of α-helices and one of β-sheets (Fig. 2B). These two domains of DS-epi1 were modelled on the crystal structure of heparinase II 19. At their boundary, they form a groove, where the substrate is positioned. Some amino acids that are essential for enzyme activity have been identified and a catalytic mechanism has been proposed. Histidine 450 abstracts the C5 proton from one side of the sugar plane of GlcA. This is followed by cleavage or glycosidic linkage between GalNAc and GlcA to generate a C4–C5 double bond containing hexuronic acid intermediate. This structure is finally protonated by histidine 205 adding a hydrogen at the side of the sugar plane that is opposite to the abstraction side. Finally, the glycosidic link is recreated. As a result of the reaction, the carboxyl group has a different spacial orientation in the IdoA epimer than in the starting GlcA. A prerequisite for activity is the presence of at least three of the four N-glycans.
