= 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.<Heparosan-N-sulfate-glucuronate 5-epimerase in IUBMB Enzyme Nomenclature>[Reaction and Comments]{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.<Heparosan-N-sulfate-glucuronate 5-epimerase in IUBMB Enzyme Nomenclature>[Comments, substrate context and exclusions]{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.<Substrate binding mode and catalytic mechanism of human heparan sulfate d-glucuronyl C5 epimerase by Claire Debarnot and others>[Overall Structures of hGlce; Figure 1A and legend]{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>.
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