 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.^[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.^[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.^[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.