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Galactose is transferred via several linkages to acceptor structures by galactosyltransferase enzymes. In prokaryotes, galactose is mainly found on lipopolysaccharides and capsular polysaccharides. In eukaryotes, galactosyltransferases, which are localized in the Golgi apparatus, are involved in the formation of several classes of glycoconjugates and in lactose biosynthesis. Although they sometimes catalyze identical reactions, prokaryotic and eukaryotic galactosyltransferases share only little structural similarities. In mammals, 19 distinct galactosyltransferase enzymes have been characterized to date. These enzymes catalyze the transfer of galactose via β1-4, β1-3, α1-3 and α1-4 linkages.
Figure 1. Protein structure of galactose.
In the human body, most of the ingested galactose is converted to glucose, which can provide 4.1 kilocalories per gram of energy, which is about the same as sucrose. Galactose can bind to glucose to make lactose (in breast milk), to lipids to make glycolipids (for example, molecules that constitute blood groups A, B and AB), or to proteins to make glycoproteins (for example, in cell membranes).
The main dietary source of galactose is lactose from milk and yogurt, which is digested to galactose and glucose. Foods containing small amounts of free galactose include low-lactose or lactose-free milk, certain yogurts, cheeses, creams, ice creams and other foods artificially sweetened with galactose. Plain natural foods (fruits, vegetables, nuts, grains, fresh meats, eggs, milk) usually contain less than 0.3 g galactose per serving.
Galactose is absorbed in the small intestine by the same mechanism as glucose, that is by the help of SGLT-1 and GLUT-2 transport proteins in the small intestinal lining.
Galactosyltransferase is a glycosyltransferase that catalyzes the transfer of galactose. An example is B-N-acetylglucosaminyl-glycopeptide b-1,4-galactosyltransferase.
The biosynthesis of disaccharides, oligosaccharides and polysaccharides involves the action of hundreds of different glycosyltransferases. These enzymes catalyze the transfer of sugar moieties from activated donor molecules to specific acceptor molecules, forming glycosidic bonds. The classification of glycosyltransferases into different sequence-based families using nucleotide diphosphate sugars, nucleotide monophosphate sugars and sugar phosphate esters and related proteins has been described. This category can be found on the CAZy (Carbohydrate-Active Enzyme) website. It is expected that the same three-dimensional folding will occur in every household. Because the 3-D structure is more conserved than the sequence, several families defined based on sequence similarity may have similar 3-D structures, thus forming a "clan".
Figure 2. Galactosyltransferase.
Glycosyltransferase family 31 comprises enzymes with a number of known activities; N-acetyllactosaminide beta-1,3-N-acetylglucosaminyltransferase; beta-1,3-galactosyltransferase; fucose-specific beta-1,3-N-acetylglucosaminyltransferase; globotriosylceramide beta-1,3-GalNAc transferase.
β-1, 4-galactosyltransferase is different from other glycosyltransferases. It is located on the mature surface of the intracellular Golgi apparatus and the plasma membrane of the cell surface, and behaves differently In the Golgi body, GalTase, like other glycosyltransferases, participates in the biosynthesis of membrane-bound and secreted sugar complexes. On the cell surface, GalTase participates in cell-cell and cell-matrix interactions as a recognition molecule.
There are more long-form GalTase transcripts encoding surface GalTase in spermatogenic cells than short-form ones, which is consistent with the results of indirect immunofluorescence detection of GalTase on the surface of spermatogenic cells. During early spermatogenesis, cell surface GalTase may be Like other cell types, promote adhesion between spermatogenic cells and supporting cells.
The expression of GalTase is regulated by the allele at the end of the T/t complex on mouse chromosome 17. The specific activity of GalTase on the surface of T/t mutant cells is 4 times that of normal cells, while embryos homozygous for T/t mutations cannot experience normal mulberries. Stage and blastocyst formation, so the cell surface GalTase may be involved in preimplantation interaction between embryonic cells and early development.
The GalTase on the sperm head of the mouse can bind to the oligosaccharide residues on the zona pellucida (ZP), and the oligosaccharide chain of the zona pellucida glycoprotein ZP3 grants the ability of the zona pellucida to bind sperm. Further studies have confirmed that GalTase and ZP3 is actually a complementary gamete receptor that mediates sperm-egg binding.
Cell surface GalTase has important biological functions in cell adhesion, migration, signal transduction and proliferation control, and the molecular mechanism of its action is worthy of in-depth study.
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