ENZYMES 47 



the presence of a catalyst. The same catalyst will effect synthesis as well 

 as decomposition; the position of equilibrium as well as the relative con- 

 centrations of reactants and products determines which reaction pre- 

 dominates. It is possible to choose conditions, in some instances, so the 

 equilibrium conditions favor synthesis. Bourquelot (1915) demonstrated 

 a-methylglucoside was readily formed from methyl alcohol and glucose in 

 the presence of yeast juice. 



CLASSIFICATION OF ENZYMES 



It is more important to classify enzymes upon the basis of function 

 rather than the site of action (endo- and exoenzymes). Many enzymes 

 catalyze reactions in which water is either a product (synthesis) or a 

 reactant (degradation). These enzymes are called hydrolases. These 

 reactions usually involve only moderate energy changes. Another class 

 of enzymes, usually intracellular, catalyze oxidation and reduction reac- 

 tions and reactions involving the scission (or formation) of carbon-to-car- 

 bon linkages. These enzymes are known as desmolyzing enzymes and 

 include oxidases, dehydrogenases, and desmolases. Energy changes 

 involved in these reactions are usually large. For more detailed classifi- 

 cations of enzymes see Gortner (1949) and Sumner and Somers (1947). 



Since an enzyme acts upon a restricted number of compounds, it is 

 convenient to name enzymes with reference to the substrate acted upon. 

 In general, enzymes are named either by adding the suffix -ase to the 

 name of the substrate or by replacing the final syllable of the name of the 

 substrate by this suffix. The following examples give the substrate fol- 

 lowed by the name of the enzyme: maltose, maltase; lactose, lactase; 

 cellulose, cellulase; starch (amylum), amylase; protein, proteinase; pectin, 

 pectinase. The suffix -ase is also used to designate classes of enzymes. 

 Thus, esterases are members of that group of enzymes which catalyze the 

 hydrolysis and synthesis of esters; oxidases are enzymes which activate 

 oxygen, and dehydrogenases are enzymes which activate the hydrogen of 

 various metabolites. An enzyme may have several names. The enzyme 

 which catalyzes the hydrolysis of sucrose is known also as saccharase and 

 invertase. Amylase is also called diastase. 



Hydrolases. The hydrolases catalyze a wide variety of reactions in 

 which water is either a reactant or a product. Hydrolysis is generally 

 thought of as a process whereby complex molecules react with water to 

 form simpler substances. Many hydrolases are exoenzymes which func- 

 tion by preparing the substrate for assimilation. Among these the follow- 

 ing should be noted : cellulase, amylase, pectinase, various disaccharidases, 

 proteinases, and peptidases. Others are endoenzymes (the same enzymes 

 in some instances), which catalyze the same or similar reactions within the 

 cells. It would be expected that the process of synthesis within the cells 



