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The crystal structure of TM-3, a small snake-venom metalloproteinase (SVMP) isolated from Taiwan habu (Trimeresurus mucrosquamatus), was determined at 1.35 Å resolution with resultant R and Rfree values of 0.181 and 0.204, respectively. The overall structure of TM-3 is an oblate ellipsoid that contains three disulfide crosslinks, Cys118–Cys197, Cys159–Cys181 and Cys161–Cys164. It exhibits the typical structural features of SVMPs and is closely related to the structure of the catalytic proteinase domain of TNFα-converting enzyme (TACE). In the present structure, the essential catalytic zinc ion was found to be replaced by a cadmium ion during crystallization, as revealed by atomic absorption analysis and X-ray data. This cadmium ion is bound to six ligands, including three conserved histidines and three water molecules, displaying the coordination geometry of a distorted octahedron. One of the water molecules is proposed to play the role of stabilizing the tetrahedral intermediate during the catalysis of SVMPs. The putative {\rm S}'_1 specificity pocket of TM-3 is relatively shallow, in contrast to the deep pockets of adamalysin II, atrolysin C and H2-proteinase, but is similar to those in acutolysin A and TACE. The shallow pocket is a consequence of the presence of the non-conserved disulfide bond Cys159–Cys181 and the residue Gln174 at the bottom of the {\rm S}'_1 pocket. The results indicate that the active-site structure of TM-3, among the know structures of SVMPs examined thus far, is most similar to that of TACE owing to their close disulfide configurations and the {\rm S}'_1 specificity pocket.

Supporting information

PDB reference: TM-3, 1kuf, r1kufsf


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