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Galactose is a hexose sugar found in the disaccharide lactose. Galactose has six carbons like glucose and differs from glucose only in the stereochemistry of one carbon, C4. The enzymes of carbohydrate metabolism are specific enough that galactose must be changed to glucose before it can enter glycolysis. To convert galactose to glucose, it is first phosphorylated by galactokinase to produce galactose-1-phosphate. Galactose is then exchanged with the glucose group in UDP-glucose to create UDP-galactose and release glucose-1-phosphate. An epimerase enzyme changes the stereochemistry of C4 in UDP-galactose, creating UDP-glucose. In the next round of the transfer reaction, this glucose is released as glucose-1-phosphate. Once released, glucose-1-phosphate is converted to glucose-6-phosphate and can enter glycolysis to generate energy. Mutation of the kinase, the transferase or the epimerase can result in clinical deficiencies in galactose metabolism known as galactosemias. Galactosemias range in severity depending on the nature of the genetic change, and are usually treated through avoidance of galactose in the diet, primarily from lactose in dairy products. (This definition may be outdated - see the DesignNote.)
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