Solution structure of candoxin, a novel three-finger toxin from the venom of Bungarus candidus

Candoxin, a novel toxin purified from the venom of the Malayan krait (Bungarus candidus), is a 66-residue polypeptide containing five disulfide bridges, and is a reversible antagonist of postjunctional nicotinic acetylcholine receptors of the neuromuscular junction. A family of structures were calculated using a combination of distance geometry and simulated annealing with nOe, hydrogen bond and dihedral angle constraints. After refinement 19 structures, which satisfy the experimental constraints, were obtained. A comparison of each of the final structures with the average structure, gives an RMSD of 1.20 angstrom for backbone atoms. Candoxin has an overall conformation similar to other three-finger toxins with a two-stranded and a three-stranded beta-sheets. In spite of the low sequence homology, the tertiary fold of candoxin closely resembles those of erabutoxin b and cobrotoxin, with the exception of the disulfide bridge in the Loop I. Though candoxin lags the most conserved Tyr at the origin of the central loop it posses the most popular 3-dimensional array of residues (W31, R35 and K49), which is reported to be critical for curaremimetic neurotoxicity. The presence of the kink at the tip of the first loop (Loop I) caused by the C6-C11 disulphide bridge, shortens the Loop and hence may isolate this portion of the molecule and prevent its interference with the rest of the molecule, and facilitate specific interaction with receptor/acceptor protein. These observations suggest that candoxin may bind to acetylcholine receptor with lower affinity compared to other neurotoxins and hence could account for its relatively lower toxicity.