CHAPTER II 

 ENZYMES 



I. DEFINITION 



In order for a reaction to take place spontaneously, we have seen that it 

 must have a negative AF. But this condition alone is not sufficient to say 

 that the reaction will take place. The petrol in the tank of a car has a very 

 negative AF for the oxidation reaction, but it remains stable in air. Simi- 

 larly, food in a grocer's shop, in contact with air, is also stable, although this 

 food is destined to provide much energy in the oxidations it will undergo, 

 in the presence of oxygen, in the human body. In order to react, most 

 molecules have to be activated. The act of bringing a lighted match to the 

 surface of the petrol accomplishes the activation. 



Conforming to the concept of activation introduced by Arrhenius, the 

 energy content of molecules is not constant but is continually changing. 

 Certain molecules, the activated molecules, have an energy higher than the 

 other molecules, and only they are capable of entering into a reaction. In a 

 solution of sucrose, for example, the number of activated molecules is 

 extrem^ely small but if the temperature is increased by 10° their number is 

 increased two or three times. These activated molecules travel faster and 

 are more labile. 



Uncatalysed reaction 



Catalysed reaction 



Reactants 



Reaction products 



Fig. 30 



When the molecules have absorbed a certain amount of energy {E in 

 Fig. 30), reaction takes place between the reactants and the reaction pro- 

 ducts are formed. Nevertheless, the need for activation can be reduced, to 



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