CATALYSIS AND ENZYME ACTION 59 



enzymes are associated with energy transformations in living 

 cells. 



Owing to the energy changes the reversible action of oxidis- 

 ing enzymes is not easy to demonstrate. To illustrate this 

 point we can refer to the union of hydrogen and oxygen to 

 form water. At ordinary temperatures hydrogen and oxygen 

 unite very slowly, but if a catalyst (spongy platinum) is present 

 the union occurs with the liberation of much energy in the 

 form of heat. It is obvious that if the action is to be reversed 

 that energy must be supplied, and we can supply this energy 

 by heating the water vapour produced by the first action. 

 This is expressed by Le Chatelier's Theorem, which in its 

 broadest form states that if one of the factors determining 

 the equilibrium of a system is changed the change that takes 

 place in the system is such as would tend to annul the alteration 

 in the factor. 



Union of hydrogen and oxygen to form water liberates heat, 

 therefore decomposition of water into hydrogen and oxygen 

 absorbs heat. Thus a rise in temperature tends to be annulled 

 by the decomposition of water vapour.* 



At 1000 C. and atmospheric pressure water vapour is 

 3Xio~ 5 per cent, dissociated and at 2500 C. and the same 

 pressure the dissociation is 3*98 per cent., or the dissociation is 

 increased one hundred thousand times by a rise of temperature 

 from 1000 2500 C.f We thus see that if we wish to reverse 

 the action of an oxidising enzyme we must furnish a supply of 

 energy. In the case of what are called reducing enzymes 

 the reversible reaction is produced by a suitable arrangement 

 so that the energy of one reaction is made to carry out another 

 (linked reactions). 



We also have a miscellaneous group of enzymes, some of 

 which are hydrolytic and others oxidising. They carry 

 out such reactions as deamidisation and the conversion of 

 a-ketonic aldehydes into a-hydroxy acids. They are kept in 

 a separate group as their main interest is independent of their 

 hydrolytic or oxidising activity. 



As the study of enzymes deals mainly with the effect of 

 various conditions on their activity we must first of all consider 

 the effect of various conditions on enzymes. The description 

 will refer in the first instance to hydrolytic enzymes and be 

 extended to the other forms afterwards. 



* H. Le Chatelier, Compt. Rend., 1884, vol. 99, p. 786. 

 t W. Nernst, Theoretical Chemistry, translated by H. T. Tizard. 

 Macmillan, 1911, p. 692. 



