CATALYSIS AND ENZYMES 331 



by greater dispersion, in others its mode of action is more specific and as yet 

 obscure. 



There is considerable doubt whether true anti-enzymes, whose nature is 

 explained in the text, have any existence. Some of the effects described as being 

 due to them are to be accounted for by changes of hydrogen ion concentration, others 

 to adsorption of the enzyme by a colloid. That of intestinal worms is a peculiar 

 substance, having none of the properties of an antibody in the sense of the 

 theory of immunity. 



Contrary to what mass action would predict, it is only in moderate concentra- 

 tions of substrate that the rate of reaction is proportional to this concentration. 

 Above a certain value, differing according to the case, the velocity of the reaction 

 either remains constant or may even decrease. The cause appears to be of various 

 nature, viscosity, adsorption-saturation of the enzyme, or removal of water. But, 

 where the question has been investigated, the composition of the system in 

 equilibrium is as the law of mass action requires, so that the anomalous effect of 

 increase of concentration only relates to the rate of change. 



The rate of enzymic reactions is greatly accelerated by rise of temperature. 

 The optimum temperature is merely that at which the increased rate due to 

 the rise is in greatest preponderance over the simultaneous increased rate of 

 destruction of the enzyme. 



The importance of regarding reversible or balanced reactions from the dynamic 

 point of view is insisted upon. 



The law of mass action shows that, in order to obtain much synthesis, 

 concentration of water must be decreased as far as possible. In the living cell 

 there are probably effective mechanisms for doing this. At the same time, if the 

 synthetic products are continually removed in any way, a small degree of synthesis 

 may result in a considerable amount of products, since the reaction is always going 

 on towards its equilibrium. 



There is no evidence for the existence of enzymes which either hydrolyse only 

 or synthesise only. In fact, if enzymes are catalysts, the one agent must do both. 



It is possible that an intermediate chemical compound may be formed between 

 the surface of the enzyme and the substrate preliminary to decomposition, but 

 there is no actual evidence that such is the case. 



The reacting substances, such as water and substrate in a hydrolytic reaction, 

 are certainly brought into very intimate contact by adsorption on the surface of the 

 enzyme, and the question is still an open one as to whether this fact, combined 

 with the special nature of the surface itself, is not a sufficient explanation of the 

 increased rate of reaction. The special nature of the surface referred to may be 

 merely physical, but the action of particular enzymes on particular substrates has 

 to be accounted for. 



The interaction of electrical forces in the action of neutral salts on adsorption 

 is to be taken into account. 



The chemical nature of enzymes is probably of very different kinds. There 

 is direct evidence that some are not proteins, and it is doubtful whether any are. 

 Some appear to be complex systems of colloids with inorganic components, or other 

 simple compounds. 



There are three stages in heterogeneous reactions diffusion, adsorption, and 

 chemical reaction. The actual rate of the reaction depends on the slowest 

 member of the series. In colloidal solutions, diffusion and adsorption are rapid, so 

 that the chemical reaction proper is the determining one, a fact which accounts 

 for the high temperature coefficient. But the rate of the chemical change itself 

 is determined by the amount of substrate adsorbed at a given moment, according 

 to the law of mass action. 



Some enzymes can be obtained in a stage of formation in which they are 



