Structures of heparin-derived disaccharide bound to cobra cardiotoxins: Context-dependent conformational change of heparin upon binding to the rigid core of the three-fingered toxin

Glycosaminoglycans (GAGs) have been suggested to be a potential target for cobra cardiotoxin (CTX) with high affinity and specificity via a cationic b elt at the concave surface of the polypeptide. The interaction of GAGs, suc h as high-molecular weight heparin, with CTXs not only can induce aggregati on of CTX molecules but also can enhance their penetration into membranes. The binding of short chain heparin, such as a heparin-derived disaccharide [Delta UA2S(1 -->4)-alpha -D-GlcNS6S], to CTX A3 from Taiwan cobra (Naja at ra), however, will not induce aggregation and was, therefore, investigated by high-resolution H-1 NMR. A novel heparin binding site on the convex side of the CTX, near the rigid disulfide bond-tightened core region of Cys38, was identified due to the observation of intermolecular NOEs between the pr otein and carbohydrate. The derived carbohydrate conformation using complet e relaxation and conformational exchange matrix analysis (CORCEMA) of NOEs indicated that the glycosidic linkage conformation and the ring conformatio n of the unsaturated uronic acid in the bound state depended significantly on the charge context of CTX molecules near the binding site. Specifically, comparative binding studies of several heparin disaccharide homologues wit h two CTX homologues (CTX Ty from Naja nigricollis and CTX A3) indicated th at the electrostatic interaction of N-sulfate of glucosamine with NH(3)(+)z eta of Lys12 and of the 2-O-sulfate of the unsaturated uronic acid with NH( 3)(+)zeta of Lys5 played an important role. These results also suggest a mo del on how the CTX-heparin interaction may regulate heparin-induced aggrega tion of the toxin via the second heparin binding site.