22 PLANT ENZYMES [ch. 



species and varieties are known. The feature of special interest in con- 

 nexion with the Yeast plant is its power of fermenting hexoses, with the 

 formation of alcohol and carbon dioxide, the process being carried out 

 by means of an enzyme, zymase. The complete reaction is generally 

 represented as follows : 



C6Hi206 = 2C02+2C2H50H 



though there is little doubt that several stages are involved, including 

 oxidation, reduction and hydrolysis. It has been known for some time 

 that phosphates are essential to the action of zymase, and the first stage 

 is probably the formation of a hexosephosphate with the accompanying 

 production of ethyl alcohol and carbon dioxide : 



2C6Hi2O6-i-2R2HPO4=C6Hi0O4(R2PO4)2-i-2C2H6OH + 2CO2 + 2H2O, 



the hexosephosphate being continually decomposed by a hydrolytic en- 

 zyme, hexosephosphatase, yielding free phosphate again : 



C6Hi0O4(R2PO4)2+2H2O = C6Hi2O6 + 2R2HPO4. 



In addition to zymase and hexosephosphatase, yeast contains the 

 enzymes, invertase, protease, peroxidase, catalase, reductase, glycogenase, 

 carboxylase, a glucoside-splitting enzyme, and some form of diastatic 

 enzyme. The carboxylase decomposes a large number of aliphatic a-keto- 

 acids, of which the most important is pyruvic acid CHg • CO * CXDOH. 

 The reaction, which is also possibly concerned in fermentation, involves 

 the formation of the corresponding aldehyde with the evolution of carbon 

 dioxide : 



CHa- CO • C00H = CH3- CHO + CO2. 



Yeast also stores, as a reserve material, the polysaccharide, glycogen, 

 which occurs in animal tissues though it is rarely found in plants : this 

 is hydrolyzed by glycogenase into a monosaccharide. Finally, yeast con- 

 tains invertase, and most species, in addition, maltase, but from a few 

 species the latter enzyme is absent. Hence yeasts are able to ferment 

 the disaccharides, cane-sugar and maltose, since they can first hydrolyze 

 them to monosaccharides. 



As in the case of the enzymes of other tissues, those of yeast can be 

 made to carry out their functions after the death of the living protoplasm. 

 The method of demonstrating this is to " kill " the cells by means of 

 drying at 25-30° C, by treatment with a mixture of alcohol and ether, 

 or by treatment with acetone and ether. In this way the protoplasm is 

 destroyed, but the enzymes remain uninjured. Yeast treated thus has 

 been termed " zymin." 



