Galactose: Definition, Function, and Examples

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Galactose Definition

Galactose is one of the three most common monosaccharides, with glucose and fructose as the other two. The most basic form of carbohydrate is monosaccharide.

Simple sugars are distinguished from more complex sugars such as oligosaccharides and polysaccharides. Glycosidic bonds allow monosaccharides to join to generate complex carbohydrates (glycosidic linkages).

What is Galactose?

Hexose monosaccharide galactose is a hexose monosaccharide. It’s a natural substance. C6H12O6 is its chemical formula in general. Galactose has a molar mass of 180.156 g/mol. The melting point is between 168 and 170 degrees Celsius. It has a crystalline structure, is water soluble, and has a sweet flavour.

Galactose vs Glucose vs Fructose

The three most frequent natural monosaccharides are glucose, galactose, and fructose. Glucose, however, is the most prevalent. C6H12O6 is the chemical formula for all three. Because of the six carbon atoms, they are classified as a hexose monosaccharide.

Fructose is a ketose, whereas glucose and galactose are both aldoses. As a result, glucose and galactose have a more physically similar structure. Despite this, the orientation of the hydroxyl group (OH) at carbon 4 can be used to distinguish glucose from galactose structurally.

Galactose also has a greater melting point than glucose. It has a melting point of 168–170 °C, compared to 146 °C for glucose. Fructose, on the other hand, has the lowest melting point of the three (103 °C). Galactose, unlike glucose, is rarely found in a free state. It is frequently found as part of larger macromolecules.

Galactose and glucose, for example, combine to make lactose (milk sugar), a disaccharide. Because glucose is more easily available than galactose or fructose, it is used more frequently in energy metabolism.

Galactose enters glycolysis when there isn’t enough glucose, but it must first be transformed into glucose 6-phosphate before proceeding to glycolysis.

Fructolysis (fructose catabolism) entails fructose phosphorylation by fructokinase to produce fructose 1-phosphate, which is then cleaved into two trioses by aldolase B: dihydroxyacetone phosphate and glyceraldehyde.

Galactose Types

Dextrogalactose (D-galactose) and Levogalactose are the two enantiomers of galactose (L-galalactose). When the glucose stereoisomer rotates plane polarised light clockwise, this nomenclature (based on Fischer projection) identifies D–.

When it rotates plane polarised light in a counterclockwise direction, it is called L–. Hydrolysis produces the dextrotatory form of galactose from milk sugar. Sugar beets, seaweeds, and nerve cell membranes all contain D-galactose. Mucilages provide it with its levorotatory form.

Galactose Metabolism

A monosaccharide, such as galactose, bonds to another monosaccharide through dehydration synthesis, which results in the release of water and the formation of a glycosidic bond. A disaccharide is formed when two monosaccharide units are joined together, but an oligosaccharide is formed when three to ten monosaccharide units are joined together.

Multiple monosaccharides are joined together to form polysaccharides. Galactose unites with another monosaccharide to generate a disaccharide in this case. Lactose, for example, is created when galactose and glucose molecules are combined.

Lactulose, a man-made disaccharide made comprised of galactose and fructose, is another option. A galactan is a polysaccharide made up of repeated galactose units in terms of polymers.

Saccharification is the process of breaking down complex carbohydrates into simpler sugars like glucose and galactose. It entails the process of hydrolysis. This involves an enzymatic action in humans and other higher animals. Lactase, a β-galactosidase enzyme, aids digestion in a diet containing galactose (e.g. lactose in dairy products).

Lactase catalyses the hydrolysis of lactose in the small intestine and breaks the -glycosidic bond, releasing glucose and galactose. In the case of ceramide-rich foods, the lactase β -glycosyleramidase complex releases galactose by breaking the β-glycosidic link in glycolipids. Lactose intolerance is caused by the lack or deficiency of lactase, which prevents lactose from being digested into simpler monosaccharides.

Lactose that is not digested in the small intestine travels to the colon, where it is fermented into lactic acid by gut bacteria. Methane and hydrogen gas are created as a result, causing discomfort, intestinal distention, and flatulence. Water is pulled into the colon by the osmotically active lactic acid, causing diarrhoea.

Lactose can be metabolised by microorganisms such as E. coli, which produces β-galactosidase from its lac operon system.

