Chapter XIV

, will find it profitable to return to

this section and review the facts here presented, as illustrating the principles and mechanism of enzyme action. But a consideration, at this time, of the relation of the molecular configuration of the sugars to their biochemical reactions cannot fail to add interest to the study of these matters from the chemical and biological standpoints.

It has been known for a long time that the dextro- and levo-isomers of a compound which contains one or more asymmetric carbon atoms are affected differently by biological agents, such as yeasts, moulds, bacteria, etc. Pasteur, as early as 1850, showed that the green mould, _Penicillium glaucum_, when growing in solutions of racemic acid (a mixture of equal molecules of _d_- and _l_-tartaric acids) uses up only the _d_-acid, leaving the _l_-form absolutely untouched. Later, it was found that the same green mould attacks _l_-mandelic acid in preference to the _d_- form; whereas the yeast, _Saccharomyces ellipsoideus_, exhibits the opposite preference for these acids.

These observations upon some of the earlier known forms of optically active organic acids led the way to a general study of this phenomenon as exhibited by the optically active soluble carbohydrates. The results of these studies may be considered in connection with the several different types of reactions which these sugars undergo, as follows:

=Glucoside Hydrolysis.=--As was pointed out in connection with the discussion of the mutarotation of glucose, this sugar may exist in either the [alpha] or the [beta] modification. Glucosides of both [alpha] and [beta] glucose are of common occurrence. The difference in molecular configuration, in such cases, may be represented by the following formulas:

R-O-C-H H-C-O-R / \ / \ / \ / \ H-C-OH \ H-C-OH \ | O | O HO-C-H / HO-C-H / \ / \ / \ / \ / C-H C-H | | H-C-OH H-C-OH | | CH_{2}OH CH_{2}OH [alpha]-Glucoside [beta]-Glucoside

The radical represented by the R may be either a common alkyl radical (as CH_{3}, C_{2}H_{5}, etc.), another saccharide group (as in the case of the disaccharides, trisaccharides, etc.), or some other complex organic group (as in the case of the natural glucosides described in