Evaluation of critical groups required for the binding of heparin to antithrombin.

Abstract

The quantitative importance of various monosaccharide residues of an octasaccharide domain of porcine mucosal heparin that are responsible for the binding of this oligosaccharide to human and bovine antithrombin was examined. Different fragments of the octasaccharide were prepared by enzymatic digestion and the avidities of these oligoaccharides for antithrombin were determined by equilibrium dialysis. The non-reducing-end and the reducing-end tetrasaccharides evidently contribute equally to the binding energy of the octasaccharide. The O6-sulfate group of the N-acetyl glucosamine moiety within the nonreducing-end tetrasaccharide is responsible for .apprxeq. 45% of the binding energy of the octasaccharide. Neither the 2 non-sulfated uronic acid groups that flank this residue nor the N-sulfated glucosamine residue on the reducing end of this tetrasaccharide sequence that bears the unique O3-sulfate substituent contribute significantly to the binding energy of the octasaccharide. The lack of sulfation of the 2 uronic acid moieties within the nonreducing-end tetrasaccharide may be required to permit the N-acetyl glucosamne O6-sulfate group to interact with a specific region on the anti-thrombin molecule. The possibility that the O3-sulfate group plays an important role in orienting this O6-sulfate group within the nonreducing-end tetrasaccharide cannot be excluded.