330 PRINCIPLES OF GENERAL PHYSIOLOGY 



They are extremely active, and explain the occurrence in the organism of 

 reactions which require, in the laboratory, powerful reagents anil high temperatures. 

 Lactose is hydrolysed by both hydrochloric acid and by an enzyme, lactase ; but 

 weight for weight, the latter is, at least, five thousand times as powerful as 

 the acid. 



These enzymes are all in the colloidal state, that is, they form a separate phase 

 of the heterogeneous system. Their action is exerted on their surface, and is 

 controlled by the amount of reagents adsorbed. 



The substances acted on by enzymes are usually called " substrates." 



The heterogeneous nature of the systems in which enzymic reactions occur is, 

 in all probability, the reason why the equilibrium position is not quite the same 

 under the action of enzyme and under that of acid. But, owing to the almost 

 complete thermo-neutrality of the hydrolytic reactions in question, an extremely 

 small amount of energy is all that is necessary to change the equilibrium to the 

 extent found. 



Whenever the equilibrium position under the action of an enzyme, or other 

 catalyst, is anywhere except at complete change, either in the direction of 

 hydrolysis or synthesis, the enzyme must accelerate both reactions, although not 

 necessarily to an equal extent. This relative degree of acceleration depends on the 

 respective chemical difficulty of the two reactions of hydrolysis and synthesis. 



Enzymes, therefore, bring about synthesis as well as hydrolysis. 



There is no sufficient evidence that the enzyme forms a constituent of the final 

 chemical equilibrium. 



Since the essential property of an enzyme or catalyst is to change the rate 

 at which a reaction proceeds, a discussion of the formulae of velocity of reactions, 

 deduced from the law of mass action, so far as applicable to the case, is introduced 

 into the text. 



When the velocity constant of an enzymic reaction is calculated by the 

 appropriate formula, it is found that it suffers, as a rule, considerable diminution 

 as the reaction progresses. This means that the activity of the enzyme is 

 decreasing. In some cases there is a spontaneous destruction of the enzyme, in 

 others it is merely temporarily paralysed by some products of the reaction. In 

 the latter case, the most frequent cause is change of the hydrogen ion concentra- 

 tion to a point above or below the optimal one. Enzymes, in fact, are very 

 sensitive to such changes. 



Some evidence has been brought forward to show that there is a special 

 chemical affinity on the part of the enzyme for the substrate or for some 

 constituent of it ; but, at present, the evidence is not very convincing. 



The relation between the concentration of an enzyme and its degree of 

 activity is an exponential one, as would be expected from the fact of its acting 

 by its surface. Small concentrations are, relatively, more active than larger 

 ones. This is to be accounted for by the fact that the rate of the reaction is 

 determined by the amount adsorbed. It is impossible to assign definite numerical 

 values to the exponents of different reactions, since they vary with the relative 

 concentration of enzyme and substrate. In the middle of the reaction, with 

 the usual amount of enzyme, it is generally just below - 2, or thereabouts. This 

 is the " square-root law," which is a rough approximation for a particular stage 

 of the reaction. 



There are certain agents which affect the rate of enzymic reactions by a special 

 action on the catalyst. Such are electrolytes, co-enzymes, and "anti -enzymes." 



Electrolytes, especially hydrogen and hydroxyl ions, have a powerful effect. 

 Neutral salts also have an influence, as a rule, of a favourable kind. 



Some enzymes require the presence of another substance in order to exert 

 their activity. This other substance, known as " co-enzyme," acts in a different 

 manner in different cases. In some it increases the active surface of the enzyme 



