Analysis of spectrometry and thermodynamics of the metallothionein in freshwater crab Sinopotamon henanense for its binding ability with different metals.

The metal binding nature of heterologously expressed metallothionein of Sinopotamon henanense (ShMT) had been demonstrated previously. In this study, we analysed the stoichiometry of ShMT yielded in vivo and exchange reactions of the Zn-ShMT with Cd, Pb and Cuin vitro via electrospray ionization time-of-flight mass spectrometry (ESI-TOF-MS), circular dichroism (CD) spectroscopy, inductively coupled plasma mass spectrometry (ICP-MS), and isothermal titration calorimetry (ITC). The results of ESI-TOF-MS analyses showed that metal-ShMT synthesized in vivo had three major forms, namely Zn-, Cd-, and Pb-ShMT. The ITC analyses of exchange reactions demonstrated that Zn-ShMT exhibited up to 6, 6, and 7 binding sites for Cd, Pb and Cu. By the analyses of the UV and CD spectra in the substitution experiments showed that the geometric structural stability of metal-ShMT could be influenced when excess of over 6, 6, or 7 equivalents of Cd, Pb, or Cu were added into Zn-ShMT. Although both the reconstructed apo-ShMT and substituted Zn-ShMT with three metal ions fitted the same MⅡ- and MⅠ-ShMT binding models for divalent and monovalent metals, the differences in their thermodynamic data suggested that discrepancies exit in their physiological functions. These results suggested that ShMT yielded in vivo had a higher storage capability for Zn and a uptake ability for Cd, and Zn-ShMT was more easy to release Zn as well as to uptake Cd, Cu, or Pb. The results presented in this work will be very valuable to understand the function(s) of ShMT not only in a normal physiological condition but also in the presence of non-essential metals in crabs.