Galactose Structure Properties Reactions and Biological Role

Galactose (milk sugar), also known as Gal, is a monosaccharide sugar that is around as sweet as glucose and 65 percent sweeter than sucrose. It's an aldohexose and a glucose C-4 epimer. Lactose is made up of galactose molecules bound to glucose molecules. Galactose has the molecular formula C₆H₁₂O₆. Galactose is a monosaccharide and glucose epimer. The hydrogen bond donor and acceptor have property values of 5 and 6, respectively. Galactose formula is C₆H₁₂O₆.

Galactan is a polymeric form of galactose found in hemicellulose that makes up the centre of galactans, a group of naturally occurring polymeric carbohydrates.

Galactose Structure 

[Image will be Uploaded Soon]

Galactose structure comes in two forms: open-chain and cyclic. There is a carbonyl at the end of the open-chain shape. Its structure can be formed with the help of the galactose formula. 

Two of the isomers have a pyranose (six-membered) ring, while the other two have a furanose (five-membered) ring. Galactofuranose is found in bacteria, fungi, and protozoa, and its exocyclic 1,2-diol is recognised by the putative chordate immune lectin intelectin. Since the transition from the open-chain to the cyclic form requires the formation of a new stereocenter at the site of the open-chain carbonyl, the cyclic form has two anomers, alpha and beta. The alcohol group is in the equatorial position in the beta form, while the alcohol group is in the axial position in the alpha form.

Properties of Galactose

• The molecular formula of galactose is C₆H₁₂O₆.

• The molecular mass of galactose is 180.156 g/mol.

• The density of galactose is 1.5 g.cm⁻³.

• Melting point of galactose is 168-170 ⁰C.

Conversion of Galactose and Glucose to Lactose

Galactose is a monosaccharide simple form of sugar. Lactose is a disaccharide formed when glucose (monosaccharide sugar) is mixed with it in a condensation reaction. Lactase and -galactosidase are enzymes that catalyse the hydrolysis of lactose to glucose and galactose. In Escherichia coli, the lac operon produces the latter.

Lactose is mainly present in milk and milk products in nature. As a result, lactose can be found in a variety of foods made from dairy-derived ingredients. Galactose metabolism, or the conversion of galactose to glucose, is carried out by three major enzymes in a process known as the Leloir pathway. Galactokinase (GALK), galactose-1-phosphate uridyltransferase (GALT), and UDP-galactose-4'-epimerase are the enzymes described in the order of the metabolic pathway (GALE).

Hexoneogenesis converts glucose to galactose in human lactation, allowing the mammary glands to secrete lactose. The majority of lactose in breast milk is made from galactose picked up from the blood, with just 356 percent coming from de novo synthesis. Glycerol also contributes to the development of galactose in the mammary gland.

Metabolism of Galactose

Glucose is more soluble than galactose, and it is less likely to form nonspecific glycoconjugates, which are molecules containing at least one sugar bound to a protein or lipid. Many people believe that this is why a mechanism for converting galactose to glucose quickly has been strongly conserved over many organisms.

The Leloir pathway is the primary pathway for galactose metabolism; however, humans and other animals have been found to have multiple alternative pathways, such as the De Ley Doudoroff Pathway. The Leloir pathway is the final step in a two-step process that transforms -D-galactose to UDP-glucose. The enzyme mutarotase converts -D-galactose to -D-galactose at the start of the process (GALM). The Leloir pathway then converts -D-galactose to UDP-glucose with the aid of three main enzymes: 

• -D-galactose is phosphorylated by galactokinase (GALK), which converts it to galactose-1-phosphate.

• Gal-1-P; Galactose-1-phosphate uridyltransferase (GALT) converts UDP-glucose to UDP-galactose by transferring a UMP group from UDP-glucose to Gal-1-P.

• Finally, UDP galactose-4'-epimerase (GALE) completes the process by interconverting UDP-galactose and UDP-glucose.

Sources of Galactose

Dairy items, avocados, sugar beets, and other gums and mucilages all contain galactose. It is also produced by the body, where it is found in glycolipids and glycoproteins in a variety of tissues; and it is a by-product of the third-generation ethanol manufacturing process (from macroalgae).

