Beta-1,4-galactosyltransferase 7 also known as galactosyltransferase I is an enzyme that in humans is encoded by the B4GALT7 gene. [5] [6] [7] Galactosyltransferase I catalyzes the synthesis of the glycosaminoglycan-protein linkage in proteoglycans. [8] Proteoglycans in turn are structural components of the extracellular matrix that is found between cells in connective tissues.
Galactosyltransferase I is one of seven β-1,4-galactosyltransferase (β4GalT) enzymes. These enzymes are type II membrane-bound glycoproteins that appear to have exclusive specificity for the donor substrate UDP-galactose; all transfer galactose in a β-1,4 linkage to similar acceptor sugars: GlcNAc, Glc, and Xyl. Each beta4GalT has a distinct function in the biosynthesis of different glycoconjugates and saccharide structures. As type II membrane proteins, they have an N-terminal hydrophobic signal sequence that directs the protein to the Golgi apparatus and which then remains uncleaved to function as a transmembrane anchor. By sequence similarity, the beta4GalTs form four groups: β4GalT1 and β4GalT2, β4GalT3 and β4GalT4, β4GalT5 and β4GalT6, and β4GalT7. The enzyme encoded by this gene attaches the first galactose in the common carbohydrate-protein (GlcA-β-1,3-Gal-β-1,3-Gal-β-1,4-Xyl-beta1-O-Ser) linkage found in proteoglycans. Manganese is required as a cofactor. This enzyme differs from the other six beta4GalTs because it lacks the conserved β4GalT1-β4GalT6 Cys residues and it is located in cis-Golgi instead of trans-Golgi. [7]
Mutations in the B4GALT7 gene that result in a defective galactosyltransferase I enzyme with reduced or absent activity are associated with Spondylodysplastic, formerly progeroid type Ehlers-Danlos syndrome. [6] [8] [9] [10] The reduced activity of B4GALT7 is associated with a reduced substitution of the proteoglycans decorin and biglycan with glycosaminoglycan carbohydrate chains, and with alterations in heparan sulfate biosynthesis, resulting in delayed wound repair, altered migration, adhesion and contractility of patient fibroblasts. [11] [12] Since mutations in B4GALT7 impair a glycosylation pathway, the resulting subtype of Ehlers-Danlos syndrome may be considered a congenital disorder of glycosylation (CDG), according to the new CDG nomenclature.
Mutations in B4GALT7 cause Larsen syndrome. [13]
Proteoglycans are proteins that are heavily glycosylated. The basic proteoglycan unit consists of a "core protein" with one or more covalently attached glycosaminoglycan (GAG) chain(s). The point of attachment is a serine (Ser) residue to which the glycosaminoglycan is joined through a tetrasaccharide bridge. The Ser residue is generally in the sequence -Ser-Gly-X-Gly-, although not every protein with this sequence has an attached glycosaminoglycan. The chains are long, linear carbohydrate polymers that are negatively charged under physiological conditions due to the occurrence of sulfate and uronic acid groups. Proteoglycans occur in connective tissue.
Glycosaminoglycans (GAGs) or mucopolysaccharides are long linear polysaccharides consisting of repeating disaccharide units. The repeating two-sugar unit consists of a uronic sugar and an amino sugar, with the exception of keratan, where in the place of the uronic sugar it has galactose. Because GAGs are highly polar and attract water, they are used in the body as a lubricant or shock absorber. Mucopolysaccharidoses are a group of metabolic disorders in which abnormal accumulations of glycosaminoglycans occur because of enzyme deficiencies. GAGs are found in vertebrates, invertebrates and bacteria.
Type III Collagen is a homotrimer, or a protein composed of three identical peptide chains (monomers), each called an alpha 1 chain of type III collagen. Formally, the monomers are called collagen type III, alpha-1 chain and in humans are encoded by the COL3A1 gene. Type III collagen is one of the fibrillar collagens whose proteins have a long, inflexible, triple-helical domain.
Galactosyltransferase is a type of glycosyltransferase which catalyzes the transfer of galactose. An example is B-N-acetylglucosaminyl-glycopeptide b-1,4-galactosyltransferase.
Collagen alpha-1(V) chain is a protein that in humans is encoded by the COL5A1 gene.
In enzymology, a galactosylxylosylprotein 3-beta-galactosyltransferase is an enzyme that catalyzes the chemical reaction
In enzymology, a glycoprotein-N-acetylgalactosamine 3-beta-galactosyltransferase is an enzyme that catalyzes the chemical reaction
In enzymology, a xylosylprotein 4-beta-galactosyltransferase is an enzyme that catalyzes the chemical reaction
Beta-1,4-galactosyltransferase 1 is an enzyme that in humans is encoded by the B4GALT1 gene.
UDP-GalNAc:beta-1,3-N-acetylgalactosaminyltransferase 1 is an enzyme that in humans is encoded by the B3GALNT1 gene.
Galactosylgalactosylxylosylprotein 3-beta-glucuronosyltransferase 3 is an enzyme that in humans is encoded by the B3GAT3 gene.
Xylosyltransferase 2 is an enzyme that in humans is encoded by the XYLT2 gene.
Beta-1,4-galactosyltransferase 2 is an enzyme that in humans is encoded by the B4GALT2 gene.
Beta-1,4-galactosyltransferase 5 is an enzyme that in humans is encoded by the B4GALT5 gene.
Beta-1,3-galactosyltransferase 5 is an enzyme that in humans is encoded by the B3GALT5 gene.
Beta-1,4-galactosyltransferase 4 is an enzyme that in humans is encoded by the B4GALT4 gene.
Beta-1,4-galactosyltransferase 3 is an enzyme that in humans is encoded by the B4GALT3 gene.
Beta-1,3-galactosyltransferase 4 is an enzyme that in humans is encoded by the B3GALT4 gene.
Carbohydrate sulfotransferase 14 is an enzyme that in humans is encoded by the CHST14 gene.
Core 1 synthase, glycoprotein-N-acetylgalactosamine 3-beta-galactosyltransferase, 1, also known as C1GALT1, is an enzyme which in humans is encoded by the C1GALT1 gene.
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