CATALYSIS AND ENZYME ACTION 67 



forms an intermediate compound with the substrate, and that 

 this breaks down with the formation of the products of the 

 reaction and the setting free of the enzyme. The other is 

 that the enzymes are colloids and the reacting substances 

 are condensed on their surface : the rate of reaction is increased 

 owing to the increased concentration of the reacting substances. 



Formation of Intermediate Compounds. In order that the 

 reaction velocity may be increased it is necessary that the 

 combined times for the formation of the intermediate com- 

 pound and for its destruction must be less than the time for 

 the reaction without the formation of an intermediate 

 compound. 



The usual example of catalysis, by means of the formation 

 of an intermediate compound is the oxidation of sulphur 

 dioxide to sulphur trioxide by means of nitrogen peroxide. 

 The nitrogen peroxide thus acting as an oxygen carrier in 

 the same way that a hydrolytic enzyme may be considered 

 to act as a water carrier. The nitrogen peroxide forms a 

 compound with the sulphur dioxide ; this decomposes, 

 forming nitric oxide and sulphur trioxide ; then the nitric 

 oxide takes up oxygen from the air and nitrogen peroxide 

 is again formed. This process can go on indefinitely, so that 

 a small amount of nitrogen peroxide can transform a large 

 amount of sulphur dioxide and the nitrogen peroxide is found 

 unchanged at the end of the experiment. The following 

 equations illustrate the reaction. 



SO 2 + NO 2 - SO 3 +NO 

 2NO + O 2 = 2NO 2 



We have seen that according to the Law of Mass Action 

 the accumulation of the products of the reaction decreases 

 the rate of the reaction owing to the reverse reaction becoming 

 more rapid. There is another way in which the rate of reaction 

 can be diminished by the product of the reaction. If the 

 enzyme action, as stated previously, is related to the structural 

 formula of the substrate, the amount of enzyme united with 

 the various substances in the solution will depend on the 

 similarity of their structural formulae to that portion of the 

 substrate to which the enzyme becomes attached. The 

 structural formula of one more than another of the products 

 of the reaction may be similar to spacial relations of the atoms 

 in that portion of the substrate which is related to the enzyme, 

 hence the enzyme will unite with this substance more readily 

 than with the other products of the reaction. Such substances, 

 by removing the enzyme from the solution, will have a more 



