72 THE ACTION OF ENZYMES 



ferments were first observed to act ; for example, Duclaux 

 named one of the enzymes which attacks the casein of 

 cheese, casease ; that affecting maltose sugar is termed 

 maltase, the ferment which changes amylum or starch is 

 an amylase, the urea ferment urease, and so on. Similarly 

 the proteid-destroying class are described as proteases. 

 This nomenclature, however, is not strictly adhered to, 

 since it is customary to retain the names trypsin and 

 pepsin for two common proteases, and the terms emulsin, 

 ptyalin are still in ordinary use. 



From many points of view it is convenient to classify 

 the enzymes according to the nature of the substance 

 which they are able to ferment. The most important 

 organic compounds which undergo fermentation are the 

 carbohydrates, proteins and fats. These form the greater 

 part of the body substance of all animals and plants, and 

 enter into the composition of food stuffs used by many 

 kinds of living organisms. It will be useful to refer 

 briefly here to the general character of the three classes, 

 and give a short account of the more commonly occurring 

 representatives in each. 



i. Carbohydrates. To this class belong cellulose, 

 starch, various kinds of sugar, and many other widely 

 distributed natural products. They all contain carbon, 

 hydrogen and oxygen, the two latter elements being 

 present in the same proportion as they exist in water : 

 hence the name hydrate of carbon or carbohydrate. 

 They may be divided into three groups : 



(i) The polysaccharoses, amyloses or starch group. 



(ii) The disaccharoses, sometimes known as the sac- 

 charose or cane-sugar group. 



(iii) The monosaccharoses, termed glucoses or the 

 grape-sugar group. 



