Enzymes: How Cells Promote Chemical Activities - 1 03 

 Table 5-1 (continued) 



General Types; 

 Specific Names 



Catalytic Action 



Comment 



5. Aminopeptidase 



Another exopeptidase; 

 specifically splits off amino 

 acids at end of chain, if 

 they possess an "exposed" 

 amino ( — NH 2 ) group 



Present in the intestinal 

 juice; trypsin, chymotryp- 

 sin, carboxypeptidase, and 

 amino peptidase have been 

 used to determine the 

 order of different amino 

 acids in the peptide chains 

 of various proteins (Fig. 

 4-16) 



Carbohydrases 



1. Salivary and 

 pancreatic 

 amylases 



2. Maltase 



3. Sucrase 



4. Lactase 



1 . Hydrolysis of polysaccha- 

 rides (starches and glyco- 

 gens) down to the disac- 

 charide (maltose) stage 



2,3, and 4. Each hydrolyzes 

 its own specific disaccha- 

 ride sugar, liberating ap- 

 propriate monosaccharides 



1. Present in saliva and pan- 

 creatic juices respectively, 

 require neutral and alka- 

 line media, respectively 



2. Liberates 2 molecules of 

 glucose 



3. Frees 1 mol glucose -f- 1 

 mol fructose 



4. Yields 1 mol glucose -\- 1 

 mol galactose 



Lipases 



1. Gastric lipase 



1,2. Hydrolysis of fats, yield- 

 ing glycerol and fatty acids 



2. Pancreatic lipase 

 (Steapsin) 



A relatively weak lipase; 

 is acid stable, however, 

 and may continue to act 

 in alkaline chyme, follow- 

 ing evacuation from stom- 

 ach (p. 310). 



A potent lipase in pan- 

 creatic juice; requires alka- 

 line medium 



CHEMICAL NATURE OF ENZYMES 



More than 300 enzymes, from a wide 

 variety of cells and tissues, have now been 

 obtained in pure crystalline form (Fig. 5-1). 

 In every case an essential part of each enzyme 

 has proved to be of protein composition. 

 Moreover, all crude (that is, unpurified) 

 enzyme extracts display protein character- 

 istics. They tend to become denatured (p. 

 88) and inactive when exposed to: (1) high 



temperature (above 40° C, typically); (2) rela- 

 tively high or low pH conditions; or (3) 

 changes in the ionic composition of the 

 medium. In short, the maintenance of cata- 

 lytic activity in an enzyme preparation ap- 

 pears to depend upon the maintenance of the 

 structural integrity of the protein compo- 

 nent, especially with reference to the second- 

 ary and tertiary aspects of such structure (p. 

 87). 



It is now generally accepted that every 



