EFFECTS OF ALTERATION OF STRUCTURE 307 



reasons why the new substance behaves differently, quantitatively or 

 qualitatively, from the original inhibitor. 



(1) The penetrability of the inhibitor into cells may be altered due to changes in 

 size, charge, or solubility; thus the new substance may reach the enzyme in 

 greater or lesser amounts than did the original inhibitor. 



(2) The new substance may be more or less readily inactivated, either spontaneously 

 or enzymically, as occurs with certain barbiturates or substituted naphthoqui- 

 nones upon lengthening the side-chain; the concentration of the new substance 

 may thus vary with time in a different manner than the original inhibitor. 



(3) The ability of the substance to alter another enzyme system may differ from 

 the parent inhibitor and if this other enzyme system is involved in what is meas- 

 ured a modified action will be observed. 



(4) The addition of the group may provide increased surface for interaction, reac- 

 tivity with some enzyme group, a favorable charge or dipole, or may sterically 

 or electrostatically interfere with the binding to the enzyme; these effects may 

 be attributed directly to the additional group. 



(5) The binding of the new substance may be different from the initial inhibitor 

 because of the changes enumerated above in the rest of the molecule as produced 

 by the presence of the added group. 



(6) The binding to the enzyme may not be significantly changed but the abiUty 

 to interfere with the enzyme reaction may be changed, such as would occur if 

 the added group were not involved in binding but sterically blocked the sub- 

 strate from the active site. 



It is frequently very difficult to establish the mechanism whereby a 

 change in molecular structure modifies an inhibitory effect because in most 

 cases many properties of the substance are changed simultaneously; for 

 example, the molecular size and configuration, the distribution of charges, 

 the penetrability through membranes, and the solubility may all be changed 

 along with an altered ability to bind to an enzyme. When the data from 

 an homologous series of inhibitory compoimds on cell respiration are im- 

 mediately interpreted in terms of differences in oil-water distribution 

 coefficient, or any other single factor, without further evidence, a certain 

 degree of naivety is implied. Indeed, the attribution of specific binding 

 energies to particular groups, as has been done previously in this chapter 

 for some enzymes, is an unsophisticated approach, justified here only by 

 the fact that isolated enzymes are at least independent of membrane per- 

 meability factors and other complications present in all tissue work, and 

 by the desire to illustrate the forces of binding in as straightforward a 

 manner as possible. However, in more critical calculations, it is well to 

 consider all the possibilities of molecular change upon altering a group 

 before assigning the binding energy difference to the groups involved. 



