Synthesis of conformationally locked L-iduronic acid derivatives: Direct evidence for a critical role of the skew-boat S-2(0) conformer in the activation of antithrombin by heparin

• Published in: Chemistry : a European journal Vol. 7; no. 22; pp. 4821 - 4834
• Main Authors: Das, SK, Mallet, JM, Esnault, J, Driguez, PA, Duchaussoy, P, Sizun, P, Herault, JP, Herbert, JM, Petitou, M, Sinay, P
• Format: Journal Article
• Language: English
• Published: WEINHEIM Wiley 19.11.2001
• Subjects: Chemistry; Chemistry, Multidisciplinary; Physical Sciences; Science & Technology; carbohydrates; MAST-CELLS; HIGH-AFFINITY; glycosylation; 2,5-DIOXABICYCLO<2.2.2>OCTANE DERIVATIVES; heparin; PENTASACCHARIDE; DERMATAN SULFATE; iduronic acid; H-1-NMR SPECTRA; BIOLOGICAL PROPERTIES; antithrombin; FORCE-FIELD; CHEMICAL-SYNTHESIS; BINDING SEQUENCE
• ISSN: 0947-6539; 1521-3765

We have used organic synthesis to understand the role of L-iduronic acid conformational flexibility in the activation of antithrombin by heparin. Among known synthetic analogues of the genuine pentasaccharidic sequence representing the antithrombin binding site of heparin, we have selected as a reference compound the methylated anti-factor Xa pentasaccharide 1. As in the genuine original fragment, the single L-iduronic acid moiety of this molecule exists in water solution as an equilibrium between three conformers C-1(4), C-4(1) and S-2(0). We have thus synthesized three analogues of 1, in which the L-iduronic acid unit is locked in one of these three fixed conformations. A covalent two atom bridge between carbon atoms two and five of L-iduronic acid was first introduced to lock the pseudorotational itinerary of the pyranoid ring around the S-2(0) form. A key compound to achieve this connection was the D-glucose derivative 5 in which the H-5 hydrogen atom has been replaced by a vinyl group, which is a progenitor of the carboxylic acid. Selective manipulations of this molecule resulted in the S-2(0)-type pentasaccharide 23. Starting from the D-glucose derivative 28, a covalent two atom bridge was now built up between carbon atoms three and five to lock the L-iduronic acid moiety around the C-1(4) chair form conformation, and the C-1(4)-type pentasaccharide 43 was synthesized. Finally the L-iduronic acid containing disaccharide 58 which, due to the presence of the methoxymethyl substituent at position five adopts a C-4(1) conformation, was directly used to synthesize the C-4(1)-type pentasaccharide 61. The locked pentasaccharide 23 showed about the same activity as the reference compound 1 in an antithrombin-mediated anti-Xa assay, whereas the two pentasaccharides 43 and 61 displayed very low activity. These results clearly establish the critical importance of the S-2(0) conformation of L-iduronic acid in the activation of antithrombin by heparin.