Despite the determination of several SBP structures, the structural difference between Zn-specific and Mn-specific proteins remain unclear [44] . There exists difference with SBPs, specific either for zinc or manganese. In the Zn-specific ZnuAs, the metal is coordinated by 3 histidines plus a water or glutamate [44, 45] . In the case of Zn-specific AdcAII, the residues involved in Zn binding are three histidines and one glutamate [25] . This is similar to the observation in Zn-specific Lbp, the zinc ion is coordinated by the side chains of three histidines and one glutamate in a slightly distorted tetrahedral geometry [28] . In the Mn-specific SBPs (PsaA and MntC), the metal is coordinated via two hisitidine residues, the third histidine is replaced by a glutamate, and the fourth position is occupied by an aspartate [23, 24] . The coordination geometry of Zn is usually tetrahedral or distorted tetrahedral (coordination number = 4) in the protein, although the coordination number can increase to five to stabilize high-energy intermediates and their flanking transition states. In contrast, the Mn 2+ ion tends to prefer hard ligands such as the carboxylate oxygens of aspartate or glutamate, and the carboxamide oxygens of asparagine or glutamine. The nitrogen atoms of histidine imidazole groups are occasionally observed as metal ligands in Mn 2+ metalloenzymes despite their borderline hardness, the metal coordination geometry is usually square pyramidal or trigonal bipyramidal (coordination number = 5) [13] . Accordingly, our structure indicated that Mn 2+ is likely to be the natural ligand. However, despite great efforts, we failed to obtain any crystals of SsTroA containing Mn 2+ .