An alternative fit : Octasaccharide (OCTA) in unbound form (figure top) and in complex with antithrombin (bottom, top and side views) is presented. OCTA includes the active GlcNS,6S‐IdoA2S ‐GlcNS,3S,6S‐IdoA2S‐GlcNS,6S sequence as an alternative to the canonical GlcNAc/NS,6S‐GlcA ‐GlcNS,3S,6S‐IdoA2S‐GlcNS,6S AT‐binding pentasaccharide. ‘Induced fit’ conformational change of the glycosidic backbone and the two IdoA2S moieties surrounding the central 3‐O‐sulfo‐glucosamine are detected.
Heparin binds to and activates antithrombin (AT) through a specific pentasaccharide sequence, in which a trisaccharide subsite, containing glucuronic acid (GlcA), has been considered as the initiator in the recognition of the polysaccharide by the protein. Recently it was suggested that sulfated iduronic acid (IdoA2S) could replace this “canonical” GlcA. Indeed, a heparin octasaccharidic sequence obtained by chemoenzymatic synthesis, in which GlcA is replaced with IdoA2S, has been found to similarly bind to and activate antithrombin. By using saturation‐transfer‐difference (STD) NMR, NOEs, transferred NOEs (tr‐NOEs) NMR and molecular dynamics, we show that, upon binding to AT, this IdoA2S unit develops comparable interactions with AT as GlcA. Interestingly, two IdoA2S units, both present in a 1C4‐2S0 equilibrium in the unbound saccharide, shift to full 2S0 and full 1C4 upon binding to antithrombin, providing the best illustration of the critical role of iduronic acid conformational flexibility in biological systems.
Wiley: Chemistry – A European Journal: Table of Contents
Authors: Stefano Elli, Eduardo Stancanelli, Zhangjie Wang, Maurice Petitou, Jian Liu, Marco Guerrini