Structural Biochemistry - Study Guide - E. A. Bessolitsyna 2015

Disaccharides consist of two monosaccharide units joined together by a covalent bond. The chemical bond linking the Monosaccharides in a disaccharide is called a glycosidic bond; it is formed through the interaction between the hydroxyl group of one sugar and the anomeric carbon atom of the second sugar. Glycosidic bonds are readily hydrolyzed by acids, but remain stable in the presence of bases. Therefore, disaccharides can be hydrolyzed to yield their free monosaccharide components by boiling them in a dilute acid.

Based on their composition, disaccharides are classified into homodisaccharides (consisting of identical monomers) and heterodisaccharides (composed of different monosaccharides).

Depending on the presence of a free hemiacetal group, disaccharides are divided into two groups: reducing and non-reducing. A free hemiacetal group can dissociate to form an alcohol and an aldehyde or ketone group, meaning the ring opens. The resulting aldehyde or ketone group can react with Metal Ions (Cu²⁺, Ag⁺) and reduce them — these are reducing disaccharides. If all free hemiacetal groups are involved in glycosidic bonds, ring opening is impossible, preventing the reduction of the aldehyde or ketone group; consequently, these sugars cannot react with metal ions and are classified as non-reducing disaccharides.

All disaccharides are Glycosides. When naming a disaccharide, it is essential to account for the fact that they are glycosides. Furthermore, the nomenclature of disaccharides requires specifying the chemical bond between the monosaccharides. This involves indicating the numbers of the carbon atoms whose hydroxyl groups participate in forming the glycosidic bond, as well as the anomeric configuration of the monosaccharide whose hemiacetal hydroxyl group forms the bond.

Practically all disaccharides serve a transport function, as they are readily soluble while remaining functionally less active than monosaccharides.

Some disaccharides, such as maltose, act as intermediate products in the Hydrolysis of Polysaccharides.

Disaccharides can be broadly categorized into plant disaccharides and animal disaccharides.

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Figure 20. Structural formulas of major disaccharides

Maltose is a homodisaccharide containing two α D-glucose residues joined by an α (1—4) glycosidic bond—meaning the bond connects the hydroxyl group at C-1 of one glucose unit to the hydroxyl group at C-4 of the other, with the hemiacetal hydroxyl in the α configuration (Figure 20). Both glucose residues in the maltose molecule exist in the pyranose form. Maltose is a reducing sugar because it contains one potentially free aldehyde group that can be oxidized. The second glucose residue in the maltose molecule can exist in either the α- or β-form. Maltose is produced by the action of salivary amylase on starch. Upon the action of maltase—an enzyme secreted by the intestinal mucosa that specifically hydrolyzes the α (1—4) bond—maltose is hydrolyzed into two molecules of D-glucose.

Lactose is a heterodisaccharide that yields D-galactose and D-glucose upon hydrolysis, with the monosaccharides linked by a β (1—4) glycosidic bond; it is found exclusively in milk (Figure 20). The presence of a potentially free carbonyl group (in the glucose residue) makes lactose a reducing disaccharide. Lactose is milk sugar and is the only disaccharide synthesized by mammals.

During Digestion, lactose undergoes Enzymatic hydrolysis through the action of lactase, an enzyme secreted by the intestinal mucosal Cells. The activity of this enzyme is very high in infants; however, in adults, intestinal lactase activity persists only in Northern Europeans and certain African populations. In the majority of adults—including East Asians, Arabs, Jews, many Africans, Indians, and Mediterranean populations—intestinal lactase activity is very low, frequently leading to lactose intolerance. This trait is genetically determined. Lactose intolerance occurs because the disaccharide can only be absorbed in the intestine after being hydrolyzed into its monosaccharide components: when lactase activity is low, unabsorbed lactose accumulates in the intestine, causing severe diarrhea and abdominal pain upon milk consumption.

Trehalose consists of two α D-glucose molecules linked by a 1—1 α glycosidic bond. It is a component of insect hemolymph and is also found in certain Fungi. Trehalose is a non-reducing disaccharide (Figure 20) and serves as the primary transport form of monosaccharides in the insect Circulatory system.

Sucrose is a heterodisaccharide composed of glucose and fructose linked by a β (1—2) glycosidic bond (Figure 20). Sucrose is a non-reducing sugar. It is synthesized by many plants, whereas higher animals lack The ability to produce it. Unlike maltose and lactose, sucrose lacks a free anomeric carbon atom because both anomeric carbons of the monosaccharide residues are linked to each other; consequently, sucrose cannot act as a reducing sugar. Sucrose is the primary intermediate product of Photosynthesis. In many plants, sugars are transported through The Vascular System from the leaves to other PARTS OF THE plant specifically in the form of sucrose. The advantage of sucrose over glucose as a transport sugar is likely due to the fact that its anomeric carbon atoms are mutually linked, protecting the molecule from attack by oxidative or hydrolytic Enzymes as it travels through the plant. Animals cannot utilize sucrose directly, but it becomes digestible following the action of the enzyme sucrase (also known as invertase), located in the cells lining the Small Intestine. This enzyme catalyzes The breakdown of sucrose into D-glucose and D-fructose, which are readily absorbed into the bloodstream.