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The term "hexose" may sometimes be used to include the deoxyhexoses, which have one or more hydroxyls (−OH) replaced by hydrogen atoms (−H). It is named as the parent hexose, with the prefix "x-deoxy-", the x indicating the carbon with the affected hydroxyl. Some examples of biological interest are L-Fucose (6-deoxy-L-galactose)
Galactose (/ ɡ ə ˈ l æ k t oʊ s /, galacto-+ -ose, "milk sugar"), sometimes abbreviated Gal, is a monosaccharide sugar that is about as sweet as glucose, and about 65% as sweet as sucrose. [2] It is an aldohexose and a C-4 epimer of glucose. [3] A galactose molecule linked with a glucose molecule forms a lactose molecule.
The galactose permease or GalP found in Escherichia coli is an integral membrane protein involved in the transport of monosaccharides, primarily hexoses, for utilization by E. coli in glycolysis and other metabolic and catabolic pathways (3,4).
Monosaccharides can be classified by the number x of carbon atoms they contain: triose (3), tetrose (4), pentose (5), hexose (6), heptose (7), and so on. Glucose, used as an energy source and for the synthesis of starch, glycogen and cellulose, is a hexose. Ribose and deoxyribose (in RNA and DNA, respectively) are pentose sugars.
After the ingestion of lactose, most commonly from breast milk for an infant or cow milk and any milk from an animal, the enzyme lactase hydrolyzes the sugar into its monosaccharide constituents, glucose and galactose. In the first step of galactose metabolism, galactose is converted to galactose-1-phosphate (Gal-1-P) by the enzyme galactokinase.
In the case of GALT, the His166 residue acts as a potent nucleophile to facilitate transfer of a nucleotide between UDP-hexoses and hexose-1-phosphates. [8] UDP-glucose + E-His ⇌ Glucose-1-phosphate + E-His-UMP; Galactose-1-phosphate + E-His-UMP ⇌ UDP-galactose + E-His [8] Two-step action of galactose-1-phosphate uridylyltransferase.
The systematic name of this enzyme class is UDP-glucose:alpha-D-galactose-1-phosphate uridylyltransferase. Other names in common use include uridyl transferase, hexose-1-phosphate uridylyltransferase, uridyltransferase, and hexose 1-phosphate uridyltransferase. This enzyme participates in galactose metabolism and nucleotide sugars metabolism.
These prefixes are attached to the systematic name of the molecular graph. So for example, D-glucose is D-gluco-hexose, D-ribose is D-ribo-pentose, and D-psicose is D-ribo-hexulose. Note that, in this nomenclature, mirror-image isomers differ only in the ' D '/' L ' prefix, even though all their hydroxyls are reversed.