ENZYMES 1 27 



There are many known catalysts consisting of large particles, though 

 apparently none of these except proteins function in biological sys- 

 tems. Thus these non-biological particulate or colloidal catalysts are 

 differentiated from enzymes by means of their origins. Ultimately the 

 synthesis of enzymes will be an accomplished fact. Probably synthetic 

 proteins with new kinds of catalytic activities will be made as well. 

 Such presently foreseeable materials will undoubtedly be classed with 

 the natural enzymes because of their structural relationship. This 

 grouping will require a slight change in the definition of enzymes. 



Classification 



As was the case with proteins in general, the classification of enzymes 

 is in a rather unsatisfactory state. Many schemes have been used for 

 diverse purposes, but as might be expected the groups often overlap. 

 Moreover, the properties classified in one system seldom bear much 

 of a relationship to other important properties. Even in this unsatis- 

 factory state, an extensive discussion of enzyme classification is desira- 

 ble because it provides coverage of much of the terminology widely 

 used in enzyme work. 



As relatively typical proteins, enzymes may be classified according 

 to the various systems used for proteins. Whenever source is con- 

 sidered to be of basic importance, it is customary to speak of the 

 enzymes of plants, bacteria, the red blood cell, and mitochondria (par- 

 ticles containing several enzymes and isolated from a variety of cells). 

 Many enzymes contain only amino acids and hence are simple proteins 

 and represent several solubility groups. Others possess additional 

 structures, thus become conjugated proteins, and are subdivided in 

 the same way as the non-catalytic complex proteins. 



Recently enzymes have been classified according to the conditions 

 governing their occurrence in cells. Many enzymes seem to be rather 

 standard equipment in the cells of a given species and have been 

 called constitutive enzymes. Others exist in low concentrations or per- 

 haps do not occur at all in particular environments. Yet when certain 

 critical substances occur in the environment, an enzyme is formed 

 seemingly to meet the need thus arising. These specially formed 

 proteins are known as adaptive or inducible enzymes. Once it was 

 thought that constitutive and inducible enzymes differed in some un- 

 known fundamental manner involving the biological control of their 

 formations. But evidence is appearing that suggests there may be no 

 major differences. Probably this classification will ultimately disap- 

 pear. 



Some workers have studied the biological stability of enzymes under 



