AbstractBiosynthesis of heparan sulfate (HS) involves conversion of D-glucuronic acid (GlcA) to L-iduronic acid (IdoA) units catalyzed by glucuronyl C5-epimerase (Hsepi). IdoA units are the favored substrate for 2-O-sulfotransferase (2OST). We used HEK293 cells as a model to investigate the effects of overexpression of these enzymes on HS structure. Overexpression of Hsepi alone resulted in an unexpected increase in HS chain length. A Hsepi point-mutant (Y168A), devoid of catalytic activity, failed to affect chain length. Moreover, the effect of Hsepi overexpression on HS chain length was abolished by simultaneous overexpression of 2OST. These findings raise novel aspects on regulation of HS biosynthesis. We propose a hypothetical enzyme-binding protein (EBP) with distinct, specific and partly overlapping binding sites, the interactions of which will determine levels of enzymes available to the biosynthetic process.