56 CHEMISTRY OF PLANT LIFE 



bichemical properties of the glucosides, so far as their hydrolysis 

 by the enzymes which are present in many biological agents is 

 concerned, depends wholly upon the molecular configuration of 

 the glucose itself. Furthermore, neither the mannosides, which 

 differ from glucosides only in the arrangement of the H and OH 

 groups attached to one of the asymmetric carbon atoms in the 

 hexose, nor galactosides in which two such arrangements are dif- 

 ferent (see configuration formulas on page 57), are attacked by 

 either maltase or emulsin. But other enzymes specifically attack 

 other dissacharides, or polysaccharides, or glucoside-like complexes. 

 For example, lactase acts energetically upon ordinary lactose and 

 all other /3-galactosides; but not upon any glucoside, mannoside, 

 etc. 



Again, neither a- nor /3-xylosides, which correspond with the 

 above-described glucosides in every particular except that the 

 HCOH group next the terminal CH^OH group is missing, are 

 hydrolyzed by either emulsin or maltase. 



These instances, selected from among many similar observa- 

 tions', clearly prove that not only the number and kind of groups in 

 the molecule, but also the arrangement of the constituent groups 

 in space, must be identical in order that the compound may be 

 acted upon by any given enzyme acting as a biological hydrolytic 

 agent. 



Fermentability. The enzyme zymase, present in all yeasts, 

 promotes the fermentation of the natural d- forms of the three 

 hexoses, glucose, mannose, and fructose, but is without effect upon 

 the artificial I- forms of the same sugars. The uniform action of 

 zymase upon these hexoses is easily explained upon the basis of 

 the same assumption which was used to account for the formation 

 of identical osazones from these sugars and their easy transforma- 

 tion into each other; namely, their easy transformation into an 

 enolic form which is identical for all three. 



Further, galactose is fermented by some yeasts (although not 

 by all), but much less readily than are the other sugars, and the 

 temperature reaction is quite different with galactose than with 

 the others. Talose and tagatose are entirely unfermentable. A 

 study of the configuration formulas for these several sugars shows 

 the explanation for these observed facts. These formulas are as 

 follows ; 



