256 2. ANALOGS OF ENZYME REACTION COMPONENTS 



the substrate), but which is resistant to chemical reaction in its complex 

 with the enzyme. It would appear that the most effective inhibitors would 

 result from modifications of the reactive groups, or of groups adjacent 

 to the reactive region, rather than changes in binding groups. Malonate 

 illustrates this in a simple way because here the — CH2CH2 — group of 

 succinate has been altered to the nonoxidizable — CHg — group, while 

 the binding — COO" groups remain. Amidation of the — C00~ groups of 

 either succinate or malonate, thereby eliminating the negative charges, 

 produces a substance that is neither a substrate nor an inhibitor because 

 the affinity for the enzyme has been lost. 



The total interaction energy between a substrate or an inhibitor and the 

 enzyme active center is the result of all the forces of attraction and repulsion 

 summed over all the participating groups. Every atom or group of a sub- 

 strate or its analog contributes to some degree to the interaction energy 

 but, practically, the binding may be attributed usually to two or three 

 groups that serve to orient the molecules on the enzyme surface. The sub- 

 traction, addition, or alteration of substrate groups may change the binding 

 energy in various ways. Modifying a region vicinal to a binding group may 

 sterically interfere with the normal approach of this group to the enzyme 

 group with which it interacts, or it may by inductive or resonance effects 

 alter the properties of the binding group, as discussed in Chapter 1-6 

 (page 304). It is worthwhile emphasizing again that a change in a particular 

 region of a molecule may produce variations in the electronic configurations 

 throughout the entire molecule, and that a change in the interaction energy 

 cannot generally be attributed solely to this altered region. Furthermore, 

 the volume and configuration of a substrate and its analog may involve 

 water of hydration, so that a group change can secondarily affect the bind- 

 ing by modifying the disposition of the bound water molecules. The in- 

 troduction of so-called neutral groups, such as hydrocarbon chains, can 

 bring about an increase in the binding energy through nonspecific van der 

 Waals' interactions, providing these groups do not interfere sterically with 

 the approach of the important binding groups to the enzyme surface. 



Most analogs of substrates have less affinity for the enzymes than do the 

 natural substrates, which is reasonable in view of the enzyme active center 

 conformation to the substrate configuration. However, occasionally an 

 analog will exhibit a much tighter binding than the substrate, the extra 

 binding energy being more than could be attributable simply to a new group 

 introduced into the molecule. In such cases it is likely that a qualitative 

 change in the binding is involved. A substrate frequently forms a covalent 

 bond with the enzyme during the catalytic reaction, and normally this 

 constitutes only an intermediate state in the sequence of changes. Certain 

 analogs may be able to form this type of bond but are unable to complete 

 the sequence, so that the analogs remain firmly attached to the enzyme. 



