466 DIGESTION 



3. Specificity. — Each enz3-me is specific in the sense that it can operate only 

 upon a certain substrate or group of substrates. This does not mean that 

 there must be a separate enzyme for every substrate, but that each enzyme 

 acts only upon substances having a certain molecular configuration. Ap- 

 parently each individual enzyme can accomplish the dissolution of one par- 

 ticular type of chemical bond. When a number of different compounds 

 possess this bond in common they can be acted upon by the same enzyme. 

 For example the enzyme emulsin can hydrolyze any jS-glycoside since the 

 chemical linkage between the sugar and non-sugar groups in all such glyco- 

 sides is the same. 



Enzyme specificity is also illustrated by the fact that different end products 

 are produced from the same substrate under the influence of different enzymes. 

 In the presence of sucrase, for example, the trisaccharide raffinose is hydro- 

 lyzed into melibiose and fructose, while in the presence of emulsin the end 

 products of the reaction are sucrose and galactose. Evidently the chemical 

 linkage at which the raffinose is split is different when sucrase is the catalyst 

 than when the reaction proceeds under the influence of emulsin. 



4. Reversibility of Action. — It has been experimentally shown that a num- 

 ber of hydrolytic enzymes catalyze the same reaction in both directions. The 

 direction which the reaction will go depends upon the relative concentration 

 of end products and substrate, and upon other conditions influencing its 

 equilibrium point. The synthesis of fats from glycerol and fatty acids under 

 the influence of the enzyme lipase has been accomplished experimentally 

 (Chap. XXIII). Similarly, it has been shown that under proper conditions 

 other enzymes such as some of the proteinases and urease can synthesize the 

 compounds which under other conditions serve as their substrates. Glyco- 

 sides have also been synthesized from the end products of their hydrolysis 

 under the influence of the enzyme emulsin. Hence it is often inferred that 

 all hydrolytic enzymes possess the capacity of catalyzing not only the hydrol- 

 ysis of a certain substrate, but also, under the proper conditions, the syn- 

 thesis of that same substrate from the end products of its hydrolysis. 



5. Heat Sensitivity. — Unlike most inorganic catalysts enzymes are in- 

 activated or destroyed at temperatures considerably below the boiling point 

 of water. At temperatures above 50° C. most enzymes in a liquid medium 

 are rapidly inactivated. Inactivation, although at a slower rate, may also 

 occur at somewhat lower temperatures. Most enzymes in a liquid medium 

 are completely destroyed at temperatures between 60° and 70° C. A few, 

 however, will endure temperatures as high as 100° C, at least for short 

 periods of time. The destruction of enzymes in this range of temperatures 

 is in all probability a heat coagulation phenomenon. 



