262 



ENZYMES 



\ 



Courtesy of Drs, M. Kunitz and J. H. Northrop and The Journal of General 



Physiology. 



Fig. 10-3. Trypsin. 



consists of a specific protein and a prosthetic group, as in the case of 

 lactic dehydrogenase, the protein part is called the apoenzyme, and the 

 prosthetic group is called the coenzyme of the complete enzyme. See 

 Table 10-1 for examples of other coenzymes. 



Classification 



Enzymes have been named on the basis of occurrence {e.g., pepsin), 

 the substance (substrate) upon which they act, the products formed by 

 their action, the nature of the linkages broken, or a particularly char- 

 acteristic type of reaction they may perform [e.g., oxidases). For ex- 

 ample, the enzyme which catalyzes the hydrolysis of sucrose has three 

 names; (1) sucrase, a name derived from the substrate, (2) invertase, so 

 named because the hydrolysis product, an equimolar mixture of glucose 

 and fructose, is called invert sugar, and (3) a-glucopyrano-^-fructofuran- 

 osidase, a term that indicates the type of linkage broken by the enzyme. 

 The variety in enzyme terminology will be apparent upon examination 

 of Table 10-1. Since enzymes catalyze such an enormous variety of 

 reactions, it is difficult to catalogue them in an exact manner. The classi- 

 fication used in Table 10-1 is empirical, but will serve as a guide to the 

 student for- the organization of the material. Like all classifications it 

 is imperfect and subject to change with advancing knowledge. 



