title: Epimerization of D-glucose to L-galactose during the biosynthesis of a sulfated L-galactan in the ascidian tunic.
abstract: The sulfated polysaccharides occurring in the tunic of ascidians are unique among known sulfated polysaccharides in that their major constituent sugar is galactose, which occurs exclusively in the L-enantiomeric form. In vitro incorporation experiments using tunic slices incubated with 14C-labeled sugars revealed that cells from this tissue epimerize D-isomers of hexose into L-galactose during the biosynthesis of their constituent polysaccharides. Compared with other hexoses, the precursor D-[14C]glucose has the highest rate of incorporation and produces the highest proportion of L-galactose units. This metabolic pathway is distinct from the epimerization of D-mannose to L-galactose through its guanosine 5'-diphosphate nucleotide, described previously in an alga and in a snail. Therefore, the epimerization of D-glucose to L-galactose in the ascidian tunic occurs through a novel metabolic route, which involves inversion of the configuration of carbon atoms 2, 3, and 5 of the hexosyl moieties.
authors.affiliation: Departamento de Bioquímica, Universidade Federal do Rio de Janeiro, Brazil.
authors.forename: P A
authors.initials: PA
authors.lastname: Mourão
mesh_terms: D000818:Animals; D005685:Galactans / Q000378:metabolism*; D005690:Galactose / Q000378:metabolism*; D005947:Glucose / Q000378:metabolism*; D007700:Kinetics; D008359:Mannose-6-Phosphate Isomerase / Q000378:metabolism*; D011134:Polysaccharides / Q000378:metabolism*; D014561:Urochordata / Q000201:enzymology / Q000378:metabolism*
chemical_list: D005685:Galactans; D011134:Polysaccharides; D008359:Mannose-6-Phosphate Isomerase; D005947:Glucose; D005690:Galactose