Heparosan-N-sulfate-glucuronate 5-epimerase

Heparosan-N-sulfate-glucuronate 5-epimerase is an enzyme that converts selected D-glucuronate residues into L-iduronate within N-sulfated heparosan. This carbon-5 epimerization changes a stereocenter while preserving the number of sugar residues. The official classification is EC 5.1.3.17.^[ref]

The surrounding chain is part of the substrate. Neighboring N-sulfated groups are required for recognition, and additional sulfate substitution can restrict the reaction. The enzyme should not therefore be described as simply acting on free glucuronate in any chemical setting.^[ref]

A 2019 study by Claire Debarnot and colleagues followed conversion with nuclear magnetic resonance using a carbon-13-labelled polymer. Under the reported conditions, the human enzyme approached approximately $38\%$ iduronate content. The researchers also compared site-directed mutagenesis variants: no activity was detected for two proposed catalytic-residue substitutions even at the stated higher enzyme concentration.^[ref]

For an illustrative 100 eligible sugar positions, a $38\%$ conversion gives 38 iduronate and 62 glucuronate positions, still 100 in total. The experiment changes chemical identity within a chain rather than extending it. The reported fraction belongs to specified conditions; it is not a universal composition of heparan sulfate.