H-C-OH HO-C-H C=O
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HO-C-H HO-C-H HO-C-H
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HO-C-H HO-C-H HO-C-H
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H-C-OH H-C-OH H-C-OH
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CH_{2}OH CH_{2}OH CH_{2}OH
Galactose Talose Tagatose

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
CH_{2}OH 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.

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