In our structure, we clearly observed the metal binding pocket provided by the buried surface between the N-terminal and Cterminal domains (Fig. 5B) . Independent experimental evidence has demonstrated that the ion bound in the metal binding pocket of recombinant SsTroA is Zn 2+ (see metal content analysis). Zinc is pentacoordinated by His 76 (bond length, 2.14 Å ), His 139 Zn coordination in our structure is best described as distorted square pyramidal [21] . The N e2 nitrogen atoms of His 76 and His 139, with the O e1 an O e2 atoms of Asp 289 generated the square planar. While the N e2 atom of His 205 is the summit of the square pyramid. The binding site is buried ,7 Å below the molecule surface and the four coordinating residues are also completely buried. This coordination geometry and the amino acid types are very similar among the Mn-binding SBP family [21, 23, 24] .