CHAPTER V11I 

 ENZYMES OR FERMENTS 



THE chemist recognizes a class of bodies of far-reaching action 

 called catalysts. A catalyst is a substance which by its presence 

 hastens a chemical reaction. A catalyst cannot start a reaction. 

 Chemical equilibrium depends on the laws of chemical dynamics, 

 on the nature of the reacting substances, their active mass or 

 concentration, and external conditions such as temperature, pressure, 

 etc. A catalyst cannot alter the equilibrium of forces, or the final 

 transformation of energy due to a reaction. It can only, so to speak, 

 oil the wheels of the machine. When dry hydrogen and oxygen gas 

 are mixed, they combine to form water, but with such slowness that 

 the reaction 2H 2 + 2 Z2H 2 O escapes observation. Finely divided 

 platinum acts as a catalyst, and enormously accelerates this reaction. 

 The catalysts may accelerate a reversible reaction in either direction. 

 Thus, in the common type of reaction, 



Acid and alcohol ^ ester and water, 



the acid ester in the presence of a great excess df water can almost 

 wholly be split into acid and alcohol, while in strong concentration 

 most of the acid and alcohol goes to form the ester. The same catalyst 

 can accelerate this reaction in either direction according to the condi- 

 tions. 



Enzymes (tv ^fyu/, in yeast) or ferments, are bodies which act like 

 catalysts, and have the power of accelerating the rate of hydrolysis 

 of certain substances or substrates. Probably all the reactions which 

 take place within living cells, or are produced in digests by the 

 secretions of the cells, are accelerated by enzymes. It is the accelera- 

 tion of any reaction which makes it manifest and effectual, for the 

 transformation of energy produced by the reaction is concentrated in 

 a short space of time. It was from the action of yeast that the 

 term " fermentation " arose. The yeast sets the must in a fer- 

 ment; it froths and bubbles. When the action of digestive juices 

 came to be studied, it was seen to be of the same ferment nature, and 

 a distinction came to be made between " organized " and " unor- 

 ganized " ferments. An organized ferment was the living cell, such 

 as yeast, which brought about fermentation by the metabolism 

 involved in its growth and multiplication. The unorganized ferment 

 was contained in the juice secreted from a cell, and acted on a substrate 

 at a distance from that cell e.g., saliva acting on starch in the mouth 

 and stomach. 



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