The Heparin−Ca2+ Interaction: Structure of the Ca2+ Binding Site

Heparin as a polyelectrolyte may exhibit cation-territorial or site-specific binding, depending on the counterion. Thus, while Ca2+ binds specifically, Na+ or Mg2+ do so territorially. We have explored the utility of computational methods that combine the calculation of the interaction energy potential with molecular dynamics simulation for the study of the cation interaction with 1. The computational procedure was designed in order to provide an accurate calculation of the interaction energy and to account simultaneously for the flexibility of the charged heparin side chains. This procedure was able to reproduce the behaviour of Na+, Ca2+, and Mg2+, suggesting that it was the combination of charge and size in Ca2+ that was responsible for its site-specific binding. Other known characteristics of this interaction were also reproduced, as was the displacement of the iduronate conformational equilibrium. The interaction potential results allowed the Ca2+ binding site between consecutive glucosamine and iduronate residues to be identified. A model disaccharide 2 was synthesised and its behaviour in the presence of Ca2+ was studied by NMR spectroscopy, confirming the location of the cation binding site within the glucosamine−iduronate disaccharide. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)