Metal binding to the SsTroA has been proposed as a major event leading to conformational changes. From our combined NMR and ANS fluorescence data, we speculated that the metalbinding pocket of apo-SsTroA and other locally disordered folding adopts a loosely folded structure in solution. This behavior explains why metal-bound states induced the decreased ANS fluorescence intensity and changes in the NMR spectra of the metal-loaded forms. These loosely ordered structures may enhance an inducible response to the changing environmental conditions or may play a pivotal role in recognizing different divalent metal ions. In some cases, binding of a target molecule or ligand drives disordered folding to a more ordered state [52] . It should be noted that the crystallographic studies on TpTroA [21, 22] have referred to a comparatively minor folding movement upon the metal binding, and structural analysis of Escherichia coli ZnuA suggests that binding and release of Zn drive small yet significant conformational changes [45] . One explanation may be that the nature of protein in solution is shown to be more dynamic than suggested by crystal structures.