496 ENZYMES 



this esterification, in all probability, is a monophosphate. 

 The ratio of carbon dioxide and hexosediphosphate required 

 by the equation is 2, but the redetermined figures give an 

 average value of 2-38. This may be explained on the assump- 

 tion that the carbon dioxide and the diphosphate are produced 

 in accordance with the equation, but part of the hexosedi- 

 phosphate is subsequently hydrolysed by the enzyme hexose- 

 phosphatase, with the formation of hexosemonophosphate 

 and an inorganic phosphate which again enters into the re- 

 action according to the first equation — 



(2) C«HioO,(PO,R.,)2 + H,0 = QHiiOslPO^R,) + R,HP04 



The rate at which this second reaction takes place is one 

 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 avail- 

 able 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, namely, fructose phosphate, from which 

 it may be concluded that glucose and mannose undergo mole- 

 cular rearrangement to the enolic modification (cf. p. 95). 

 The behaviour of fructose is qualitatively the same as glucose, 

 but quantitatively there is a considerable difference. Thus 



