58 BIOLOGICAL CHEMISTRY 



catalyst is added. Bayliss has compared these two views by 

 means of a mechanical simile. A weight placed on an inclined 

 plane may move slowly downwards or remain at rest. On 

 adding oil the slowly-moving weight moves more quickly and 

 the weight that was at rest may commence to move. The 

 catalyst may be compared to the oil and it may be said to 

 decrease the resistance of the reaction. 



ENZYMES 



It is generally assumed that most chemical transformations 

 in cells are controlled by enzymes, and as enzymes seem to be 

 present in all cells the study of -enzymes is of immense im- 

 portance in Biological Chemistry. In what follows the 

 enzymes will be discussed on the assumption that they act as 

 catalysts by increasing the rate of chemical reactions. 



The chemical composition of the various enzymes is un- 

 known and we can demonstrate the presence of an enzyme 

 only by its effect on the rate of reaction. The nomenclature 

 of enzymes depends upon the kind of chemical change that is 

 accelerated. Certain names which were introduced in the 

 early period of enzyme studies are so well known in the 

 literature that they are retained, but now names are made 

 by adding the suffix -ase to the name of the substance acted 

 upon (substrate). This method does not give any indication 

 of the nature of the chemical change which is accelerated, but 

 this disadvantage is not great as the chemical nature of the 

 substrate usually indicates the nature of the reaction ; in some 

 cases the ending -ase is qualified by some other term. 



The enzymes show several well marked groups. The first 

 group consists of those which act by adding a molecule of 

 water with the decomposition of the substrate into two sub- 

 stances. This is the group of the hydrolytic enzymes. The 

 changes produced involve very little energy change, hence 

 they are easily reversible and these are the enzymes that are 

 used as a preliminary to the transference of material from one 

 place to another, e.g. in the process of digestion. 



The hydrolytic enzymes can be subdivided according to the 

 nature of the materials upon which they act ; for example, 

 amyloclastic or those which act upon starch, steatoclastic or 

 those which act upon fats and proteoclastic or those which 

 act upon proteins.* 



The second group is that of the oxidising enzymes which act 

 by accelerating the rate of oxidation of the substrate. This 

 type of reaction is accompanied by energy changes and these 

 * H. E. Armstrong, Proc. Roy. Soc., 1904, vol. 73, p. 500. 



