ENZYMES. FERMENTATION 415 



THE SYNTHETICAL ACTION OF ENZYMES. 



The majority of the chemical changes effected by enzymes are hydrolytic 

 changes. The substrate consists of a compound which can be hydrolysed 

 into two or more constituents and in most instances the organic compound 

 has been synthesised from its constituents. The reactions are reversible. 

 The typical example of a reversible reaction is the formation and hydrolysis 

 of methyl acetate : 



CH 3 OH + CH 8 COOH < H 2 O + CH 3 . 



These reactions have been measured and it has been found that an 

 equilibrium is reached from whichever side the reaction is started when the 

 composition of the mixture of the four substances is 



$ mol. ester + f mol. water -J- mol. alcohol + mol. acid. 



Other similar reactions have also been measured and their equilibrium 

 positions have been determined. Reversible reactions proceed according to 

 the Law of Mass Action. The effect of a catalyst upon reversible reactions, 

 as it effects the reaction to the same degree from both sides, is not to alter 

 the position of equilibrium of the reaction. The final position in the case of 

 enzymes is usually reached when the products of hydrolysis make up over 90 

 and sometimes nearly 100 per cent, of the mixture. This is on account of 

 the large proportion of water present. Enzymes as catalysts should there- 

 fore accelerate the reaction in both directions, i.e. be capable of synthesising 

 the compounds which they hydrolyse. The synthetic power of enzymes has 

 been demonstrated in only a few cases, e.g. that of maltase by Croft Hill, of 

 lipase, of emulsin and of trypsin. The demonstration of the synthetical 

 action of enzymes is difficult as the equilibrium point is generally so near the 

 point of complete hydrolysis. It can be shown most easily in the cases of 

 lipase and of emulsin. The following experiment devised by Bayliss shows 

 the synthetical action of emulsin : 



1 8 parts of pure anhydrous glucose are dissolved in 12 parts of water and 

 cooled. 40 parts of dry glycerol and 3 parts of emulsin are added. The 

 rotation of the mixture (+ 2*83) is taken immediately the mixture is made. 

 The mixture is kept at 47 for seven days and the rotation again observed 

 (+ -80); in fifteen days the rotation is - -16, which corresponds to 75 percent. 

 of synthesis. A control experiment is made with emulsin alone omitting the 

 glucose. In order to show that glucose has not disappeared by other re- 

 actions, the mixture is diluted with 2 to 3 volumes of water and a fresh 

 quantity ("5 gm.) of emulsin is added. The rotation of the solution is taken 

 immediately and again after 2 or 3 days when hydrolysis is complete. It 

 will be found to be the same as the original rotation of the mixture, allowing 

 for the dilution. 



Croft Hill and Bayliss point out that though the synthetical reaction is so 

 small it suffices for synthesis in nature ; the synthetical product is usually a 

 colloid and insoluble ; it is removed from the reaction and synthesis will con- 

 tinue. Bayliss considers- that we have no cause for believing that there are 

 enzymes which act specially as synthetical catalysts of the natural compounds. 



