ENZYMES 147 



TYPICAL ENZYMES 



The number of known enzymes is very large and still increasing 

 rapidly. For both reasons a complete listing of enzymes is impractical. 

 However, a few of the more widely studied representatives of the six 

 major groups of the classification system have been chosen as typical 

 examples, beginning with the various types of hydrolytic enzymes. 

 Many of the enzymes mentioned below will be discussed again in later 

 chapters. 



Proteases 



These enzymes catalyze the partial hydrolysis of proteins, peptones, 

 and peptides; in fact, almost anything containing the required specific 

 peptide bond. However, the rates of action are generally much higher 

 for proteins than for small peptides. The proteases, also called pro- 

 teinases or proteolytic enzymes, probably occur in almost all cells. 

 They are especially abundant in those plant and animal tissues which 

 hydrolyze proteins extensively. Each protease is characterized by 

 specificity toward particular peptide bonds. Most are rather specific, 

 hydrolyzing only a few types of peptide linkages. 



Pepsin and rennin are gastric enzymes functioning at the relatively 

 low pH of the stomach. Rennin is found in the stomachs of young 

 animals like the calf and the lamb, where it clots milk and starts the 

 hydrolysis of milk proteins. As the animal grows older the rennin 

 diminishes and is replaced by pepsin. This latter enzyme is secreted 

 in the form of the precursor pepsinogen, which is converted to active 

 pepsin in the presence of acid. Pepsin plays a role in digestion in 

 birds and mammals by specifically attacking peptide bonds of the type 



— NHCHCO— NHCHCO— 



I t 1 



R I Ar 



where R signifies the side chain of any of the usual amino acids but 

 Ar represents only aromatic groups. Bonds involving the carboxyl 

 groups of aromatic amino acids are not attacked unless attached to 

 other aromatic amino acids. 



Trypsin and chymotrypsin are secreted by the pancreas in the form 

 of inactive precursors which are both activated catalytically by trypsin 

 itself. These and related enzymes function best in neutral or slightly 

 alkaline solutions. Trypsin acts on the peptide bonds involving the 



