470 ENZYMES 



ratio to, or is a constant fraction of, the amount of substance 

 unchanged ; on plotting the amounts changed as ordinates 

 against the time as abscissae there is accordingly obtained 

 what is known as a logarithmic curve. 



In the case of organic catalysts, the rate of the reaction 

 is dependent on the proportional relationship between the 

 enzyme and substrate, provided that the conditioning factors, 

 other than those under consideration, are kept constant and 

 that no disorganization of the reaction, such as autocatalysis 

 and reversibility, takes place. It is not infrequently supposed 

 that there is a union between the enzyme and its substrate ; 

 and since the enzyme does not itself enter into the reaction, 

 its part is that of an accelerator, this union is dissolved when 

 the change in the substrate, hydrolysis, for example, has been 

 effected, and the molecule of enzyme is available to accelerate 

 the hydrolysis of another molecule of substrate. If a fer- 

 mentation be set up by adding a small amount of enzyme to 

 an excess of substrate, the proportionate relationship between 

 them will change with time. In the beginning the substrate 

 will be in excess, midway parity will be reached, and beyond 

 this point the proportion of enzyme to substrate will increase 

 until there is no more substrate to be hydrolysed. 



As regards the rate of the reaction no one law is applicable 

 throughout ; in the earliest phase the rate of the reaction is 

 in linear proportion to the amount of enzyme, i.e. the rate 

 will be doubled if the amount of enzyme be doubled, this also 

 obtains at the later phase where the enzyme is in great excess, 

 but here the substrate must be increased to obtain an increased 

 rate. To mention a few examples : Horace Brown and 

 Glendinning * found that equal amounts of starch were hydro- 

 lysed by diastase in equal times during the earlier part of 

 the reaction, in other words, the course of the reaction was 

 expressed by a straight line ; as the reaction proceeded, how- 

 ever, it became logarithmic, or, in other words, at the com- 

 mencement, when the concentration of the substance being 

 hydrolysed is great as compared with that of the enzyme, the 

 reaction is Hnear and not in accordance with the law of mass 

 * Brown and Glendinning : " J. Chem. Soc, Lond.," 1902, 81, 392. 



