124 



ENZYMES 



TABLE XX— continued 



Lactose and melibiose are both glucose galactosides differing only 

 in the position occupied by the hydroxyl of the glucose molecule 

 united to the galactoside. As galactosides, both are slowly hydro- 

 lysed by crude emulsin (known to be a mixture of at least three 

 enzymes). Lactase is, however, without action on melibiose, and 

 melibiase does not split milk sugar. Till further experimental 

 work has been done attempted explanation of these facts is mere 

 guesswork. Fischer has suggested that the enzyme is to its 

 substrate as a key is to its own particular lock. The evidence at 

 present available does not altogether lend itself to this explanation. 

 It looks as if a careful study of the alterations brought about in the 

 configuration of colloids by slight modifications of the surrounding 

 conditions might lead towards an acceptable explanation of 

 specificity. (See also Optical Activity, p. 126.) Weight is given 

 to this suggestion by examination of the synthesising power of 

 enzymes. Since enzymes accelerate reactions that would take 

 place without them, and since, theoretically, all reactions are 

 reversible, the synthesis of complex bodies from their constituents 

 might be expected by the aid of the same enzyme as brought about 

 the splitting of the complex. That is, a lipase should not only split 

 a fat into fatty acid + glycerol, but should regenerate fat from fatty 

 acid + glycerol. 



A reversible or balanced reaction is one in which, under definite 

 conditions, there is a certain equilibrium point at which the 

 amount of material being broken down is exactly balanced by the 

 amount being built up. For example, take a stoppered bottle 

 half full of water. Two processes are going on simultaneously, 

 (a) Liquid water is undergoing vaporisation and the gaseous 

 hydrol is passing into the air, {b) Gaseous water particles are 

 passing from the air into the water to form, say, dihydrol. When 

 the air is saturated with humidity for that particular temperature, 

 exactly the same number of water molecules will leave the water 



