= Chondroitin-glucuronate 5-epimerase
<Chondroitin-glucuronate 5-epimerase> changes D-<glucuronate> residues in a <chondroitin> chain into L-<iduronate> residues in a dermatan chain. Its classification is EC 5.1.3.19. The reaction alters carbon-5 stereochemistry without adding a sugar residue or changing the residue's elemental formula.<Chondroitin-glucuronate 5-epimerase in IUBMB Enzyme Nomenclature>[Reaction and accepted name]{ref}
This connects polymer modification to <dermatan sulfate> formation. It is a different enzyme assignment from <heparosan-N-sulfate-glucuronate 5-epimerase>, whose substrate belongs to another polysaccharide pathway. Similar names do not make the polymer substrates interchangeable.<Chondroitin-glucuronate 5-epimerase in IUBMB Enzyme Nomenclature>[Comments, distinction from EC 5.1.3.17]{ref}
In 1982, <A. Malmström> and <L. Åberg> reported distinct uronosyl-epimerase activities associated with dermatan- and heparan-sulfate biosynthesis in microsomal preparations from cultured fibroblasts. The comparison made substrate context experimentally relevant rather than treating all carbon-5 epimerization as one nonspecific activity.<Biosynthesis of dermatan sulphate. Assay and properties of the uronosyl C-5 epimerase by A. Malmström and L. Åberg>[Abstract, two microsomal epimerase activities]{ref}
For a constructed 50-position substrate segment, conversion of ten eligible glucuronate residues yields ten iduronate residues and 40 unchanged positions. The chain still contains 50 positions. A measurement sensitive only to total sugar amount or shared molecular mass cannot establish that this stereochemical conversion occurred.
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