The Chemistry of Plant Life

by Roscoe Wilfred Thatcher

Again, neither α- nor β-xylosides, which correspond with the above-described glucosides in every particular except that the HCOH group next the terminal CH₂OH group is missing, are hydrolyzed by either emulsin or maltase.

These instances, selected from among many similar observations, 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 l- 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 transformation 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:

It will be noted that in the case of glucose, mannose, and fructose, the configuration is identical at every point except at the aldehyde end of the chain, and that here the two groups readily arrange themselves into the same enolic form for the three sugars. Galactose differs from these three sugars only in the arrangement of the H and OH groups attached to one of the other carbon atoms (the third from the alcoholic end); the difficulty of its fermentation indicates that some molecular rearrangement to bring this group into its proper configuration must precede the fermentation process. The fact that it is the third HCOH group which thus undergoes rearrangement is significant because of the participation of these parts of molecules in groups of threes in many biological processes, as will be mentioned elsewhere. Talose is unfermentable, even though the arrangement of its upper three groups is the same as in the galactose and the lower three the same as in mannose.

If further proof that fermentability depends upon molecular configuration were needed, it is furnished by the fact that no pentose is fermentible, even though the stereo-arrangement of each of the four alcoholic groups in the molecule is identical with the corresponding groups in a fermentible hexose.

Oxidation by Bacteria.—The bacillus Bacterium xylinum contains an enzyme, or enzymes, which promote the oxidation of the aldehyde group of an aldose sugar to COOH, or of one alcoholic CHOH group next the terminal CH₂OH group of a hexatomic alcohol to C=O. But these oxidizing enzymes affect only those compounds in which the OH groups are on the same side of the two asymmetric carbon atoms next the end of the molecule where the oxidation takes place, as indicated in the following groupings.

The configuration of the remainder of the molecule is immaterial to action by these oxidizing bacteria; hence, the enzymes in this case are apparently concerned only with the configuration arrangement of a portion of the molecule, instead of with the whole hexose grouping, as in the cases of the other reactions which have been thus far considered.