The atomic resolution structure of bucandin, a novel toxin isolated from the Malayan krait, determined by direct methods.

Bucandin is a novel presynaptic neurotoxin isolated from Bungarus candidus (Malayan krait). It has the unique property of enhancing presynaptic acetylcholine release and represents a family of three-finger toxins with an additional disulfide in the first loop. There are no existing structures from this sub-category of three-finger toxins. The X-ray crystal structure of bucandin has been determined by the Shake-and-Bake direct-methods procedure. The resulting electron-density maps were of outstanding quality and allowed the automated tracing of 61 of the 63 amino-acid residues, including their side chains, and the placement of 48 solvent molecules. The 0.97 A resolution full-matrix least-squares refinement converged to a crystallographic R factor of 12.4% and the final model contains 118 solvent molecules. This is the highest resolution structure of any member of the three-finger toxin family and thus it can serve as the best model for other members of the family. Furthermore, the structure of this novel toxin will help in understanding its unique ability to enhance acetylcholine release. The unique structure resulting from the fifth disulfide bond residing in the first loop improves the understanding of other toxins with a similar arrangement of disulfide bonds.