GENERAL CHARACTERISTICS OF THE ENZYMES 203 



if the term "velocity" is doubled in magnitude, while (a x) remains 

 unaltered, then k must have been doubled in magnitude. When the 

 percentage of effective collisions is doubled, therefore, k is doubled, 

 and so forth. 



We have hitherto not considered the part which the Acid plays in 

 bringing ahout the hydrolysis. Neutral water only hydrolyses sugar 

 extremely slowly, so slowly that the velocity is negligible in comparison 

 with the velocity of hydrolysis in acid solutions. At the completion of 

 hydrolysis the acid is unaltered and is available for bringing about 

 further and, apparently, unlimited hydrolysis. The acid does not, 

 therefore, communicate any energy to the system; it merely increases 

 the Percentage of Effective Collisions of molecules of sugar with mole- 

 cules of water. Such an action is termed a Catalytic Action, and the 

 agent which brings about the acceleration, in this instance the acid, 

 is termed a Catalyzer or Catalyst. The Mechanism of Catalysis is, in 

 this instance, not perfectly clear, but judging from the analogy afforded 

 by the mode of action of many other catalysts, we may conclude that 

 a compound of the disaccharide with acid is formed and that it is this 

 compound which actually undergoes hydrolysis. In many cases of 

 catalysis such compounds of the catalyst with the substance undergoing 

 decomposition, or Substrate, have been isolated and identified, so that 

 we feel justified in assuming that if such compounds are not readily 

 detectable in an instance of catalysis such as that afforded by the 

 hydrolysis of cane-sugar by acids, the reason is that only a minute 

 trace of the compound of the catalyst and the substrate is present in 

 the mixture at any moment 



The quantity of the compound of the substrate with the catalyst 

 which is present at any moment in the mixture, must, however small, 

 be proportional to the concentration of the catalyzer and also to the 

 concentration of the substrate, for if two substances A and B combine 

 to form a third AB the quantity of this compound formed must be 

 determined, as the Guldberg and Waage Mass-law requires, by the 

 equation : 



Mass of A X mass of B = constant X mass of AB 



If, then, the concentration (=mass per unit- volume) of the catalyzer 

 be kept constant, the quantity of the substrate-catalyzer compound, 

 in this instance the compound of cane-sugar with acid, must be directly 

 proportional to the concentration of the still unaltered cane-sugar and 

 decrease as it decreases. Since, for all practical purposes of measure- 

 ment, it is only the molecules of sugar-acid compound which are under- 

 going hydrolysis it follows that the velocity of hydrolysis must, if the 

 same proportion of these compound molecules is decomposed in each 

 instant, fall off in direct proportion to the concentration of still un- 

 altered sugar, or in other words the equation : 



Velocity of hydrolysis = k(a x) 



