THE NATURE OF ENZYME ACTIVE CENTERvS 197 



The chymotrypsinogen -> chymotrypsin transformation is similar in that 

 initially a single peptide is removed; in this case there is evidence that 

 there is a rather radical alteration in the polypeptide configuation as a 

 result of this splitting, more of the chain assuming a helical configuration. 

 The bonds split or the peptides removed may be considered as impeding 

 the development of the configuration necessary for the active site (Neurath, 

 1957). One can also anticipate results pertinent to the composition of 

 active sites in the modifications of enzyme synthesis that will soon be 

 possible. 



Finally, one must consider that the active site includes the water mol- 

 ecules that are associated with the protein groups and the atmosphere of 

 ions that surrounds the charged groups on the site. During reaction of a 

 substrate or inhibitor with the active site, water and ions must often be 

 displaced and thus contribute to the final energy of interaction and the 

 kinetics. If they are not displaced they will modify the configuration of 

 the active site and the electrical fields emanating from the protein groups. 



Topography of Active Centers 



Most active sites are regions in which the sequence and characteristic 

 configurations of the polypeptide chain form a pattern of amino acid res- 

 idues that allows a participant in the enzyme reaction to be bound and 

 undergo electronic distortion. These sites may be more or less closely as- 

 sociated with water, specific metal ions, or nonspecific ions of the envi- 

 ronment. The configuration of the site, both spatially and electrically, 

 provides the close fit between substrate and enzyme that is usually neces- 

 sary for activation and confers the high degree of specificity that is exhibited 

 by most enzymes. It is not necessary that the site be complementary to 

 the entire substrate molecule; certain groups or regions only may interact 

 with the enzyme, as is evident from specificity and inhibitor data, and it 

 is the corresponding pattern of groups on the active site that is pertinent. 

 It has been postulated by Pauling (1948) that the active site may not be 

 complementary to the normal state of the substrate but to the substrate 

 in the activated complex, thereby reducing the activation energy and 

 accelerating the reaction. This concept is of particular interest in connec- 

 tion with the interactions of molecules structurally related to the substrate. 

 Furthermore, variations in activity towards various substrates should 

 not be interpreted solely in terms of binding but also with respect to 

 the ability to form the appropriate activated complex; in other words, 

 there may be differences in both the concentration of ES and in the rate 

 constant k^. 



There is evidence that most substrates must be bound to the active site 

 at two or more points for reaction to occur (Levine, 1954 a, b) and that 

 frequently there are two or more groups involved in the displacements 



