50 GENERAL PHYSIOLOGY OF BACTERIA 



ically in the sense that they transform more material than the 

 organisms require : this phenomenon is exhibited in the extensive lique- 

 faction of gelatin by proteolytic bacteria, as B. proteus. The organism 

 which elaborates such an exo-enzyme probably derives but little energy 

 from its activity, and; conversely, probably expends comparatively 

 little energy in the elaboration and secretion of the exo-enzyme. 



Endo-enzymes. Comparatively little is known of the endo-enzymes : 

 it is generally believed that they are comparatively non-diffusible, at 

 least in an active state, and that they are non- or but slightly filter- 

 able. This suggests that they are relatively complex in their mole- 

 cular aggregation. Their function is probably to act upon the nutrient 

 substances which the cell has assimilated, partly to liberate energy 

 from them, and partly to participate in the organization of the cell 

 constituents. These endo-enzymes work economically in contra- 

 distinction to the exo-enzymes in the sense that the substrate is appar- 

 ently changed by them in proportion to the requirements of the cell. 

 Endo-enzymes may be obtained from bacterial cells when the latter 

 disintegrate, provided the rupture of the cells is not accomplished by 

 violent chemical means. Probably the phenomena of autolysis which 

 many bacteria exhibit when they are placed in an environment free 

 from food may be due, in part at least, to the autodigestion of the 

 organisms by their endo-enzymes. 



Classification of Enzymes. Enzymes are usually classified according 

 to the substrate they act upon : thus, proteolytic enzymes, or proteases, 

 split proteins or protein derivatives into simpler compounds; carbo- 

 hydrolytic enzymes split starches or polysaccharides into simpler 

 carbohydrates; fat-splitting ferments, lipases, split fats into glycerin 

 and fatty acids. The above enzymes are hydrolytic in character, 

 that is, they effect cleavage of protein or carbohydrate or fat or of 

 glucosides by splitting the molecule into simpler molecules which 

 simultaneously take up hydrogen and oxygen in the proportions to 

 form water, thus: 



i. 



CH 2 NH 2 CO-NHCH 2 COOH + H 2 O (+ enzyme) = CH 2 NH 2 COOH + CH 2 NH 2 COOH. 

 Glycyl-glycine. Glycine. Glycine. 



2. 



Ci2H 22 Ou + H 2 O ( + lactase) = C 6 Hi 2 O 6 + C 6 Hi 2 O. 

 Lactose. Dextrose. Galactose. 



3. 



CH 2 O-CO-CH 3 CH 2 OH 



I | 



CHO-CO-CHs + 3 H 2 O ( + lipase) = CHOH + 3 CHaCOOH 



Acetic acid. 



CH 2 O-CO-CH 3 CH 2 OH 



Triacetin. Glycerin. 



