ ions per 

 molecule (assuming a molecular weight of 420,000) and myosin B about half 

 this number. The effects of pH on this binding indicate the participation 

 of cationic groups and a " chelation " type of binding: 



-coo- 



-H3N - + K+ 



-coo- 



K+ ■•• H,N 



H^ 



There are two types of binding sites, one involving the imidazole group of 

 histidine and one an amino group. The thermodynamic constants for K"^ are: 



where K is the dissociation constant. The high binding energy would also 

 indicate some type of ])inding other than simple ion-ion interaction. The 

 binding is, of course, markedly dependent on pH; there is essentially a 

 competition between K"^ and H"*" ions for the nitrogen group. That appro- 

 priate conjunctions of anionic and cationic groups on a protein is necessary 

 for such binding is shown by the failure of actin and seralbumin to bind 

 either K+ or Na"^. 



DISTORTION ENERGY 



The binding of an inhibitor to an enzyme may involve modifications in 

 the structure of either or both components and such changes from the nor- 

 mal state will require expenditure of energy. These energy changes should 

 be incorporated in the expression for the over-all energy of interaction but 

 it is generally impossible to evaluate the magnitude of the contributions. 



