Research progress on metallothioneins: insights into structure, metal binding properties and molecular function by spectroscopic investigations

Metallothioneins (MTs) are low molecular weight, cysteine-rich proteins with an exceptionally heavy metal coordination capacity. Because of their ability to bind metals and to scavenge oxidant radicals, MTs are considered to play a role in metal homeostasis, metal detoxification and control of the oxidative stress. Although their high heterogeneity on the expression patterns, metal binding abilities and primary structure suggest very diverse functional specializations, the structural and functional studies have been mainly devoted to vertebrate and fungal MTs and their canonical cysteine-metal clusters. This chapter will be focused on the new methodological procedures settled for the structural characterisation of some metallic MT aggregates. Five zinc complexes of the invertebrate and plant MTs, poorly described up to now in the literature, in addition to one well studied MT from vertebrate family, will be analysed and the new discovered structural features of metal-MT clusters, together with the perspectives on MT research, will be commented. In fact, the application of several spectroscopic techniques such as Raman and IR spectroscopies, Circular Dichroism, can provide new structural information eventually related to the function of the metal binding. Recombinant expression in E.coli has allowed the biosynthesis of intact metal-MT complexes, well corresponding to native forms, in sufficient quantity and purity for analytical spectrometric and spectroscopic characterization. The spectroscopic analyses of the in vivo-synthesised metal-MTs have recently demonstrated the participation of extra-protein ligands, such as chloride and sulfide ions, in the metal-MT coordination environment of vertebrate, invertebrate and plant MTs. Recently, some new insights on the structure of metal-MT complexes has been obtained by using Raman and IR spectroscopies, powerful techniques in protein studies. Despite the potentialities of these techniques, to our knowledge they have been scarcely used in MT conformational studies until now. The use of these spectroscopies has resulted to be very useful to approach unambiguously two basic structural points poorly described in MTs: the participation of chloride, sulfide ions and His residues to the metal-coordination sphere and the presence of secondary structure elements. In particular, ordered secondary structures, oppositely to what has been commonly accepted, are present in MTs from vertebrate, invertebrate and plant MTs, and could develop crucial roles in the determination of the functional properties of MTs.