13 julio, 2024

Disaccharides: characteristics, structure, examples, functions

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

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

Carbohydrates are the most abundant organic substances in nature and are present in all plants. The cellulose that makes up the structure of plant cell walls is a carbohydrate, just 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 as: (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. Physiologically important disaccharides in humans and other animals are sucrose, maltose, and lactose.

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Characteristics and structure

Being carbohydrates, disaccharides are made up 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 the reason 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 a 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 one glucose and one fructose, maltose gives rise to two glucoses, and lactose to one galactose and one glucose.

examples

Saccharose

Sucrose is the most abundant sugar in nature and sIt is made up of the monosaccharides glucose and fructose.. It is found in plant juices 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, losing this ability as they mature. In fact, humans who are capable of digesting dairy products as adults have a mutation that allows them to do so.

That’s why so many people are lactose intolerant; Human children, like other mammals, did not have the ability to digest lactose in infancy until this mutation became present in certain populations around 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 made up of two glucose units and is formed when the enzyme amylase hydrolyzes starch present in plants. In the digestive process, salivary amylase and pancreatic amylase (amylopepsin) break down the 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.

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 such as 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 quick energy for flight when it breaks down.

chitobiose

It consists of two linked molecules of glucosamine. 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 is used in biochemical research in order to study enzyme activity.

It is also found in chitin, which forms walls of fungi, exoskeletons of insects, arthropods, and crustaceans, and is also found in fish and cephalopods such as octopus 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 (1 → 4) bond

Lactulose (galactose + fructose)

Lactulose is a synthetic (man-made) sugar that is not absorbed by the body, but is broken down in the colon into products that absorb water in the colon, softening the stool. Its primary use is to treat constipation.

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

Isomaltose (glucose + glucose Isomaltase)

Produced during the digestion of starch (bread, potatoes, rice), or produced artificially.

Isomaltulose (Glucose + Fructose Isomaltase)

Sugar cane syrup, and honey is also produced artificially.

Trehalulose

Trehalulose is a man-made 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 dental caries.

chitobiose

It is the disaccharide repeating unit in chitin, differing 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 analogue of lactulose, used as a sweetener. It is also a laxative and is used to treat constipation.

Turanosa

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.

soforosa

A disaccharide present in a sophorolipid.

gentiobiosa

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

leucrose

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

rutinous

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, diffuses passively within the intestinal cell and most of it is converted to glucose before entering the bloodstream.

Lactase, maltase and sucrase are the enzymes located in 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 children, but in some populations it stops being 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. When it reaches 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 is known as lactose intolerance.

Glucose and galactose are absorbed by a common sodium-dependent mechanism. First, there is active sodium transport that moves sodium out of the intestinal cell across 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 inside 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 entry of glucose or galactose. If there is no sodium or its concentration in the tubular 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.

functions

Ingested disaccharides enter the organism 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, skeletal muscles and 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 humans and…

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