380 ENZYMES 



in stages, the first of which is the formation of hexose phos- 

 phate which takes place during the first period of temporary 

 acceleration. 



(i) 2 C 6 H 12 6 + 2 R 2 HP0 4 = 2 C0 2 + 2C 2 H 6 + C 6 H 10 O 4 (PO 4 R2) 2 + 2 H 2 O 

 This reaction only takes place provided that the enzyme and 

 the co-ferment are present ; soluble phosphates alone are un- 

 able to promote the fermentation in a mixture of the enzyme 

 and glucose. 



The hexose phosphate is continually being hydrolysed by 

 an enzyme, hexose-phosphatase, yielding a free phosphate 

 which again enters into combination with hexose: 



C 6 H 10 4 (P0 4 R 2 ) 2 + 2 H 2 = C 6 H 12 6 + 2 R 2 HPO 4 



The rate at which this second reaction takes place is the 

 determinating factor in the fermentation rate when glucose is 

 fermented by yeast-extract. There is an optimum concentra- 

 tion of phosphate which produces a maximum initial rate of 

 fermentation ; beyond this optimum a further addition of 

 phosphate depresses the fermentative activity. If the available 

 amount of phosphate in a mixture of sugar, ferment and co- 

 ferment be very small, the total fermentation is greatly reduced, 

 but if to such a mixture a little phosphate be added, there is 

 an enormous increase, as much as 700 per cent, in the total 

 fermentation, even after discounting an amount of carbon 

 dioxide equivalent to the phosphate added. 



With regard to other sugars, Harden and Young found 

 that mannose and fructose are freely fermented by yeast-extract, 

 fructose being fermented more quickly than mannose and 

 mannose rather more quickly than glucose. Also the total 

 weight of carbon dioxide given off from an excess of sugar by 

 the action of a given volume of yeast-juice was slightly greater 

 with fructose than with glucose, whilst that evolved from 

 mannose was less than from glucose. No matter what sugar 

 is used, glucose, fructose or mannose, the hexose phosphate 

 is the same. The behaviour of fructose is qualitatively the same 

 as glucose, but quantitatively there is a considerable difference. 

 Thus the optimum concentration of phosphate for the fermenta- 

 tion of fructose is from I - 5 to 10 times as great as the optimum 

 for glucose, and the maximum rate of fermentation of fructose 

 is 2 to 6 times as great as that of glucose. 



Fructose also behaves in the presence of other sugars as 



