ENZYMES 137 



in ilic disintegration, calleil autolysis, of dcail cells. Still other systems 

 oi this type seem to play roles in the metamorphosis of insects. 



FACTORS AFFECTING ENZYME ACTIVITY 



The rate of reaction in an enzyme-substrate system depends first 

 of all on the natures of both the reactants and the catalyst. The fun- 

 damental basis of this dependence is certainly chemical, at least in 

 part, but there may be important physical factors like size. Although 

 the problem has been given much consideration, little has been 

 learned. 



Aside from this innate dependency, the nature of the environment 

 in which the system exists has great influence. In this case the experi- 

 mentalist has a better opportunity for understanding. He can alter 

 the environment almost at will and observe the effect produced. 



Solvent 



The solvent is important, first because many enzymes are active 

 only in solution (colloidal dispersion) and dissolve only in water. 

 Others act while in the solid state in the particulates of cells, but the 

 jubstrates are soluble only in water. All enzymes seem to require 

 the presence of water for catalytic activity. Indeed water is either a 

 reactant or a product in a great many enzymatic reactions. The addi- 

 tion of solvents miscible with water may retard the rate of reaction at 

 moderate concentrations but not stop it until the concentration be- 

 comes high. 



Added inert, uncharged solutes have elTects similar to those of the 

 liquids above, slight at low concentration, important at high concen- 

 tration. When salts are added, the resulting changes in ionic strength 

 may modify the reaction rate sharply. Some enzyme systems are quite 

 sensitive to changes in ionic strength, but others are not. Actual 

 behavior depends upon the effect of ionic strength on the structures 

 of the enzyme and substrate and perhaps on the complex between 

 them. Little is known on this point. 



Concentrations of Enzyme and Substrate 



Enzyme-catalyzed reactions may be represented by 



Reactants -j- enzyme "=;■ complex i=; products + enzyme 

 This overall process may involve more than one reactant or substrate 



