DISACCHARIDES: CHARACTERISTICS, STRUCTURE, EXAMPLES, FUNCTIONS - BIOLOGY - 2024

The disaccharides are carbohydrates which are also called double sugars. They have important functions in the diet of man as main sources of energy. These can be of vegetable origin, such as the sucrose of sugar cane and the maltose present, and of animal origin such as the lactose present in the milk of mammals, among others.

Carbohydrates or sugars are the so-called carbohydrates or carbohydrates, which are water-soluble substances composed of carbon, oxygen and hydrogen with the general chemical formula (CH2O) n.

Representation of the structure of the disaccharide Lactose (Source: Telliott at English Wikipedia via Wikimedia Commons)

Carbohydrates are the most abundant organic substances in nature and are present in all plants. The cellulose that constitutes the structure of plant cell walls is a carbohydrate, like the starches in grains and tubers.

They are also found in all animal tissues, such as the blood and milk of mammals.

Carbohydrates are classified into: (1) monosaccharides, which cannot be hydrolyzed into simpler carbohydrates; (2) in disaccharides, which when hydrolyzed produce two monosaccharides; (3) in oligosaccharides, which give 3-10 monosaccharides by hydrolysis and (4) in polysaccharides, whose hydrolysis results in more than 10 monosaccharides.

Starch, cellulose, and glycogen are polysaccharides. Disaccharides of physiological importance in humans and other animals are sucrose, maltose, and lactose.

Characteristics and structure

Being carbohydrates, disaccharides are composed of carbon, oxygen and hydrogen. In general, oxygen and hydrogen in the structure of most carbohydrates are in the same proportion as they are in water, that is, for every oxygen there are two hydrogens.

That is why they are called "carbohydrates or carbohydrates." Chemically, carbohydrates can be defined as polyhydroxylated aldehydes (R-CHO) or ketones (R-CO-R).

Aldehydes and ketones have a carbonyl group (C = O). In aldehydes, this group is attached to at least one hydrogen and, in ketones, this carbonyl group is not attached to hydrogen.

Disaccharides are two monosaccharides linked by a glycosidic bond.

Disaccharides such as maltose, sucrose and lactose, when subjected to heating with dilute acids or by enzymatic action, hydrolyze and give rise to their monosaccharide components. Sucrose gives rise to a glucose and a fructose, maltose gives rise to two glucose and lactose to a galactose and a glucose.

Examples

Saccharose

Sucrose is the most abundant sugar in nature and is composed of the monosaccharides glucose and fructose.. It is found in the juices of plants such as beets, sugar cane, sorghum, pineapple, maple and to a lesser extent in ripe fruits and the juice of many vegetables. This disaccharide is easily fermented by the action of yeasts.

Lactose

Lactose, or milk sugar, is made up of galactose and glucose. Mammalian milk is high in lactose and provides nutrients for babies.

Most mammals can only digest lactose as infants, and they lose this ability as they mature. In fact, humans who are able to digest dairy products in adulthood have a mutation that allows them to do so.

This is why so many people are lactose intolerant; Humans, like other mammals, did not have the ability to digest lactose in childhood until this mutation became present in certain populations about 10,000 years ago.

Today, the number of people who are lactose intolerant varies widely between populations, ranging from 10% in Northern Europe to 95% in parts of Africa and Asia. The traditional diets of different cultures reflect this in the amount of dairy products consumed.

Maltose

Maltose is composed of two glucose units and is formed when the enzyme amylase hydrolyzes the starch present in plants. In the digestive process, salivary amylase and pancreatic amylase (amylopepsin) break down starch, giving rise to an intermediate product that is maltose.

This disaccharide is present in corn sugar syrups, malt sugar and sprouted barley and can be easily fermented by yeast action.

Trehalose

Trehalose is also made up of two glucose molecules like maltose, but the molecules are linked differently. It is found in certain plants, fungi, and animals like shrimp and insects.

The blood sugar of many insects, such as bees, grasshoppers, and butterflies, is made up of trehalose. They use it as an efficient storage molecule that provides fast energy for flight when it breaks down.

Chitobiosa

It consists of two linked glucosamine molecules. Structurally it is very similar to cellobiose, except that it has an N-acetylamino group where cellobiose has a hydroxyl group.

It is found in some bacteria, and it is used in biochemical research to study enzyme activity.

It is also found in chitin, which forms walls of fungi, insect exoskeletons, arthropods, and crustaceans, and is also found in fish and cephalopods such as octopuses and squid.

Cellobiose (glucose + glucose)

Cellobiose is a product of hydrolysis of cellulose or cellulose-rich materials, such as paper or cotton. It is formed by joining two beta-glucose molecules by a β bond (1 → 4)

Lactulose (galactose + fructose)

Lactulose is a synthetic (artificial) sugar that is not absorbed by the body, but instead breaks down in the colon into products that absorb water in the colon, thus softening the stool. Its primary use is to treat constipation.

It is also used to lower blood ammonia levels in people with liver disease, as lactulose absorbs ammonia in the colon (eliminating it from the body).

Isomaltose (glucose + glucose Isomaltase)

Trehalulose is an artificial sugar, a disaccharide composed of glucose and fructose linked by an alpha (1-1) glycosidic bond.

It is produced during the production of isomaltulose from sucrose. In the lining of the small intestine, the enzyme isomaltase breaks down trehalulose into glucose and fructose, which are then absorbed in the small intestine. Trehalulose has a low potency to cause tooth decay.

Chitobiosa

It is the disaccharide repeating unit in chitin, which differs from cellobiose only in the presence of an N-acetylamino group on carbon-2 instead of the hydroxyl group. However, the non-acetylated form is often also called chitobiose.

