Lecture IV. 29 



cannot. Like colloids many are coagulated by heat, and this 

 coagulation being irreversible the enzyme is no longer active. 

 Many may be coagulated by alcohol, and when this coagulation 

 is reversible, the enzyme thus precipitated may be again dispersed 

 in water and is able to carry on its action. Thus extraction with 

 water and coagulation with alcohol form a very useful method in 

 the production of enzyme preparations. 



It is now very generally held that the reactions which enzymes 

 accelerate are reversible actions, i.e. reactions in which a dis- 

 integrating and synthesising action are going on simultaneously, 

 and in which the final condition is a state of equilibrium, or 

 balance struck, between these two actions. The enzyme ac- 

 celerates both actions, and consequently the final condition is 

 the same whether attained automatically or by the aid of the 

 enzyme. When either of these actions is much more rapid than 

 the other the change appears complete or almost so ; when the 

 two are more evenly balanced the change is incomplete. Sup- 

 pose, for instance, an enzyme is in contact with a substrate A 

 and accelerates its hydrolysis so that it disintegrates into two 

 products B and C, and at the same time the enzyme accelerates 

 the combination of B and C into A. At the beginning of the 

 reaction only a little of the two products has been formed. Con- 

 sequently but little synthesis takes place and this synthesis is 

 completely dominated by the formation of these two products 

 under the action of the enzyme on the large quantities of A 

 still available. But as the reaction goes on, owing to disin- 

 tegration, there is less of A available and consequently less of 

 B and C is formed, while the synthesising action increases as B 

 and C become more abundant. Evidently an equilibrium point 

 will be reached where the two opposing actions will balance 

 each other. It is also clear that the relative rates of synthesis 

 and disintegration will determine whether the reaction will 

 appear incomplete or complete. Evidently, if the products of 

 one of the actions are continually removed from the reaction 

 as quickly as they are formed, the change will become complete 

 in that direction. This condition seems to be often realised 

 in the changes effected in the metabolism of plants, so that 

 an enzyme which, under ordinary laboratory conditions where 

 the products are not removed, acts as a disintegrating agent 

 may in the plant cell if all the products of synthesis are removed 

 act as a synthesiser. 



Being in the colloidal state enzymes present enormous 

 surfaces in the water in which they are dispersed, and the 



