148 GENERAL BIOCHEMISTRY 



carboxyl groups of lysine or arginine. Chymotrypsin splits the peptide 

 bonds at the carboxyl end of the aromatic amino acids. Both have 

 hydrolytic activity for methyl and ethyl esters of peptides, although the 

 biological importance of this action is unsettled. Insectivorous plants 

 have related enzymes. 



Papain from papaya, ficin from fig, and bromelin from pineapple 

 are typical of the papainase group of proteolytic enzymes from plants. 

 Many require free — SH groups for activity and are inactivated by 

 heavy metals or oxidation. These enzymes are widely distributed in 

 fruits, microorganisms, and seeds. Occasionally they are encountered 

 in the digestion of gelatin, thus preventing the setting of gelatin 

 desserts. The papainases have a wide range of specificity, attacking 

 many different peptide bonds. 



Most of the preceding proteases are exocellular enzymes and func- 

 tion after secretion by the cell. Another important group, though 

 not well known, called cathepsins, act inside cells. These enzymes 

 are thought to be inactive while the cells live. When a cell dies its 

 pH drops, activating the cathepsins, which hydrolyze the cellular 

 proteins as a part of the change resulting from death. 



Esterases and Lipases 



These enzymes catalyze the hydrolysis of esters and lipides into 

 the individual acids and alcohols. They are very widely distributed 

 in both plants and animals, but much less is known of most of them 

 than of the proteases. 



Pectase or pectinase attacks pectin, and tannase attacks the complex 

 tannins. Both enzymes are known only from plants. Choline esterase 

 from animals is important in the conduction of nerve impulses 

 hydrolyzing acetylcholine to choline and acetic acid. 



CH3COOCH2CH2N+(CH3)3 + H2O ^^^ 



esterase 



CH3COOH + HOCH2CH2N+(CH3)3 



The enzyme is biologically so important to higher animals that some 

 of its inhibitors are chemical warfare agents called the nerve gases. 



Lipases act on the lipides of both plants and animals. These 

 enzymes yield the separate components of both fats and oils for further 

 metabolic transformations. Pancreatic lipase (steapsin) is fairly typi- 

 cal, playing an important role in animal digestive tracts. It is 

 activated by calcium ions. Since the substrates are ordinarily not 

 water soluble, the enzymatic action is markedly accelerated by emul- 

 sifying agents which help disperse the non-polar substrate molecules. 



