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In this article we will discuss about the substances that liberate one or more monosaccharides on hydrolysis.
The Oligo- and Polysaccharides:
Holosides therefore consist exclusively of monosaccharides. Depending on the number of molecules of monosaccharides liberated during hydrolysis there are disaccharides, trisaccharides, etc. We will study particularly two groups of compounds: the disaccharides and the polysaccharides.
1. Disaccharides:
There are 2 types of natural disaccharides depending on the mode of linkage of the 2 molecules of monosaccharides:
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i. Reducing Disaccharides:
The hemiacetal group of one of the monosaccharides is involved in an oside linkage with an alcoholic hydroxyl of the second monosaccharide. The reducing character of the first monosaccharide has disappeared, but that of the second remains; this imparts a reducing character to the disaccharide molecule.
ii. Non-Reducing Disaccharides:
The two monosaccharides are linked by their hemiacetal groups; the disaccharide has no reducing power; this immediately indicates its structure.
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On the contrary, to determine the structure of a reducing disaccharide, it must be subjected to a methylation, then a hydrolysis and the tri- and tetra-O-methyl-monosaccharides obtained must be characterized (only the free hydroxyls will have been methylated).
For example in the case of maltose, the hydrolysis of permethylmaltose will give 2, 3, 4, 6-tetra-O-methyl-glucose and 2, 3, 6-tri-O-methyl glucose. On the other hand, an oxidation or a reduction followed by hydrolysis, will determine the monosaccharide whose reducing group has remained free.
Glucose, for example, will give gluconic acid or sorbitol. Lastly, the use of specific enzymes (α- and β-glucosidases, α- and β-galactosidases, etc.) will indicate the configuration of the monosaccharide involved by its hemiacetal group.
As in the case of monosaccharides, there are nomenclature rules giving a precise description of the structure of oligo-saccharides. The use of initials for designating monosaccharides considerably simplifies the expression of the structure of oligosaccharides (see below for example, for maltose or lactose).
A. Reducing Disaccharides:
a) Maltose:
It is an intermediate product of the hydrolysis — acid or enzymatic — of polysaccharides like starch and glycogen. It is composed of the union of 2 molecules of D-glucose by a α-1,4-glucosidic linkage; it is a 4-D- glucopyranosyl-α-D-glucopyranose (see fig. 4-15); for simplicity, it may be written, Glc (α 1-4) Glc, or α D-Glcp-(1 → 4)-D-Glcp.
The hemiacetal group remaining free (on the right in the figure) can, in solution, exist under 2 configurations which are in equilibrium, the α-form (α-maltose) and the β-form (β-maltose). The other hemiacetal group, on the contrary, is fixed in the α-1, 4-glucosidic linkage. This hemiacetal group which has remained free is responsible for the reducing power of maltose.
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b) Lactose:
It is found in milk of mammals. It is formed by the union of a molecule of D-galactose (involved by its hemiacetal group) and one of D-glucose (involved by its hydroxyl in position 4); it is a β-1, 4-galactosidic linkage (which will be hydrolyzable by a β-galactosidase). Lactose is a 4-D- glucopyranosvl-β-D-galactopyranose (see fig. 4-16).
For convenience, it may be written Gal (β1-4) Glc or β-D-Galp-(1 → 4)-D-Glcp. As in the case of maltose, there will be, in solution, equilibrium between the 2 possible forms, depending on the configuration taken by the hemiacetal group which has remained free (that of glucose, on the right in the figure). This hemiacetal group is responsible for the reducing power of lactose.
B. Non-Reducing Disaccharides:
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Sucrose (Sometimes Called Saccharose):
This is the most abundantly distributed non-reducing disaccharide and it is the only compound examined here. Found in numerous plants, it is particularly abundant in sugar-beet and sugarcane. It is formed by the union of a molecule of D-glucose and a molecule of D-fructose, both involved by their hemiacetal group in the oligosaccharide linkage.
There is, therefore, only one possible configuration and no reducing power. Sucrose is the 1-α-D-glucopyranosyl-2-β-D-fructofuranoside (see fig. 4-17). Let us note that fructose is here in the furanose form, while the pyranose form predominates in free fructose.
Sucrose is dextrorotatory ([α]D = +66°7). When hydrolyzed, either by acids or intestinal invertase, it gives a mixture of equimolecular quantities of D(+) glucose (in mutarotation equilibrium of the α and β-pyranoses forms, [α]D = +52° approximately) and D(-) fructose (in mutarotation equilibrium, [a]D = -92° approximately); the levorotation of fructose is therefore greater than the dextrorotation of glucose, so that the mixture obtained is levorotatory (contrary to the initial sucrose); for this reason it has been called invert sugar.
2. Trisaccharides:
The most widely distributed is raffinose, present in numerous plants, particularly in beet where it co-exists with sucrose (its name also indicates that it can be obtained from the fractions eliminated during the refining of beet sugar).
It is composed of one molecule of α-galactose (pyran form) linked by its reducing group to the primary alcohol group of a molecule of glucose which is involved by its reducing group with the reducing group of a molecule of fructose (these two molecules therefore form a molecule of sucrose). Raffinose is therefore an α-D-galactopyranosyl (1 → 6)-α-D-glucopyranosyl (1 → 2)- β-D-fructofuranoside, i.e. a non-reducing trisaccharide.
3. Polysaccharides:
Polyholosides, or polyosides, or polysaccharides are formed by the condensation of a large number of molecules of monosaccharides, either all identical (homopolysaccharides), or of different types (heteropolysaccharides).
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A. Homopolysaccharides:
Glucosans (composed exclusively of glucose units) must be mentioned first.
a) Starch:
It is the carbohydrate reserve form in plants; it is generally found in the form of starch grains whose morphology varies according to the plant species.
