Investigations on metallothioneins: an help from Raman spectroscopy

MTs are low molecular weight metalloproteins (6-7 kDa) characterized by a high content in Cys. They are ubiquitously distributed among all living organisms and have an outstanding heavy-metal binding capacity. Overall, the general structural properties that characterize all MTs are the formation of metal-thiolate clusters involving terminal and bridging cysteinyl thiolate groups. Recently, it has been shown that also exogenous ligands such as inorganic ions (i.e. sulfide or chloride ions) can participate in the coordination sphere of metals in MTs. Some Zn- and Cd-complexed MTs, representative of different MT families, enclosing the well-known vertebrate and echinoderm, and the poorly understood nematode, diptera, molluscan, protozoa and plani MTs, have been analysed by Raman Spectroscopy fact, despite the well-known potentialities of this techniques, to our knowledge it has been scarcely used in MT structural studies until now. Raman spectroscopy has resulted to be very useful in shedding light on the secondary structures eventually present in MTs and the ligands involved in metal coordination. The oxidation state of Cys residues and their participation in the metal chelation can be clearly defined, as well as the eventual involvement of His residues. As regards exogenous metal ligands such as sulfide anions, their presence can be identify by some marker bands whose intensity is linearly correlated with sulfide/metal molar ratio. Finally, Raman can be also an useful tool for providing information on the favourite sites of the radical attack and radical-induced modification in protein folding. In conclusion, Raman spectroscopy, in coupling with analytical techniques, can be one of the most promising experimental strategies in the research on new hints on MTs.