Antibody BCF2 against scorpion toxin Cn2 from Centruroides noxius Hoffmann: Primary structure and three-dimensional model as free Fv fragment and complexed with its antigen

The antibody BCF2 generated against the mammal-specific toxin Cn2 of the sc orpion Centruroides noxius Hoffmann neutralizes the effect of both the toxi n and the venom. We cloned and sequenced the genes coding for the Fv fragme nt of BCF2, A three-dimensional (3D) model of the Fv fragment was generated using a knowledge-based approach. Furthermore, a 3D model of the complex C n2-BCF2 was built using the nuclear magnetic resonance (NMR) structure of C n2 and experimental results on a putative epitope region around the N and C termini, The initial complex conformations were submitted to a new refinem ent procedure of rigid-body energy minimization combined with flexible-side -chain molecular dynamics. The final complex, selected after an extensive e valuation, uses the loop 7-11 as the central part of the epitope, The gener ated complex allows the following conclusions: 1) the neutralizing capacity of BCF2 toward the venom of C, noxius might rather be caused by the high v enom concentration and toxicity of Cn2 than by a broad specificity, 2) the region involved in the binding of Cn2 to the Na+ channel, should overlap wi th the employed epitope region, and 3) contact residues SerL91, AsnL92, Leu H50, AspH56, TyrH95, and TyrH98 of BCF2 are candidates for mutations to bro aden its specificity. (C) 1999 Wiley-Liss, Inc.