8 ENZYMES 



2. The action of electrolytes. — All enzymes are very sensi- 

 tive to the reaction of the medium in which they work. 

 There is for every enzyme a certain H-ion concentration 

 in which it displays a maximum activity. This is readily 

 understood when we consider the effect of electrolytes 

 upon the colloid particles. Agglomeration of particles 

 must lead to diminution of surface upon which adsorption 

 can take place. 



3. Specificity. — This is a characteristic feature of enzymes. 

 Each enzyme brings about only one kind of reaction, and 

 acts either upon only one particular substance or only one 

 class of substances. Enzymes are indeed commonly named 

 after the bodies upon which they act. There are the proteo- 

 lytic enzymes, which hydrolyse proteins ; lactase, which 

 acts upon lactose; arginase, which hydrolyses arginine. 

 In this respect enzymes differ from inorganic catalysts 

 only in degree. The specificity of enzymes is not absolute, 

 as was once supposed. Further, specificity is found among 

 inorganic catalysts, although to a far less extent. 



The high specificity of enzymes is beUeved by some to 

 depend upon a close structural resemblance between enzyme 

 and substrate, these fitting hke lock and key. The view 

 is also held that an enzyme consists of two parts — an active 

 principle related structurally to the substrate, and a non- 

 specific colloid which merely serves to provide a surface 

 upon which the active principle can come into contact 

 with the substrate. 



4. Reversibility of Action. — When a reaction is reversible, 

 an inorganic catalyst which quickens it in one direction 

 quickens it in the other to the same extent. The catalyst, 

 therefore, does not influence the equihbrium point. For 

 instance, in the reaction — 



Ethyl acetate + water '^ ethyl alcohol + acetic acid, 



the equiUbrium-point is the same whatever the amount 

 of the catalyst HCl present. It depends only upon the 

 relative velocity of the two reactions, that is to say, upon 