Galactose is absorbed by intestinal cells (enterocytes) via a sodium-dependent glucose transporter, which is similar to the ATP-driven transport mechanism that absorbs glucose. As a result, during intestinal absorption, glucose competes with galactose. Galactose leaves the intestinal cells and enters the circulation via glucose transporter-mediated transport (Glut-).

A two-phase procedure is used to convert galactose to glucose. The enzyme mutarotase converts β -D-galactose into α-D-galactose in the first phase. -D-galactose is transformed to uridine diphosphate (UDP)-glucose in the final step. The Leloir pathway is frequently used during the last phase.

α-D-galactose is phosphorylated by galactokinase to create galactose 1-phosphate in this route. The uridine monophosphate (UMP) group is then added to galactose 1-phosphate by the enzyme galactose-1-phosphate uridyltransferase, resulting in UDP-galactose.

The enzyme UDP galactose-4′-epimerase then interconverts UDP-galactose to UDP-glucose. The De Ley Duodoroff pathway is an alternative to the Leloir pathway in humans and other species. Galactose that has been transformed to glucose is one method galactose enters the glycolytic pathway. As a result, the entire reaction would be as follows:

Galactose + ATP → Glucose-1-phosphate + ADP + H+

The isomerization of glucose 1-phosphate to glucose 6-phosphate is catalysed by phosphoglucomutase. Galactose metabolism takes place in the liver in humans.

Some glucose molecules are converted to galactose in humans and other mammals so that there is more galactose to mix with glucose to generate lactose. This is especially crucial during the milking process.

Lactose is secreted as milk by the mammary gland, especially during breastfeeding. Note that galactose can also be acquired through dietary sources. Hexoneogenesis is the de novo production of glucose and galactose in the mammary gland.

Galactan is a galactose polymer found in hemicelluloses. Galactose monomers bind together to generate galactans in plants like axlewood ( Anogeissus latifolia) and acacia trees.

Glycosylation is the process of adding a carbohydrate component to proteins and lipids, such as galactose. Galactose is a sugar that is found in a variety of glycolipids and glycoproteins. It could, for example, be a component of cerebroside (a glycolipid comprised of a carbohydrate and a sphingolipid). Glucocerebrosides and galactocerebrosides have glucose and galactose carbohydrate residues, respectively.

Galactose Disorders

Galactosemia is a disease caused by improper galactose metabolism. It’s a rare metabolic condition. A heritable genetic mutation influencing the manufacturing of a Leloir pathway enzyme, galactose 1-phosphate uridyl transferase, is one of the most common causes. Galactosemics should avoid eating a diet high in galactose (and lactose). It could cause diarrhoea, vomiting, and finally cirrhosis if not treated.

Galactose Function

Galactose is a type of monosaccharide that has a variety of biological functions. It can be used as a substitute for glucose when the latter is insufficient to meet an organism’s metabolic needs. It has the ability to enter glycolysis and produce energy. It must, however, go through several preliminary stages before entering the glycolytic pathway.

Galactose is a component of lactose, a milk disaccharide. Lactose is biosynthesized by humans and other milk-producing animals from galactose and glucose.

Milk is an essential source of nutrition, particularly for newborns. Because galactose is a component of cerebrosides, they are referred to as galactocerebrosides rather than glucocerebrosides, which contain glucose rather than galactose.

Galactocerebrosides are abundant in neural tissues and are the primary glycosphingolipid in the brain, which is why galactose is known as the brain sugar. Sulfatide is galactose that has been sulfated afterwards.

Immune response and neurological system signalling are also influenced by sulfatides. Galactose can be found in a variety of plants, including flaxseed mucilages and sugar beet.

A galactose polymer found in the hemicellulose of plants such as axlewood ( Anogeissus latifolia) and acacia trees is known as galactan.

Galactose Citations
  • Galactose-Induced Skin Aging: The Role of Oxidative Stress. Oxid Med Cell Longev . 2020 Jun 17;2020:7145656.
  • D-Galactose-induced accelerated aging model: an overview. Biogerontology . 2019 Dec;20(6):763-782.
  • Galactose metabolism and health. Curr Opin Clin Nutr Metab Care . 2015 Jul;18(4):422-7.


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