ENZYMES 131 



Enzymes are commonly assigned to one of a series of groups based 

 on the type of reaction concerned. The number and definitions of 

 these groups vary with the interests of the authors concerned. The 

 subdivision of the major groups is based on the relationships of the 

 substrates involved. The scheme of Table 6-1 represents one classifica- 

 tion of enzymes. 



A number of these classes contain many different individual enzymes 

 and may be further subdivided. The oxidases, for example, may be 

 grouped into the oxidases containing iron, those containing copper, 

 and those lacking an attached metal. This procedure introduces a 

 new type of classification, according to the nature of the special at- 

 tached or associated structures. Thus, certain of the oxidases are 

 metal enzymes, specifically either iron or copper enzymes. 



In the above cases the metal is rather firmly bound to the protein 

 and is not removed by dialysis under physiological conditions. Groups 

 thus firmly bound are called prosthetic groups. Besides metals there 

 are other types of prosthetic groups, including phosphoric esters and 

 complex organic structures called porphyrins, usually associated with 

 iron in enzymes. In fact all the non-amino acid components of the 

 conjugated or complex proteins may be termed prosthetic groups, al- 

 though not all types are represented among the enzymes themselves. 



The same or similar prosthetic groups are attached to quite differ- 

 ent protein components or moieties. Since the resulting enzymes differ 

 in their activities, the catalytic properties of an enzyme of this type 

 depend on both the prosthetic group and the protein moiety. 



In other enzyme systems associated rather than attached structures 

 are important for catalytic action. These associated materials are 

 rather readily separated from the enzyme themselves. They are called 

 cofactors and are discussed more extensively on page 139. 



Catalytic Action 



Enzymes are distinguished from other proteins by their catalytic 

 action, and this property is the most important one enzymes have. 

 The diversity of reactions catalyzed is known to be great (see page 

 130), and our knowledge is still increasing. Since enzymes function 

 biologically, most of the reactions involve organic compounds, at least 

 in part. A few enzymes catalyze the reaction of inorganic substrates, 

 as in the following: 



2H2O2 -^ 2H2O + O2 

 2H + H3O+ + S04= -^ HSO3- + 2H2O 

 2H + NO3- -^ NO2- + H2O 



