Fundamentals of Biochemistry - A. A. Anisimov 1986

Carbohydrates
Oligosaccharides (First-order polysaccharides)

6.3.1. Nomenclature. Oligosaccharides contain from 2 to 10 monosaccharide residues linked together by glycosidic bonds. They are readily soluble in Water and have a sweet taste. Depending on the number of simple sugar molecules making up the oligosaccharide, they are classified into Disaccharides, trisaccharides, tetrasaccharides, etc. Based on the composition of their monosaccharide residues, they are divided into homooligosaccharides (containing identical monomers) and heterooligosaccharides (containing different monosaccharide units). Structurally, oligosaccharide molecules can be either branched or linear.

There are several principles for constructing the nomenclature of oligosaccharides. According to one approach, for reducing oligosaccharides, the base name is assigned to the monosaccharide residue with a free hemiacetal hydroxyl group, while all units linked to it are treated as substituents. The substitution positions and the configuration of the linked Monosaccharides are specified. For non-reducing sugars, the entire compound is designated as a glycoside.

According to another nomenclature system, the names of the units are written in sequence, connected by an arrow and numbers indicating which atoms of adjacent residues are linked (sometimes a comma is used instead of an arrow). In both cases, the suffix -ose in the names of substituent monosaccharides is changed to -yl, whereas for the monosaccharide chosen as the base name, the suffix becomes -ide, provided the residue lacks a free hemiacetal hydroxyl group. If such a group is present, the -ose suffix is retained.

Finally, There is a more concise system for denoting The Structure of oligosaccharides, similar to the notation used for Proteins AND Peptides. It is often employed for higher oligosaccharides with A large number of units. Each monosaccharide residue is denoted by two or three Latin letters. The configuration (D or L) and ring type are also indicated by letters (p for pyranose, f for furanose). Numbers indicate the linking atoms, and an arrow shows the direction of the bond. However, in laboratory and industrial practice, common trivial names of oligosaccharides are more frequently used; many of these indicate either the Origin of the sugar or its properties, much like monosaccharides, with The addition of the suffix -ose.

Below are the trivial names of several oligosaccharides along with their designations According to the three systems described above:

Lactose 1) 4-O-(β-D-galactopyranosyl)-D-glucopyranose;

2) β-D-galactopyranosyl-(1→4)-D-glucopyranose;

3) β-D-Galp (1→4)—Glp

Sucrose 1) 2-O-(α-D-glucopyranosyl)-β-D-fructofuranoside;

2) α-D-glucopyranosyl-(1→2)-β-D-fructofuranoside;

3) α-D-Glp(1→2) β-D-Fruf.

6.3.2. Characteristics of Individual oligosaccharides. Among oligosaccharides, disaccharides are the most widely distributed in nature. A disaccharide molecule can be formed by linking two hexoses, two pentoses, or a hexose and a pentose. Some disaccharides contain identical monosaccharide residues. For example, maltose, cellobiose, and trehalose yield only glucose upon Hydrolysis, whereas the differences in The properties of these three disaccharides stem from the fact that they contain different glucose isomers (the α-form in maltose, the β-form in cellobiose) and that the glucose molecules are linked differently (maltose vs. trehalose). Depending on the mode of linkage between the two monosaccharide residues, all disaccharides are divided into two types.

Maltose-type disaccharides. In this type of disaccharide, the bond between the two monosaccharide residues is formed via the hemiacetal hydroxyl of one molecule and a hydroxyl group at another position (most commonly the 4th or 6th) of the second monosaccharide. Such disaccharides contain one free hemiacetal hydroxyl group and are therefore capable of reducing Fehling's solution. In aqueous solutions, they exhibit mutarotation. A maltose-type disaccharide molecule exists in solution as a mixture of α- and β-forms; in crystalline form, these isomers can be obtained separately. The main representatives of this type are maltose, isomaltose, cellobiose, lactose, melibiose, and gentiobiose.

Maltose (α-D-glucopyranosyl-(1→4)-D-glucopyranose) is formed during the enzymatic Hydrolysis of Starch by β-amylase. It is present in large quantities in malt and malt extracts, which is why it is commonly known as malt sugar. The question of its occurrence in plants in vivo remains debatable. The β-form is predominant.

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Cellobiose (β-D-glucopyranosyl-(1→4)-D-glucopyranose)

serves as the primary structural unit of plant fiber (Cellulose). Within cellulose, it occurs in the β-form.

Lactose (β-D-galactopyranosyl-(1→4)-D-glucopyranose) is found in significant amounts exclusively in milk (milk sugar), from which it is extracted. For instance, it accounts for 4–5.5% of cow's milk and 5.5–8.4% of human breast milk. Lactose is rare in higher plants, although it has been detected in the pollen tubes of certain plant species.

Sucrose-type disaccharides. These disaccharides are characterized by the absence of reducing power and mutarotation. This is because the glycosidic bond is formed through the hemiacetal hydroxyl groups of both monosaccharide components. The principal representatives of this disaccharide group are sucrose and trehalose.

Sucrose (α-D-glucopyranosyl-(1→2)-β-D-fructofuranoside) is extremely widespread in the plant kingdom. It is found in leaves, stems, roots, fruits, berries, and tubers. It acts as the primary transport form of CARBOHYDRATES in most plants and serves as a storage form in crops such as sugar beets and sugarcane, which serve as raw Materials for its industrial production. Sucrose is readily hydrolyzed in an acidic environment due to the presence of the furanose form of fructose in its molecule. Its rate of hydrolysis is approximately 1,000 times greater than that of other disaccharides under identical conditions.

During acid hydrolysis or under the action of the enzyme invertase (β-D-fructofuranosidase), sucrose yields equal amounts of α-D-glucose and β-D-fructofuranose. This process is accompanied by a shift in the optical Rotation of the polarization plane from right to left (dextrorotatory to levorotatory). This phenomenon is known as inversion, and the resulting product is called invert sugar or invert. Inversion occurs because sucrose itself, which has a specific rotation of [α]²⁰_D = +66.5°, breaks down into glucose ([α]²⁰_D = +52.5°) and fructose with a specific rotation of –92°, resulting in a Reversal of the optical rotation sign to negative. Honey is a natural invert sugar, as it primarily consists of equal parts of glucose and fructose.

Trehalose (α-D-glucopyranosyl-(1→1)-α-D-glucopyranoside) is found in Fungi, Algae, and ergot sclerotia. This disaccharide Functions as the major carbohydrate in the hemolymph of many insects.

Trisaccharides and other oligosaccharides. Among trisaccharides, raffinose, gentianose, and melezitose occur naturally. They primarily function as reserve carbohydrates in plants, serve as a transport form of sugars in certain species (rafinose), or represent excretions of aphids and other sap-sucking insects feeding on plant leaves (melezitose).

Raffinose (a-D-galactopyranosyl-(1→6)-a-D-glucopyranosyl-(1→2)-β-D-fructofuranoside) contains monosaccharide residues linked via hemiacetal hydroxyl groups, and therefore does not exhibit reducing properties or mutarotation.

Many plants contain the tetrasaccharide stachyose. In ash trees, it serves as the transport form of carbohydrates.