Crystallization and preliminary X-ray analysis of candoxin, a novel reversible neurotoxin from the Malayan krait Bungarus candidus

Candoxin, a novel three-finger toxin from Bungarus candidus, is a reversible antagonist of muscle (αβγδ) but a poorly reversible antagonist of neuronal α7 nicotinic acetylcholine receptors. It has a molecular weight of 7344 Da, with 66 amino-acid residues including ten half-cystines. The fifth disulfide bridge is located at the tip of loop I (Cys6-Cys11) instead of in loop II as found in other α-neurotoxins. Interestingly, candoxin lacks the segment cyclized by the fifth disulfide bridge at the tip of the middle loop of long-chain neurotoxins, which was reported to be critical for binding to α7 receptors. As a first step to determining its three-dimensional structure, candoxin was crystallized by the hanging-drop vapour-diffusion technique in conditions around 1.5 M sodium chloride, 10%(v/v) ethanol. The crystals formed belonged to the hexagonal system, space group P6₂22, with unit-cell parameters a = 54.88, b = 54.88, c = 75.54 Å, α = β = 90, γ = 120°, and diffract to a resolution of 1.80 Å. The crystallographic asymmetric unit contains one molecule of candoxin, with an estimated solvent content of 44.6%. Attempts to solve these structures by molecular-replacement methods have not been successful and a heavy-atom derivative search has been initiated.