GalactoseEdit
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Galactose is a naturally occurring simple sugar (monosaccharide) of the aldohexose class. It is the C-4 epimer of glucose, differing in the orientation of the hydroxyl group at carbon 4. In nature, galactose occurs predominantly as a constituent of lactose, the disaccharide that provides the main dietary source of galactose in mammals. Beyond lactose, galactose is present in glycoproteins, glycolipids, and various plant and microbial polysaccharides. In human nutrition, galactose enters circulation mainly after lactose digestion by lactase in the small intestine; in metabolic terms, it can be converted to glucose-1-phosphate via the Leloir pathway and fed into glycolysis or glycogen synthesis. For historical and clinical reasons, galactose metabolism is a classic example used to illustrate how specific enzymatic steps govern metabolic flux and hereditary disease. See also monosaccharide and glucose.
Biochemistry and metabolism
Structure and properties
Galactose is a hexose sugar and an aldose, so its backbone consists of six carbons with an aldehyde group at carbon 1. As the C-4 epimer of glucose, it shares much of glucose’s chemistry but differs in the orientation of the hydroxyl group on carbon 4. This seemingly small difference has important consequences for how galactose is transported, phosphorylated, and metabolized in cells. For context, galactose is related to other sugars such as fructose and glucose within the broader family of carbohydrates.
Transport and uptake
Galactose is taken up by cells from the circulation via specific transporters and, in the gut, by sodium-dependent transporters. The primary intestinal uptake involves sodium-dependent glucose transporters, which also move galactose alongside glucose, and other facilitated transporters contribute to cellular entry in different tissues. See for example SGLT1 and related transport systems. Once inside cells, galactose can be phosphorylated and routed into metabolic pathways.
Leloir pathway (galactose metabolism)
The canonical route for metabolizing dietary galactose in animals is the Leloir pathway, named after the scientists who described the series of enzymatic steps. The core steps are:
- Galactose is phosphorylated by galactokinase to galactose-1-phosphate. See galactokinase.
- Galactose-1-phosphate is converted to glucose-1-phosphate and UDP-galactose via galactose-1-phosphate uridylyltransferase. See GALT (galactose-1-phosphate uridylyltransferase).
- UDP-galactose is epimerized to UDP-glucose by UDP-galactose 4'-epimerase. See GALE (UDP-galactose 4'-epimerase).
- Glucose-1-phosphate is isomerized to glucose-6-phosphate and can enter glycolysis or glycogen synthesis. See glucose-6-phosphate and glycolysis.
This pathway integrates galactose metabolism with central carbohydrate metabolism and is essential for energy production and for providing sugar donors (like UDP-galactose) for biosynthetic processes, including glycosylation of proteins and lipids. For broader context, see UDP-galactose and glycoprotein biosynthesis.
Metabolic fates and roles
Beyond the Leloir pathway, galactose serves as a substrate for the synthesis of UDP-galactose, a key donor in glycosylation reactions that build glycoproteins and glycolipids. In mammals, galactose-derived intermediates can contribute to energy production, glycogen storage, and the elaboration of extracellular and cell-surface carbohydrates. In bacteria and plants, galactose residues are common components of polysaccharides and glycoconjugates, underscoring galactose’s wide biological role.
Occurrence and dietary sources
In humans, galactose is most commonly encountered as part of lactose, the disaccharide formed from glucose and galactose and abundant in mammalian milk. Lactose hydrolysis by lactase yields glucose and galactose for absorption. Dietary sources and metabolism link galactose to the broader family of milk-derived carbohydrates. Beyond lactose, galactose is present as a subunit in various glycoproteins and glycolipids, and it forms part of plant cell wall polysaccharides such as galactans and pectin components. See lactose and glycoprotein for related topics.
The availability of galactose in the diet has implications for certain inherited metabolic disorders. In particular, conditions that impair galactose metabolism require dietary management to minimize galactose intake. See galactosemia for more on clinical implications and management.
Clinical significance
Galactose handling and disorders
A cluster of inherited metabolic disorders affects galactose metabolism, most notably galactosemia. Classic galactosemia arises from deficiencies in galactose-1-phosphate uridylyltransferase (GALT) and leads to impaired conversion of galactose-1-phosphate to glucose-1-phosphate, with consequences for infant feeding and liver, renal, and neurodevelopmental outcomes. Other disorders include galactokinase deficiency (GALK1 deficiency) and epimerase deficiency (GALE deficiency). Each disruption alters how galactose is processed and can produce distinct clinical manifestations, ranging from cataracts and milder metabolic disturbances to life-threatening illness in neonates if not addressed promptly. See galactosemia and the specific enzyme deficiencies galactokinase and GALE.
Diagnosis and management
Newborn screening programs often test for galactosemia or related metabolic abnormalities, enabling early dietary intervention. Management typically involves strict dietary restriction of galactose and lactose, especially in severe forms, along with nutritional support to prevent deficiencies and promote growth. In clinical practice, long-term care emphasizes monitoring liver function, growth, and neurodevelopment, as well as addressing potential complications such as cataracts in relevant subtypes. See newborn screening and lactose for connected topics.
History
Galactose and its metabolism have a long history in biochemistry as a model system for understanding nucleotide-sugar donors, enzyme specificity, and metabolic regulation. The discovery and characterization of the Leloir pathway drew on work in carbohydrate chemistry and metabolism that linked dietary sugars to cellular energy and biosynthesis. See history of carbohydrate chemistry for broader context.