Galactose

Galactose is a monosaccharide sugar that is about as sweet as glucose and approximately 65% as sweet as sucrose[1]. It is an aldohexose and a C-4 epimer of glucose, meaning that it has the same molecular formula as glucose but with a different arrangement of atoms[1]. Galactose is a component of glycoproteins found in nerve tissue and is sometimes referred to as "brain sugar".

Galactose is commonly found in dairy products, avocados, sugar beets, and other gums and mucilages. It is also synthesized by the body and forms part of glycolipids and glycoproteins in several tissues. In addition, galactose is a by-product of the third-generation ethanol production process from macroalgae.

When combined with glucose through a condensation reaction, galactose forms the disaccharide lactose. The hydrolysis of lactose to glucose and galactose is catalyzed by the enzymes lactase and β-galactosidase. Galactose metabolism, which converts galactose into glucose, is carried out by the three principal enzymes in a mechanism known as the Leloir pathway.

Galactose was first isolated and studied by Louis Pasteur in 1856, and its configuration was determined by Emil Fischer and Robert Morrell in 1894. It is classified as a monosaccharide, an aldose, a hexose, and is a reducing sugar[2].

Galactose plays an important role in various physiological processes, including galactosylation of ceramide during myelin sheath synthesis of Schwann cells and synthesis of heparin/heparan sulfates[4]. It is formed endogenously in human cells, and a 70 kg adult male can synthesize up to 2 grams of galactose per day[4].

Galactosemia is an inborn genetic metabolic disorder that emerges as a result of the impaired processing of galactose[4]. It is subclassified into several types, including classic galactosemia, Duarte galactosemia, and epimerase-deficiency galactosemia[4]. The treatment for galactosemia is to use a formula based on the sugar sucrose rather than milk with lactose, and a galactose-free diet is critical only in infancy[2][4].

In addition to its role in galactosemia, galactose has been linked to skin aging through the activation of aldose reductase, which leads to oxidative stress[4]. It is also involved in glucose-galactose malabsorption, a rare condition in which the body cannot absorb the sugars glucose and galactose, resulting in severe diarrhea[5].

Citations:

[1] https://en.wikipedia.org/wiki/Galactose

[2] https://chem.libretexts.org/Bookshelves/Biological_Chemistry/Supplemental_Modules_%28Biological_Chemistry%29/Carbohydrates/Monosaccharides/Galactose

[3] https://www.britannica.com/science/galactose

[4] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7317321/

[5] https://medlineplus.gov/genetics/condition/glucose-galactose-malabsorption/