An NMR and molecular modeling study of the site-specific binding of histamine by heparin, chemically modified heparin, and heparin-derived oligosaccharides

The diprotonated form of histamine binds site-specifically to heparin, a hi ghly sulfated 1-->4 linked repeating copolymer comprised predominantly of 2 -O-sulfo-alpha-L-iduronic acid (the I ring) and 3-deoxy-2-sulfamido-6-O-sul fo-alpha-D-glucupyranosyl (the A ring). The binding is mediated by electros tatic interactions. The structural features of histamine and heparin, which are required for the site-specific binding, have been identified from the results of H-1 NMR studies of the binding of histamine by six heparin-deriv ed oligosaccharides and four chemically modified heparins and molecular mod eling studies. The results indicate that the imidazolium ring of diprotonat ed histamine is critical for directing site-specific binding, while the amm onium group increases the binding affinity. The imidazolium ring binds with in a cleft, with the A ring of an IAI triad at the top of the cleft, and th e I rings forming the two sides. The H3 proton of the A ring is in the shie lding cone of the imidazolium ring. The carboxylate group of the I-ring at the reducing end of the IAI triad and possibly the sulfamido group of the A -ring are essential for site-specific binding, whereas the 2-O-sulfate grou p of the I ring and the 6-O-sulfate group of the A ring are not. The result s indicate that histamine binds to the IAI triad with the I rings in the C- 1(4) conformation. Also, the configuration of the carboxylate group is crit ical, as indicated by the absence of site-specific binding of histamine by the related IAG sequence, where G is alpha-D-glucuronic acid. The molecular modeling results indicate that the N1H and N3H protons of the imidazolium ring of site-specifically bound histamine are hydrogen bonded to the carbox ylates of the I rings at the nonreducing and reducing ends of the IAI trisa ccharide sequence.