Conformational transitions induced in heparin octasaccharides by binding with antithrombin III

The present study deals with the conformation in solution of two heparin octasaccharides containing the pentasaccharide sequence GlcN_NAc,6S-GlcA-GlcN_NS,3,6S-IdoA_2S-GlcN_NS,6S [AGA*IA; where GlcN_NAc,6S is N-acetylated, 6-O-sulfated α-D-glucosamine, GlcN_NS,3,6S is N,3,6-O-trisulfated α-D-glucosamine and IdoA_2S is 2-O-sulfated IdoA (α-L-iduronic acid)] located at different positions in the heparin chain and focuses on establishing geometries of IdoA residues (IdoA_2S and IdoA) both inside and outside the AGA*IA sequence. AGA*IA constitutes the active site for AT (antithrombin) and is essential for the expression of high anticoagulant and antithrombotic activities. Analysis of NMR parameters [NOEs (nuclear Overhauser effects), transferred NOEs and coupling constants] for the two octasaccharides indicated that between the ¹C₄ and ²S₀ conformations present in dynamic equilibrium in the free state for the IdoA_2S residue within AGA*IA, AT selects the ²S₀ form, as previously shown [Hricovini, Guerrini, Bisio, Torri, Petitou and Casu (2001) Biochem. J. 359, 265–272]. Notably, the ²S₀ conformation is also adopted by the non-sulfated IdoA residue preceding AGA*IA that, in the absence of AT, adopts predominantly the ¹C₄ form. These results further support the concept that heparin-binding proteins influence the conformational equilibrium of iduronic acid residues that are directly or indirectly involved in binding and select one of their equi-energetic conformations for best fitting in the complex. The complete reversal of an iduronic acid conformation preferred in the free state is also demonstrated for the first time. Preliminary docking studies provided information on the octasaccharide binding location agreeing most closely with the experimental data. These results suggest a possible biological role for the non-sulfated IdoA residue preceding AGA*IA, previously thought not to influence the AT-binding properties of the pentasaccharide. Thus, for each AT binding sequence longer than AGA*IA, the interactions with the protein could differ and give to each heparin fragment a specific biological response.