= N-sulfated heparosan
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<N-sulfated heparosan> is a modified form of <heparosan> in which glucosamine nitrogen positions have undergone deacetylation and <N-sulfation of glucosamine>. The chain retains alternating uronic-acid and amino-sugar positions; the extent of modification must be stated for a particular preparation. N-sulfation changes a nitrogen substituent, rather than adding another sugar residue.<Heparosan-N-sulfate-glucuronate 5-epimerase in IUBMB Enzyme Nomenclature>[EC5.1.3.17: systematic substrate description and Comments]{ref}
The polymer is an informative substrate for <heparosan-N-sulfate-glucuronate 5-epimerase>. Neighboring N-sulfated sugar residues help determine which <glucuronate> positions can be converted into <iduronate>; additional oxygen-linked sulfate substitutions can restrict the reaction. The substrate is therefore a local arrangement within a chain, rather than free <glucuronate> considered in isolation.<Heparosan-N-sulfate-glucuronate 5-epimerase in IUBMB Enzyme Nomenclature>[Comments: required N-substitution and excluded O-sulfated contexts]{ref}
A 2019 study followed conversion of carbon13-labelled <N-sulfated heparosan> with <nuclear magnetic resonance> and compared enzyme variants. The assay connects a change in sugar configuration with a defined polymer substrate, rather than merely measuring total carbohydrate.<Substrate binding mode and catalytic mechanism of human heparan sulfate d-glucuronyl C5 epimerase by Claire Debarnot and others>[Figure1A: labelled-polymer NMR assay and enzyme variants]{ref}
For an original bookkeeping example, a segment with 30 complete two-sugar repeats contains 60 sugar residues before and after modifying ten eligible nitrogens. Epimerizing five uronic-acid positions still leaves 60 residues. Chain extension, nitrogen substitution and uronic-acid configuration are three different operations.
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