In most cases it is composed of 2 constituents:
1. Amylose, a polysaccharide with linear chains, is composed of D-glucose units joined by α-1, 4 glucosidic linkages (as in maltose); its molecular weight varies from 150 000 to 600 000. These chains are of varying lengths and can combine through hydrogen bonds between the hydroxyls, thus forming rather compact structures.
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2. Amylopectin (or isoamylose) is a polysaccharide whose molecular weight can reach several millions; it is formed of main chains identical to those of amylose (α-1, 4 glucosidic linkages) but with side chains joining them — by α-1, 6 glucosidic linkages — which have the same structure as the main chains (see fig. 4-18) and which are 20 to 25 glucose residues long.
Some starches contain only amylopectin. In those containing amylose and amylopectin, the respective proportions of these two constituents and the length of chains vary according to the type of starch.
b) Glycogen:
It is the storage form of glucose in animals (it occurs especially in the liver and muscles). The structure of glycogen is the same as that of amylopectin (see fig. 4-18); however, glycogen is often branched to a greater degree and therefore contains more α-1, 6-glucosidic linkages.
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Besides, the average length of chains varies from 10 to 15 glucose residues. Its molecular weight can attain several tens of millions. It has been recently proved that glycogen is conjugated to proteins through the OH group of tyrosine residues.
c) Cellulose:
This is the substance mainly responsible for the structure of cell walls of plants. It is not hydrolyzable by enzymes present in the digestive tract of man and has therefore no food value unlike starch (however, the ruminants and various xylophagous insects can utilize cellulose because they have in their digestive tract, microorganisms whose enzymes hydrolyze this polysaccharide).
Cellulose consists of long chains composed of D-glucose units joined by β-1, 4-glucosidic linkages (and not α-1, 4 as in amylose). These chains (see their structure in figure 4-19) are closely attached to one another by hydrogen or Van der Waals type bonds thus forming compact and insoluble fibrous structures which permit the industrial use of cellulose (paper fibers, fabrics, etc.)
d) Dextrans:
These polysaccharides consist of α-glucose units linked in 1, 6. Shorter chains are grafted (by 1, 4 linkages) on this main chain. Dextrans are found in various bacteria.
The lengthening of chains is catalyzed by dextran-sucrase, an enzyme which can split a sucrose molecule, liberating fructose and adding glucose at the end of a dextran chain which is thus lengthened by one unit. Dextrans are used for chromatography by gel filtration; some dextrans (of molecular weight between 50,000 and 100,000) are used in therapeutics as substitute for blood plasma.
e) Chitin:
It is a polymer of N-acetylated glucosamine; its molecules are joined in linear chains by β-1, 4 linkages. In combination with mineral salts and proteins, it forms the exoskeleton of Arthropoda.
In addition to glucosans, the homopolysaccharides also include:
1. Arabans (consisting of α-arabofuranose units generally linked in 1, 5) found in pectic matters or pectins, present in fruits and responsible for the phenomenon of jelly formation.
2. xylans (composed of xylopyranose units) found in lignified plant tissues;
3. fructosans, like inulin (consisting of fructofuranose units) found in tubers of some plants;
4. pectins are polymers of D-galacturonic acid (α-1,4 linkages); some of their carboxylic groups are in the form of methylesters.
B. Heteropolysaccharides:
Their hydrolysis can liberate not only various neutral monosaccharides but also uronic acids, osamines and sialic acids.
In this group are found plant polysaccharides the structure of which is often not yet fully understood, for example, gums (branched structures containing D-galactose, L-arabinose, L-rhamnose and D-glucuronic acid), hemicelluloses (composed of D-glucose, D-xylose, L-arabinose and D-glucuronic acid), mucilages (containing D-mannose, D-galactose, sometimes esterified by sulphuric acid).
To this group belong the polysaccharides constituting the capsule of bacteria like pneumococci or streptococci; each type of pneumococcus for example has its own polysaccharide which confers on it an antigenic specificity.
As an indication, the structure of the polysaccharide of pneumococcus III is given below:
Lastly, this group sometimes includes oligo- or polysaccharides which, in the native state, are conjugated either with proteins or with lipids. In this respect they rather belong to the group of heterosides. We shall examine some of them in our study of glycoproteins: acid mucopolysaccharides of the connective tissue, like chondroitin sulphuric acids, murein, etc.
Heteropolysaccharides:
The hemiacetal group of a monosaccharide can react with a compound which is not of carbohydrate nature. A heteropolysaccharide is then obtained. The non-saccharide part is called aglucone. Depending on the nature of the saccharide, there are glucosides, galactosides, etc.
Let us first recall a simple example of heteropolysaccharide, mentioned in connection with arguments supporting the cyclic structure of saccharides, i.e. the combination of the pseudo-aldehyde group of glucose with the alcohol group of methanol, giving either α-D-methylglucoside, or β-D- methylglucoside. Having studied the cyclic structures of monosaccharides, we may now state that these 2 compounds can exist either in the furanose form or in the pyranose form.
Heteropolysaccharides are very widely distributed in plants. A large number have pharmacodynamic properties and are used in therapeutics. We shall just mention the heterosides having cardiotonic properties (especially in the digital), the aglucones of which are derivative of sterols.
Let us recall to memory, the glucurono-conjugates which are heteropolysaccharides, the monosaccharide of which is glucuronic acid.
It may be observed that in most cases, one has O-heterosides where an alcoholic or phenolic hydroxyl of the aglucone is combined with the reducing group of a monosaccharide. But nucleosides and nucleotides, where the reducing group of ribose or deoxyribose is linked with the nitrogen of a purine or pyrimidine base, can be considered as N-heterosides.
It must be pointed out that glycoproteins and glycolipids are also heterosides, the carbohydrate part of which is called glycan.
