NATURE OF ENZYMES 7 



colloids. When in " solution " in water they exist as 

 particles containing a small amount of water suspended 

 in water. vSome of their properties are, as we shall see, 

 referable to their colloidal nature. There is considerable 

 evidence to show that they act by providing a large surface 

 upon which the molecules of the substrate are adsorbed. 

 The concentration of the substrate thus brought about 

 leads, by the law of mass action, to the acceleration of a 

 reaction which otherwise would take place only at an 

 infinitely slow rate. In favour of the existence of ad sorption 

 compounds as an intermediate stage, is the fact that some- 

 times an enzyme is more resistant to heat when in presence 

 of its substrate. Again, the fact that certain enzymes 

 may function even in a medium in which they are insoluble, 

 is best explained on the assumption that adsorption com- 

 pounds are formed. 



In their surface effects enzymes strongly resemble the 

 metals in a finely divided state. Colloidal platinum effects 

 a rapid combination of hydrogen and oxygen ; colloidal iron 

 greatly accelerates the oxidising action of hydrogen peroxide. 



We now have to consider the factors which influence 

 enzyme action, showing how they lend support to the idea 

 that enzymes are colloidal in structure and catalytic in 

 function. 



1. The effect of temperature. — At 0° C, enzymes are 

 reduced to inactivity, but are not destroyed. As the 

 temperature rises they become more active. This, however, 

 is only one particular instance of the general rule that 

 molecular activity increases with rise of temperature. 

 At a temperature equal to or shghtly above body-tempera- 

 ture, enzymes display their maximum activity. This is 

 the so-called optimum temperature. Beyond this point 

 their activity wanes, owing to their gradual destruction. 



Destruction by heat does not constitute any distinction 

 between enzymes and inorganic catalysts. It is a property 

 of the enzyme, which is shared by some inorganic catalysts 

 of colloidal nature — for instance, colloidal platinum, 



