Is N ‐sulfation just a gateway modification during heparan sulfate biosynthesis?

Several biologically important growth factor–heparan sulfate (HS) interactions are regulated by HS sulfation patterns. However, the biogenesis of these combinatorial sulfation patterns is largely unknown. N ‐Deacetylase/ N ‐sulfotrasferase (NDST) converts N ‐acetyl‐ d ‐glucosamine residues to N ‐sulfo‐ d ‐glucosamine residues. This enzyme is suggested to be a gateway enzyme because N ‐sulfation dictates the final HS sulfation pattern. It is known that O ‐sulfation blocks C5‐epimerase, which acts immediately after NDST action. However, it is still unknown whether O ‐sulfation inhibits NDST action in a similar manner. In this article we radically change conventional assumptions regarding HS biosynthesis by providing in vitro evidence that N ‐sulfation is not necessarily just a gateway modification during HS biosynthesis.