, etc.

Again, neither [alpha]- nor [beta]-xylosides, which correspond with the
above-described glucosides in every particular except that the HCOH group
next the terminal CH_{2}OH 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:

CHO CHO CH_{2}OH CHOH
| | | |
H-C-OH HO-C-H C=O C-OH
| | | |
HO-C-H HO-C-H HO-C-H HO-C-H
| | | |
H-C-OH H-C-OH H-C-OH H-C-OH
| | | |
H-C-OH H-C-OH H-C-OH H-C-OH
| | | |
CH_{2}OH CH_{2}OH CH_{2}OH CH_{2}OH
Glucose Mannose Fructose Enol

CHO CHO CH_{2}OH
| | |