ENZYMES 149 



The natural dispersing agents involved are called bile salts and are 

 secreted by the liver. 



Carbohydrases 



This group includes those enzymes hydrolyzing the various glycosidic 

 linkages between sugar units or between sugar and alcohol. The 

 bonds affected are of the acetal type and are split during digestive 

 processes. Carbohydrases are universally distributed and usually 

 possess relatively sharp specificities. For example, some attack only 

 acetals having the (3 configuration of a particular monosaccharide sub- 

 unit in the substrate molecule. 



The amylases attack the 1,4-a-glucosidic linkages of starch and 

 glycogen. a-Amylase splits bonds near the middle of the amylose or 

 amylopectin molecules, yielding mainly large fragments, dextrins, but 

 liquefying starch gels in the process. j8-Amylase hydrolyzes the same 

 bonds but works from the ends of the chains inward, breaking off 

 maltose segments one after another. This enzyme splits amylose 

 completely but does not proceed past the branches in amylopectin. 

 The 1,6-a-glucosidic linkages of these branches prevent action past 

 that point in the amylopectin residue. Further digestion requires the 

 activity of another enzyme, 1,6-a-glucosidase, in removing the branches. 



Cellulase hydrolyzes the 1 ,4-/3-glucoside bonds of cellulose to form 

 a disaccharide, cellobiose, which is then split to glucose by cellobiase. 

 These two enzymes, cellulase and cellobiase, are important in the 

 digestion of cellulose by micro-organisms. Cellulase is not synthesized 

 by animals which, therefore, cannot utilize cellulose directly. Rumi- 

 nants and horses, for example, possess bacterial flora in the rumen and 

 intestine, respectively, capable of degrading cellulose into fragments 

 digestible by these animals. 



Maltase and emulsin are representative of the enzymes specific for 

 small carbohydrates. Maltase (a-glucosidase) cleaves maltose and a 

 variety of substances that are acetals of a-D-glucose, including a-methyl 

 glucoside. Emulsin (/3-glucosidase) requires acetals of the /? configura- 

 tion. In this case, sugars other than glucose exist in susceptible sub- 

 strates. 



Phosphatases 



These enzymes hydrolyze either esters or anhydrides of phosphoric 

 acid. The types of substrates include monoesters like glucose-6-phos- 

 phatc, dicsters of phosphoric acid like lecithins and nucleic acids, and 

 anhydrides like acetyl phosphate, pyrophosphate, and adenosine 

 triphosphate (structure on page 141). 