Lactitol

It is a crystalline alcohol C12H24O11 obtained by hydrogenation of lactose. It is a disaccharide analog of lactulose, used as a sweetener. It is also a laxative and is used to treat constipation.

Turanose

A reducing disaccharide organic compound that can be used as a carbon source by bacteria and fungi.

Melibiosa

A disaccharide sugar (C12H22O11) formed by partial hydrolysis of raffinose.

Xylobiose

A disaccharide consisting of two xylose residues.

Suffocating

A disaccharide present in a soforolipid.

Gentiobiosa

Gentiobiose is a disaccharide consisting of two D-glucose units linked by a β-type glycosidic bond (1 → 6). Gentiobiose has many isomers that differ by the nature of the glycosidic bond that connects the two glucose units.

Leucrose

It is a glycosylfructose consisting of an α-D-glucopyranosyl residue linked to D-fructopyranose through a bond (1 → 5). An isomer of sucrose.

Routine

It is a disaccharide present in glycosides.

Caroliniaside A

Oligosaccharides that contain two monosaccharide units linked by a glycosidic bond.

Absorption

In humans, ingested disaccharides or polysaccharides such as starch and glycogen are hydrolyzed and absorbed as monosaccharides in the small intestine. Ingested monosaccharides are absorbed as such.

Fructose, for example, passively diffuses into the intestinal cell and most is converted to glucose before entering the bloodstream.

Lactase, maltase and sucrase are the enzymes located at the luminal border of the cells of the small intestine responsible for the hydrolysis of lactose, maltose and sucrose respectively.

Lactase is produced by newborn infants, but in some populations it is no longer synthesized by the enterocyte during adult life.

As a consequence of the absence of lactase, lactose remains in the intestine and drags water by osmosis towards the intestinal lumen. Upon reaching the colon, lactose is degraded by fermentation by bacteria in the digestive tract with the production of CO2 and various acids. When consuming milk, this combination of water and CO2 causes diarrhea, and this is known as lactose intolerance.

Glucose and galactose are absorbed by a common sodium-dependent mechanism. First, there is an active transport of sodium that removes sodium from the intestinal cell through the basolateral membrane into the blood. This lowers the sodium concentration within the intestinal cell, which generates a sodium gradient between the lumen of the intestine and the interior of the enterocyte.

When this gradient is generated, the force that will drive sodium along with glucose or galactose into the cell is obtained. In the walls of the small intestine, there is a Na + / glucose, Na + / galactose cotransporter (a symporter) that depends on sodium concentrations for the entry of glucose or galactose.

The higher the concentration of Na + in the lumen of the digestive tract, the greater the influx of glucose or galactose. If there is no sodium or its concentration in the tube lumen is very low, neither glucose nor galactose will be adequately absorbed.

In bacteria such as E. Coli, for example, which normally obtain their energy from glucose, in the absence of this carbohydrate in the medium they can use lactose and for this they synthesize a protein responsible for the active transport of lactose called lactose permease, thus entering lactose without being previously hydrolyzed.

Features

Ingested disaccharides enter the body of animals that consume them as monosaccharides. In the human body, mainly in the liver, although it also occurs in other organs, these monosaccharides are integrated into the metabolic chains of synthesis or catabolism as needed.

Through catabolism (breakdown) these carbohydrates participate in the production of ATP. In the synthesis processes they participate in the synthesis of polysaccharides such as glycogen and thus form the energy reserves present in the liver, in skeletal muscles and in many other organs.

They also participate in the synthesis of many glycoproteins and glycolipids in general.

Although disaccharides, like all ingested carbohydrates, can be sources of energy for man and animals, they participate in multiple organic functions as they form part of the structures of cell membranes and glycoproteins.

Glucosamine, for example, is a fundamental component of hyaluronic acid and heparin.

Of lactose and its derivatives

Lactose present in milk and its derivatives is the most important source of galactose. Galactose is of great importance as it is part of cerebrosides, gangliosides and mucoproteins, which are essential constituents of neuronal cell membranes.

Lactose and the presence of other sugars in the diet favors the development of the intestinal flora, which is essential for digestive function.

Galactose also participates in the immune system as it is one of the components of the ABO group in the wall of red blood cells.

Glucose, product of the digestion of lactose, sucrose or maltose, can enter the body to the synthesis pathway of pentoses, especially the synthesis of ribose which is necessary for the synthesis of nucleic acids.

In plants

In the majority of higher plants, disaccharides are synthesized from triose phosphate from the photosynthetic carbon reduction cycle.

These plants mainly synthesize sucrose and transport it from the cytosol to the roots, seeds and young leaves, that is, to areas of the plant that do not use photosynthesis in a substantial way.

This is how sucrose synthesized by the photosynthetic carbon reduction cycle and that which originates from the degradation of starch synthesized by photosynthesis and accumulated in chloroplasts, are two nocturnal sources of energy for plants.

Another known function of some disaccharides, especially maltose, is to participate in the mechanism of transduction of chemical signals to the flagellum motor of some bacteria.

In this case the maltose first binds to a protein and this complex then binds to the transducer; as a result of this binding, an intracellular signal is produced directed to the motor activity of the flagellum.

References

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3. Guyton, A., & Hall, J. (2006). Textbook of Medical Physiology (11th ed.). Elsevier Inc.
4. Murray, R., Bender, D., Botham, K., Kennelly, P., Rodwell, V., & Weil, P. (2009). Harper's Illustrated Biochemistry (28th ed.). McGraw-Hill Medical.
5. Rawn, JD (1998). Biochemistry. Burlington, Massachusetts: Neil Patterson Publishers.