

CATALYSIS AND ENZYMES 309 



that there are new ones continually being discovered. How far many of these 

 new ones are really such, and not new capabilities of old enzymes, may sometimes 

 be a matter of doubt. In the following list, the name of the appropriate 

 substrate is placed in brackets after that of the enzyme : amylase (starch), 

 maltase (maltose and a-glucosides), emulsin (/3-glucosides), pepsin (proteins in 

 acid medium), trypsin (proteins in alkaline medium), urease (urea), arginase 

 (arginine), lipase (esters), peroxidase (organic peroxides, including that of hydrogen), 

 and so on. We may also divide enzymes into classes according to the nature 

 of the chemical change accelerated. The majority add or remove the elements 

 of water, and may be called hydrolysiny from the one aspect of their activity. 

 All of those mentioned above, with the exception of peroxidase, belong to this 

 large class. Those that cause activation of oxygen or of hydrogen, bringing about 

 oxidations and reductions, will be dealt with in Chapter XX. There is another 

 class which appear simply to break up a complex molecule, although it is probable 

 that this is done by a series of changes, involving oxidation, reduction, and 

 hydrolysis, as in the case of the zymase system of yeast, converting glucose into 

 alcohol and carbon dioxide 



C a H 12 O tf = 2C. 2 H 6 



Whether an enzyme merely accelerates a spontaneous reaction it is impossible 

 to state as a general rule. But there are certainly some reactions which proceed 

 slowly by themselves and are accelerated by enzymes : the esters in water 

 may be mentioned. In other cases, the change, rapid under the action of an 

 enzyme, is, at ordinary temperatures, too slow to be detected, although it can 

 be made to proceed at a measurable rate by raising the temperature ; the hydro- 

 lysis of cane-sugar and of salicin by water are cases in point. When a reaction 

 can be made obvious by heat, it is justifiable to conclude that it is not entirely 

 absent at ordinary temperatures. 



In such cases of solutions in water, the question arises as to whether the spontaneous 

 change might be due to the catalytic action of the small quantity of hydrogen and hydroxyl 

 ions always present. Consideration will show, however, that, even when a reaction is 

 proceeding under the influence of one catalyst, if it is further accelerated by another 

 substance, this second is no less an additional catalyst ; except in those cases where the 

 second acts by increasing the activity of the first, and is inactive alone. 



A more important point is the question of the relation of the enzyme to 

 the fatal products. In some cases the enzyme has been recovered at the end 

 of the reaction unchanged, as the acid in ester reactions. In other cases it 

 disappears, partially or entirely. This disappearance, however, is found to 

 be due to the instability of the enzyme itself. That it does not form a component 

 of the final equilibrium is shown by the numerous experiments in which it has 

 been found that the total amount of change is independent of the amount 

 of enzyme added, which would be impossible in the other case. A series of 

 curves illustrating this fact will be found in my monograph (1913, 2), which 

 shows also how the rate of the change depends on the amount of the catalyst. 

 Another case, using the synthetic aspect of the action of emulsin, is given in 

 Fig. 84 below. 



If the enzyme formed a component of the final equilibrium, the position of this 

 equilibrium would be altered by mass action if more enzyme were added after 

 its attainment. This, in point of fact, does not happen (Bayliss, 1913, 1, p. 246). 



Certain views as to the attainment of what has been called a "false equilibrium," in which 

 the final result appears to be in proportion to the concentration of the enzyme, will be found 

 discussed in a paper by myself (1913, 1). It will suffice to say here that careful examination 

 of the experimental facts shows that they do not compel us to make an assumption of this 

 kind, and are, for the most part, to be accounted for by destruction of the enzyme before it 



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has had time to carry the reaction as far as the equilibrium position. Naturally, the nicm- 

 enzyme is present at first, the faster the reaction proceeds ; and, moreover, it will be 

 longer before the whole of the enzyme has disappeared. 



The fact that the position of equilibrium is found to be the same whether 

 we start from the system consisting only of substrate or only of products, 

 is again of considerable importance as regards the proof that we are dealing 



