DEFINITION OF ENZYMES 73 



Enzymes act by catalysis and hence are often stated to be " select- 

 ive colloidal catalysts, present in living cells and destroyed by 

 heat." A catalyzer is "a substance which alters the velocity of 

 a chemical reaction without undergoing any apparent physical or 

 chemical change itself and without becoming a part of the product 

 formed." It is a well-known fact that the speed of many chemical 

 reactions is accelerated by catalyzers; for example, the inversion of 

 cane sugar by acid and the numerous reactions affected by platinum. 

 Negative catalysis is not as common, but the stopping of theslow 

 oxidation of phosphorus in air by a trace of ether vapor may be 

 taken as an example. The general characteristics of catalysts are 

 admirably illustrated by Bayliss: 



"There are certain phenomena which, at first sight, might be 

 confused with those of catalysis, but which must be carefully dis- 

 tinguished from them. A mechanical model will serve to make 

 this clear. If a brass weight of, say 500 grams, be placed at the 

 top of an inclined plane of polished plate-glass, it will be possible 

 to find a slope of the plane such that the weight will slowly slide 

 down. This represents any reaction taking time to complete. 

 If now the bottom of the weight be oiled (oil-catalyst) the rate of 

 its fall will be greatly increased. We see, that in either case, the 

 weight if placed at the top of the plane does not remain there, but 

 sooner or later reaches the bottom. It may, however, be kept at 

 the top by some kind of catch or trigger arrangement, in which case 

 it will remain there indefinitely until the catch is released. The 

 amount of energy lost by the weight in its fall, being the product 

 of its weight and the vertical height from which it has fallen, is in 

 no way affected by the work required to remove the obstacle pre- 

 venting its fall, nor is the rate at which it falls when set free. A 

 typical instance of such a 'trigger' action is that of supersaturated 

 solutions, which remain for any length of time unchanged unless 

 infected with a crystal. It has, moreover, been shown by B. 

 Moore (1893) that the rate at which the solidification of supercooled 

 glacial acetic acid moves along a tube is independent of the quantity 

 of crystals placed at one end to start the process. Not so with 

 true catalytic action; although the work done by our sliding weight 

 is in no way affected by the amount of catalyst (oil) used, the rate 

 of the fall is, within limits, directly proportional to it, and this is a 

 property of catalysts in general. 



"It cannot be expected that a rough model of this kind would 

 show all of the characteristics of catalytic phenomena, but there 

 are two instructive points shown by it in addition to those already 

 spoken of. The first is the disappearance of the catalyst by stick- 

 ing to the glass as the weight slides down. An analogous phe- 

 nomenon is often met with in catalytic processes, as will be seen 

 later. The second point is one of importance with regard to 



