What is Galactose Meaning?
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
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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.
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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."
FAQs on Galactose
1. What is galactose and what is its primary function in the body?
Galactose is a type of sugar known as a monosaccharide, meaning it is a single sugar unit. It is an aldohexose and a C-4 epimer of glucose. Its primary functions in the body include serving as a source of energy after being converted to glucose and acting as a crucial building block for glycolipids and glycoproteins found in many tissues, particularly in the brain and nervous system.
2. What is the chemical formula and basic structure of galactose?
The chemical formula for galactose is C₆H₁₂O₆, the same as glucose. Structurally, it exists in two forms: an open-chain form with a carbonyl group at the end, and a more stable cyclic form (a pyranose ring). This cyclic structure is formed when the hydroxyl group on carbon 5 attacks the aldehyde group on carbon 1, creating a hemiacetal.
3. What is the main difference between galactose and glucose?
The main difference between galactose and glucose lies in the stereochemistry at a single carbon atom. They are epimers of each other at carbon-4 (C-4). This means the orientation of the hydroxyl group (–OH) at the C-4 position is different: in glucose it is in one position (equatorial), while in galactose it is in the opposite position (axial).
4. How does galactose combine with glucose to form lactose?
Galactose combines with glucose through a condensation reaction to form the disaccharide lactose, commonly known as milk sugar. This reaction involves the formation of a β-1,4 glycosidic bond between the anomeric carbon of a galactose molecule and the hydroxyl group on carbon 4 of a glucose molecule, releasing one molecule of water in the process.
5. What are some common food sources of galactose?
Galactose is most famously found in milk and dairy products as a component of lactose. It is also found in other foods, sometimes in its free form. Common sources include:
- Dairy products (milk, cheese, yogurt)
- Avocados
- Sugar beets
- Certain gums and mucilages
The body also produces galactose internally for various biological functions.
6. Why is galactose often called 'brain sugar'?
Galactose is often referred to as 'brain sugar' because it is a vital component of glycolipids (like cerebrosides) and glycoproteins. These molecules are essential for the structure and function of the central nervous system, particularly in the brain and the myelin sheaths that insulate nerve cells, ensuring proper nerve impulse transmission.
7. Is galactose a reducing sugar? Explain why.
Yes, galactose is a reducing sugar. This property is due to the presence of a free hemiacetal group in its cyclic structure. This hemiacetal can open up to form an aldehyde group in its open-chain form. The aldehyde group can be readily oxidised, allowing galactose to act as a reducing agent in chemical reactions, such as the Benedict's or Tollens' test.
8. How does the human body metabolise galactose, and what happens if this process fails?
The human body primarily metabolises galactose through a process called the Leloir pathway, which converts it into glucose-1-phosphate, a readily usable energy intermediate. If this pathway fails due to a genetic mutation in one of the key enzymes (like GALT), it results in a serious condition called galactosemia. In galactosemia, galactose builds up to toxic levels in the blood, leading to severe health issues like liver damage, cataracts, and brain damage.
9. Compare the structures of galactose and lactose.
The primary structural difference is that galactose is a monosaccharide (a single sugar unit), while lactose is a disaccharide (composed of two sugar units). Lactose is formed when a molecule of β-galactose is chemically bonded to a molecule of glucose. Therefore, galactose is a building block, whereas lactose is a larger, more complex carbohydrate made from that block.
10. What is the significance of the alpha (α) and beta (β) anomers of galactose?
The alpha (α) and beta (β) anomers describe the orientation of the hydroxyl (–OH) group on the anomeric carbon (C-1) after galactose forms a cyclic structure. In α-galactose, the –OH group is in the axial position (typically drawn pointing down), while in β-galactose, it is in the equatorial position (typically drawn pointing up). This distinction is critically important as it determines the type of glycosidic bonds the sugar can form. For instance, the bond in lactose specifically requires the β-anomer of galactose.
