= N-sulfation of glucosamine
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<N-sulfation of glucosamine> attaches a sulfur-containing group at the <nitrogen> of an eligible glucosamine residue. In <heparan sulfate> biosynthesis, this changes nitrogen substitution within a <polysaccharide>; it does not add a new sugar residue or change which carbon atoms form the backbone.
The enzyme <heparan sulfate glucosamine N-sulfotransferase> transfers a group from the activated donor <PAPS> to polymer-associated <glucosamine>. Its official reaction produces the N-sulfated sugar position and adenosine3′,5′-bisphosphate. This specifies both the donor and the modified atom, rather than treating free inorganic <sulfate> as a complete account of the reaction.<Heparan sulfate glucosamine N-sulfotransferase in IUBMB Enzyme Nomenclature>[EC2.8.2.8: accepted name, Reaction and PAPS glossary]{ref}
An N-acetylated position must first lose its <acetyl group> before that nitrogen can undergo the corresponding N-sulfation. Nitrogen substitution also matters downstream: <heparosan-N-sulfate-glucuronate 5-epimerase> recognizes particular neighboring N-sulfated arrangements when changing <glucuronate> into <iduronate>.<Heparosan-N-sulfate-glucuronate 5-epimerase in IUBMB Enzyme Nomenclature>[EC5.1.3.17 Comments: N-deacetylation/N-sulfation and neighboring-residue requirements]{ref}
For an original ideal stoichiometric example, modifying 12 eligible nitrogen positions consumes 12 <PAPS> donor molecules. A chain segment that initially contained 40 sugar residues still contains 40 afterward. Counting sulfur incorporation and counting chain extension therefore measure different processes; counting total sulfur also cannot locate which nitrogen or oxygen positions were modified.
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