Structure of scorpion toxin variant-3 at 1.2 A resolution

The crystal structure of the variant-3 protein neurotoxin from the scorpion Centruroides sculpturatus Ewing has been refined at 1.2 A resolution using restrained least-squares. The final model includes 492 non-hydrogen protein atoms, 453 protein hydrogen atoms, eight 2-methyl-2,4-pentanediol (MPD) solvent atoms, and 125 water oxygen atoms. The variant-3 protein model geometry deviates from ideal bond lengths by 0.024 A and from ideal angles by 3.6 degrees. The crystallographic R-factor for structure factors calculated from the final model is 0.192 for 17,706 unique reflections between 10.0 to 1.2 A. A comparison between the models of the initial 1.8 A and the 1.2 A refinement shows a new arrangement of the previously poorly defined residues 31 to 34. Multiple conformations are observed for four cysteine residues and an MPD oxygen atom. The electron density indicates that disulfide bonds between Cys12 and Cys65 and between Cys29 and Cys48 have two distinct side-chain conformations. A molecule of MPD bridges neighboring protein molecules in the crystal lattice, and both MPD enantiomers are present in the crystal. A total of 125 water molecules per molecule of protein are included in the final model with B-values ranging from 11 to 52 A2 and occupancies from unity down to 0.4. Comparisons between the 1.2 A and 1.8 A models, including the bound water structure and crystal packing contacts, are emphasized.