PROPERTIES OF ENZYMES 77 



Thus it is that the oxids of nitrogen serve to convert the sulphur 

 dioxid to the trioxid and in the presence of air reverts to the original 

 condition and again repeats the cycle. While in the Gay-Lussac 

 tower the nitrosul-sulphuric acid is formed : 



N 2 O 3 + H 2 SO 4 = 2NO 2 HOSO 2 + H 2 O 

 2NO 2 HOSO 2 + 2 H 2 O = 2H 2 SO 4 + N 2 O 3 



Where there are a number of steps in a reaction, as is the case with 

 the above, it is necessary, as pointed out by Ostwald, that the sum 

 of all the reactions in the catalyzed system are more rapid than are 

 the changes in the uncatalyzed. 



The classic illustration of an organic reaction of this type is that 

 afforded by the production of ether from alcohol. In this process 

 sulphuric acid is employed as catalyzer and as well known this first 

 combines with alcohol with the formation of ethyl-sulphuric acid. 



HO C 2 H 6 O 



\ \ 



C 2 H 5 OH + SO 2 = HOH + S0 2 



/ / 



HO HO 



Alcohol. Sulphuric acid. Ethyl-sulphuric acid. 



The ethyl-sulphuric acid reacts with another molecule of alcohol 

 forming ether and regenerating sulphuric acid. 



C 2 H 5 O HO 



\ \ 



SO 2 + C 2 H 6 OH = SO 2 + C 2 H 5 - O - C 2 H 6 



/ / 



HO HO 



Ethyl-sulphuric acid. Alcohol. Sulphuric acid. Ether. 



Similar combinations occur with the enzymes, for it is found that 

 sucrase will withstand uninjured a temperature 25 C. higher in 

 the presence of sucrose than in its absence. It is difficult to see 

 how this could happen unless the enzyme entered into some sort of 

 union with the sugar. 



Intimately connected with the subject of combination of enzyme 

 with substrate is that of specificity, an example of which is seen 

 in the fact that certain enzymes act only on carbohydrates, others 

 on fats, and still others on proteins. The group of those trans- 

 forming carbohydrates is further subdivided into specific enzymes 

 each of which has the power of acting alone upon only one sugar. 

 This property is so specific that in many cases the enzyme will 

 act upon one optically active compound leaving the opposite optical 

 isomer untouched. This led Fischer to the formulation of his 

 famous simile of the "lock and key" relationship. In this he 

 considers that the enzyme and its substrate must have an inter- 

 relation, such as the key has to the lock; otherwise, the reaction 



