194 CHEMISTRY OF PLANT LIFE 



THE NATURE OF ENZYME ACTION 



The mechanism by which an enzyme accomplishes its catalytic 

 effects has been the object of extensive studies during recent years, 

 especially since the discovery by Biichner that enzymes could be 

 isolated in solutions entirely free from the disturbing influence of 

 growing cells. Several theories concerning the mode of this 

 catalytic action have been advanced. The earliest and simplest 

 of these was that the enzyme simply creates an environment favor- 

 able for the particular chemical reaction to take place, as by 

 exposing large surfaces of the substance in question to the action 

 of the hydrolytic, or other effective, agent, by means of surface 

 adsorption of the substrate material on the colloidal enzyme. 



However, more recent investigations clearly indicate that there 

 is an actual combination between the substrate material and the 

 enzyme, which combination then breaks down with a resultant 

 change in the substrate material and a freeing of the enzyme 

 for repeated recombination with additional substrate, with the 

 net result that the chemical change in the substrate material is 

 enormously accelerated. That such a combination between sub- 

 strate and enzyme actually exists has been demonstrated in two 

 different ways: (a) experimentally, by mixing together solutions 

 of an enzyme and of its substrate, each of which is filterable 

 through paper or through a porous clay filter, with the result that 

 the active material in the combined solutions will not pass through 

 these same filters; and (6) mathematically, by a study of the curves 

 representing the reaction velocities of typical reactions which are 

 proceeding under the influence of an enzyme, which show that so 

 long as there is a large excess of substrate material present, the 

 accelerating influence of the catalyst is uniform over given suc- 

 cessive periods of time, but that when the quantity of substrate 

 material becomes smaller than that which permits the maximum 

 combining power of the enzyme to be exercised, the reaction 

 velocity immediately slows up. 



Again, the fact that the specificity of the action of an enzyme, 

 i.e., the limitation of the action of that enzyme to a specific single 

 compound or group of similar compounds, is definitely related to 

 the molecular configuration of the molecule of the substrate, as has 

 been found to be true in all those cases where the molecular con- 

 figuration of the substrate material has been established (see 



