28 2. THE KINETICS OF ENZYME REACTIONS 



interactions generally: electrostatic attraction between ionic groups, dipole 

 interactions, forces arising from polarization in local electric fields, disper- 

 sion forces, covalent bonds, and hydrogen bonds. Any one or a com- 

 bination of these may be involved in the total interaction energy. The 

 binding of inhibitors is quite comparable and a more detailed discussion 

 of these interaction forces will be presented in Chapter 6 in connection with 

 inhibition. 



It is probable that most substrates are bound to the active center by 

 two or more interaction loci or attachment points; i.e., several regions on 

 the substrate molecule interact with the enzyme. This is attested to by the 

 high specificity of most enzymes and is necessary in order that the substrate 

 assume the proper configuration for reaction. In some cases it would appear 

 that the entire substrate molecule, or at least one side of it, fits fairly ac- 

 curately a complementary configuration on the enzyme, since the alteration 

 of any group on the molecule changes the binding characteristics. The sit- 

 uation here is probably not too different from the reaction between antigen, 

 or hapten, and antibody, where conformity to within 1-2 A is requisite for 

 maximal interaction. This multipoint attachment of substrate increases 

 the possibilities of interference by inhiljitors, since the blocking or modifi- 

 cation of only one group out of several may be sufficient to reduce the reac- 

 tion rate significantly or totally. 



The problem of activation arises because it is generally stated that the 

 substrate is activated upon being bound. The term activation has several 

 meanings; it would be more satisfactory if its use were restricted to those 

 states of high energy level and temporary abnormal electron configuration 

 implied in transition state kinetics. If this view is maintained, it is evident 

 that the usual ES complex does not represent an activated state, since 

 its energy level is generally lower (it is really a more stable state of the 

 enzyme and substrate considered thermodynamically) and its atomic and 

 electronic configuration is normal (its structure may be defined in terms of 

 ordinary interatomic distances and electron positions). The ES complex, 

 however, does pass through a truly activated state EX* in the course of the 

 reaction, and sometimes perhaps through a series of such activated states 

 (Fig. 2-5). In some reactions the products may be formed unbound to the 

 enzyme from the transformation of the EX* complex, but in other cases 

 the products form complexes with the enzyme that are comparable to the 

 ES complexes. 



Despite the fact that the substrate molecule may not be truly activated 

 when bound in the ES complex, it is obvious that it is in a different state 

 from that free in solution, inasmuch as binding implies forces that must 

 alter the electronic configuration and over-all structure to some extent, 

 increasing the probaljility of reaction with another molecule. It might be 

 said that binding increases the probability of activation. There is also the 



