The Chemistry of Plant Life

by Roscoe Wilfred Thatcher

Melizitose, a trisaccharide which, in crystallized form, has the formula, C₁₈H₃₂O₁₆·2H₂O, occurs in the sap of Larix europea and in Persian manna, and has recently been found in considerable quantities in the manna which collects on the twigs of Douglas fir and other conifers. When hydrolyzed, it yields one molecule of fructose and one of turanose, a disaccharide containing fructose and glucose linked together in a slightly different way than they are in sucrose. Turanose itself is a reducing sugar, but when linked with fructose to form melizitose its reducing properties are destroyed. Melizitose is a very sweet sugar.

TETRASACCHARIDES

A complex saccharide, known as stachyose, which is found in the tubers of Stachys tuberifera, is said by some investigators to be a tetrasaccharide and by others to have the formula C₃₆H₆₂O₃₁·7H₂O (i.e., a hexasaccharide). It is a crystalline solid, with a faintly sweetish taste, and a specific rotatory power of +148°. When hydrolyzed it yields glucose, fructose, and two (or more) molecules of galactose.

THE RELATION OF THE MOLECULAR CONFIGURATION OF SUGARS TO THEIR BIOCHEMICAL PROPERTIES

As will be pointed out later (see [Chapter XIV]), all chemical reactions which are involved in vital phenomena, including those of plant growth and metabolism, are controlled by enzymes. The biochemical reactions which the soluble carbohydrates undergo afford such excellent illustrations of the relation of the molecular configuration of an organic compound to the possibility of the action of an enzyme upon it, that it seems desirable to discuss this relationship at this point, rather than to postpone it until after the nature of enzyme action has been considered. Undoubtedly, the student, after he has studied the nature of enzymes and their mode of action, as presented in [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 α or the β modification. Glucosides of both α and β glucose are of common occurrence. The difference in molecular configuration, in such cases, may be represented by the following formulas:

The radical represented by the R may be either a common alkyl radical (as CH₃, C₂H₅, 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 [Chapter VI]). But, in every case, the glucoside is easily hydrolyzed by the enzyme maltase (or α-glucase) if the molecular arrangement is that represented by the α-attachment, or by the enzyme emulsin (or β-glucase) if the glucoside is of the β type; but emulsin is absolutely without effect upon α-glucosides, and maltase does not produce the slightest change in β-glucosides. These statements hold true regardless of the nature of the group which is represented by the R in the formulas above. Hence, the biochemical 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 different (see configuration formulas on [page 57]), are attacked by either maltase or emulsin. But other enzymes specifically attack other disacharides, or polysaccharides, or glucoside-like complexes. For example, lactase acts energetically upon ordinary lactose and all other β-galactosides; but not upon any glucoside, mannoside, etc.