Effect of metal binding (Zn+2 or Cd+2) on the secondary structure and metal clusters of in vivo-sinthesized plant QsMT metallothionein.

Metallothioneins (MTs) are low molecular weight cysteine-rich proteins able to coordinate metal ions. Metal-MT chelation is mainly achieved by formation of metal-thiolate clusters but also by non-proteic ligands, such as chloride and sulfìde ions. Plant Quercus suber metallothionein (QsMT) differs from animal MTs in sequence organisation consisting of two Cys-rich regions separated by a Cys-devoid spacer domain containing one His residue. In contrast with the current knowledge on the 3D structure of animal MTs, there is an appalling lack of structural data on plant MTs. Zn+2 and Cd+2 complexes of QsMT were obtained by in vivo synthesis, in order to obtain physiological representative aggregates, and characterized by spectrometric and spectroscopic methods. The composition of the metallic aggregates were determined by ICP-AES, GC-FPD and ESI-MS. The results indicated that Cd-QsMT contains significantly higher amounts of metal and sulfìde ions that Zn-QsMT. Raman andir measurements revealed that, oppositely to what is known for other MTs, metal-QsMT contains significant amounts of secondary structures, predominantly B-sheet and B-turns elements, whereas it lacks a-helices. Interestingly, the properties of the metallic clusters formed are strongly dependent on the metal bound, Zn+2 or Cd+2. According to analytical results, the Raman spectra showed that Cd-QsMT is able to inglobe an the higher content of sulfide ions than Zn-QsMT, and suggested different structures of the complexes depending on the metal bound . Raman spectra also evidenced the participation of His in metal chelation in Cd-QsMT but not in Zn-QsMT. The differences observed between Zn+2 and Cd+2 metallic clusters suggest for QsMT a hybrid homeostatic/detoxificating role which would be developed through the acquisition of novel properties and metal-binding abilities depending on the metal coordinated.