ENZYMES 129 



perhaps into two. For example, pepsin catalyzes the partial hydrolysis 

 of proteins. It is called a protease or proteolytic enzyme or more gen- 

 erally a hydrolase, a class including all enzymes catalyzing hydrolyses. 

 Pepsin also splits certain peptides and thus could be called a pep- 

 tidase. Thus it becomes apparent that the classification of enzymes is 

 not completel) standardized and may be adapted to the needs of the 

 user. 



In enzyme chemistry one of the reactants is always of particular in- 

 terest for reasons of structure, occurrence, or ultimate biological utili- 

 zation. This reactant is normally referred to as the substrate of the 

 enzyme in (}uestion and is a principal focus of attention. Again in 

 the case of pepsin, a protein and water react during the enzymatic 

 process. The water is very abundant in cells, of course, and is of less 

 specific interest in work on this reaction system than is the protein. 

 Hence the protein is known as the substrate of pepsin, although the 

 water is clearly a fundamental necessity in the reaction. 



The general theory of catalysis states that catalysts merely alter 

 reaction rates but never initiate reactions that would not occur in the 

 absence of the catalyst. In practice this concept is difficult to prove or 

 even apply because many reactions go so slowly that products cannot 

 be detected until a catalyst is added. The reaction 



2H2 + 02-^ 2H2O + energy 



is theoretically a spontaneous one with its equilibrium position at 

 room temperature far in the direction of water. Yet oxygen and hy- 

 drogen apparently can exist together indefinitely until a catalyst is 

 introduced. In cases of this kind including a number of biological 

 reactions, it is difficult to decide whether a catalyst initiated a reac- 

 tion or merely accelerated it enormously. Possibly concern over this 

 problem is akin to tilting with windmills, but only time and knowl- 

 edge can decide. 



On the other hand, there are many catalyzed reactions that do 

 proceed at slow but spontaneous rates. In these examples, there can 

 be no question of initiation of reaction since the instant reactants are 

 combined the process starts. Perhaps the catalyst could initiate the 

 reaction, but it never does if the reactants are mixed before the catalyst 

 is added. When one reactant is combined with the catalyst, addition 

 of the second reactant starts the process. Since reaction would begin 

 without the catalyst, it seems impossible to attribute any triggering 

 action to the catalyst itself. 



As mentioned on page 113, enzymes are commonly denoted by the 

 suffix ase except for a few long-established names for some of the hy- 



