202 BIOLOGICAL CHEMISTRY 



The reaction is very rapid at first but it soon falls to a slower 

 rate. Addition of phosphates increases the rate once more 

 until the phosphate is used up, and even then the final slow rate 

 may be greater than it was before the addition of phosphate. 



One of the stages of the reaction is the union of sugar with 

 phosphoric acid to form a hexosephosphate. The hexose- 

 phosphate is hydrolysed into sugar and phosphate by an 

 enzyme called hexosephosphatase. The rate of hydrolysis 

 of hexosephosphate depends upon its concentration. Free 

 phosphate seems to be necessary for the action of zymase 

 upon glucose, hence addition of phosphate increases the rate 

 of conversion of sugar into alcohol, but the phosphate is soon 

 removed by conversion into hexosephosphate. The rate now 

 falls off, but owing to the hydrolysis of hexosephosphate free 

 phosphate is being formed continuously and the rate of 

 alcohol formation becomes uniform. .The initial rise in the 

 rate of fermentation depends upon the amount of phosphate 

 added and it is found that each molecule of phosphoric acid 

 added gives rise to one molecule of carbonic acid. 



The final rate of fermentation depends upon the rate at 

 which phosphoric acid is liberated from hexosephosphate, and 

 this, like other enzyme actions, depends upon the concentration 

 of hexosephosphate and hexosephosphatase. If the latter is 

 present in excess the addition of fresh phosphate, leading to a 

 higher concentration of hexosephosphate, will cause a larger 

 continuous supply of free phosphate, and the final rate of 

 fermentation will be greater than the final rate before the 

 addition of fresh phosphate. If, on the other hand, the 

 hexosephosphate is already in excess the addition of more 

 phosphate will cause an initial rise in rate of fermentation, 

 but the final uniform rate will not be increased. 



There are therefore two stages, an early stage in which the 

 rate of reaction depends upon the concentration of free 

 phosphate and a later slower stage the rate of which depends 

 upon the rate of formation of free phosphate from hexose- 

 phosphate by hexosephosphatase. The fermenting complex 

 is composed of two parts, the enzyme and co-enzyme. The 

 latter of these is not destroyed, whilst the former is de- 

 stroyed by heating. The phosphate is believed to* play 

 the part of co-enzyme. The enzyme, zymase, cannot 

 ferment the di-saccharides cane sugar and maltose, but the 

 intact yeast contains invertase and maltase, whereby these 

 sugars are hydrolysed into the monosaccharides glucose and 

 fructose, which can be fermented by zymase. 