Effects of Galactose

Chronic systemic exposure to D-galactose accelerates senescence in mice, rats, and Drosophila (aging). In rodents, large doses of D-galactose (120 mg/kg) have been shown to reduce sperm concentration and motility, and it has been widely used as an ageing model when administered subcutaneously. A potential correlation between galactose in milk and ovarian cancer has been indicated by two studies. Other studies have found no connection, even when galactose metabolism is impaired. More recently, a pooled study conducted by the Harvard School of Public Health found no direct link between lactose-containing foods and ovarian cancer, as well as statistically insignificant increases in risk for lactose intake of 30 g/day. To determine potential risks, further research is needed.

Galactose may play a role in the treatment of focal segmental glomerulosclerosis, according to some ongoing research (a kidney disease resulting in kidney failure and proteinuria). This effect is most likely due to galactose binding to the FSGS factor.

Galactose is a part of the antigens found on blood cells that determine blood group in the ABO system. There are two monomers of galactose on the antigens of O and A antigens, while there are three monomers of galactose on the antigens of B antigens.

Galactose-alpha-1,3-galactose (alpha-gal), a disaccharide made up of two galactose units, has been identified as a possible allergen found in mammal meat. Lone star tick bites can cause an alpha-gal allergy.

Diagnostic Test for Galactose

When red cell galactose-1-phosphate is elevated but GALT is common, GALE deficiency should be suspected. An increased total blood galactose level with usual GALT activity can result in an irregular result on newborn screening. Epimerase in erythrocytes is used to validate the diagnosis. Epimerase activity is decreased in heterozygous parents, which may aid in the assessment. More research into GALE activity in transformed lymphoblasts and red cell galactose-1-phosphate on and off dietary galactose can help to better characterise the disorder. GALE gene sequencing has been the most rapid way of deciding whether or not babies at risk are affected in families with the extreme type of GALE deficiency.

Metabolism of Galactose

In the liver, gal is primarily converted to glucose l-phosphate and then to glucose 6-phosphate. A small alternative pathway exists, but it has yet to be identified. Galactokinase phosphorylation is the first and most important step. The enzyme exists in two genetically distinct isoforms, each of which has a different tissue distribution. Cataracts can develop in childhood or early adulthood as a result of galactitol accumulation in the lenses of people who have a defective galactokinase 1. The conversion of UDP by UDP-glucose-hexose-i-phosphate uridylyltransferase is the next step in Gal metabolism. Epimerized UDPGal to UDP-glucose with UDP-glucose-4′-epimerase. This acts like an autocatalytic process, with a net conversion of Gal 1-phosphate to glucose 1-phosphate, since UDP-glucose provides the UDP for the next Gal 1-phosphate molecule. Phosphoglucomutase, a magnesium-dependent enzyme, converts glucose 1-phosphate to the easily metabolised intermediate glucose 6-phosphate. In the presence of Gal abundance, NADPH-dependent aldehyde reductase will reduce Gal to galactitol as an alternative.

[Image will be Uploaded Soon]

Some Important Points about Galactose

• Galactose is a monosaccharide.

• The chemical name or IUPAC name of galactose is (3R,4S,5R,6R)-6-(hydroxymethyl)oxane-2,3,4,5-tetrol.

• Galactose is reducing sugar.

• Galactose is a glucose optical isomer.

• Galactosemia is a genetically inherited failure to properly break down galactose due to a mutation in one of the Leloir pathway enzymes. As a consequence, even small amounts of glucose are toxic to galactosemics.

Did You Know?

• Galactose is a simple sugar that is naturally converted to energy in the liver. This sugar is commonly found in human diets and serves a variety of functions. The essential Galactose function is the production of energy since it is a precursor to glucose production.

• The metabolic conversion of D-glucose to D-galactose provides the galactose needed by the human body. It's a key component of the glycolipids that occur in the brain and in nerve cells' myelin sheaths. As a result, it's also known as "brain sugar."