152 UNITY AND DIVERSITY IN BIOCHEMISTRY 



varying degrees, by the presence of catalysts, which, in effect, lower the 

 activation-energy barrier. In Nature, reactions do not take place at high 

 temperatures, and most of them only occur because of the presence of the 

 organic catalysts which we call enzymes. 



As we saw in the preceding chapter, one of the characteristics of the 

 biochemical machine is a constant opposition to the establishment of 

 thermodynamic equilibrium. 



A cell, even if after a given period we do not observe any change in its 

 composition, is not in a state of thermodynamic equilibrium, but is in a 

 stationary state of flux in which the velocities of synthesis and breakdown, 

 for example, are equilibrated. Such an equilibrium is the result of the 

 control of the velocities in question. The control of these velocities is the 

 work of the very many specific catalysts which each cell contains. 



II. ENZYMES AND ACTIVATORS 



It was in 1926 that an enzyme, urease, was crystallized for the first time 

 by Sumner. Since then, many other enzymes have been crystallized and 

 the list is constantly increasing. All these purified enzymes are proteins. 

 However, as long ago as 1897, Gabriel Bertrand introduced the name 

 "coenzyme" or "coferment" for those metal ions whose presence was 

 indispensable for the action of certain enzymes. 



It soon became evident that certain enzymes were heteroproteins in 

 nature, that is that they contained a prosthetic group firmly attached to the 

 protein. For example the haem in catalase. Also there are a number of 

 cofactors in the absence of which biocatalysis does not occur. At the present 

 time the term coenzymes refers to these small organic molecules which are 

 indispensable for the performance of biocatalysis but which are not in- 

 cluded in the enzyme molecule in the state in which it is isolated from the 

 cell. Hence enzymes exist which are inactive in the absence of the co- 

 enzyme. 



In addition, when an ion, whether attached firmly or not to the protein, is 

 indispensable for its biocatalytic action, it is called an activator. The study 

 of the mechanism of enzyme action, of coenzymes and activators, is in full 

 swing at the present time. We shall, therefore, content ourselves for the 

 time being with the above terminology which covers extremely diverse 

 mechanisms. Some examples of these general biochemical mechanisms will 

 be taken and described in the pages which follow. 



III. CLASSIFICATION OF ENZYMES 

 A. Hydrolases 



These catalyse reactions of the type : AB + HgO -^ AGH + HB. 

 These enzymes can be considered as catalysing the transfer of a trans- 

 ferable group with water playing the role of specific acceptor. They can 



