194 6. INTERACTIONS OF INHIBITOES WITH ENZYMES 



sideration will first be given to the structures and properties of active cen- 

 ters; the various types of intermolecular interactions will then be discussed; 

 the application of such interactions to biologically important substances 

 and groups will be outlined; finally, the role of these interactions in enzyme 

 inhibition will be presented. 



THE NATURE OF ENZYME ACTIVE CENTERS 



Accurate visualization of the interaction between an inhibitor molecule 

 and an enzyme depends in part upon a spatial concept of the properties of 

 the surface of the enzyme and especially some idea of the topography of 

 the active centers where catalysis occurs. The active region should, how- 

 ever, not be thought of solely in terms of molecular configuration; beyond 

 the electronic contours of the groups there are complex and often strong 

 electrical fields emanating from the atoms and ions composing the active 

 center. The interaction and orientation of an inhibitor molecule with res- 

 pect to an enzyme are primarily dependent on the field pattern. 



Composition of the Active Center 



The active center comprises that region of the enzyme surface where the 

 reactants are bound, interact, and are chemically altered. It may be made 

 up of several specific sites where the individual components of the reaction 

 are bound, e.g., substrate site, coenzyme site, and activator site. An active 

 site may be very small and simple (perhaps a single group) or it may ex- 

 tend over a molecularly large region of the enzyme and itself contain many 

 interacting groups and multiple binding points. There are basically three 

 types of active site: (a) polypeptide chains only, (6) a complex of a metal 

 ion with protein, and (c) a nonpeptide prosthetic group bound firmly 

 to the protein. Enzymic catalysis is most intimately related to the poly- 

 peptide composition of the active center. 



The active region is usually only a small fraction of the total enzyme 

 protein and it has been stated that the remainder of the protein is without 

 importance in the catalysis. Indeed the irradiation of trypsin led Augen- 

 stine (1959) to conclude that the active surface area was 2.5% of the total 

 area. Since the total area is about 4,050 A^ (on the basis of a molecular 

 weight of 20,000), the active center would be approximately 100 A^ in 

 area, which would implicate four surface amino acids. However, in living 

 cells it is likely that few enzymes are free in solution and it is possible that 

 much of the protein is involved in the organization of the enzymes into 

 functionally efficient structures; the noncatalytic portion of the enzyme 

 could thus be important in the operation of more complex metabolic se- 

 quences or cycles. Furthermore, those regions of the enzyme surrounding 



