**1. Introduction**

Many of the elements are play a significant role in all kind of organisms and pose some functional outcomes in living organisms. Based on this outcome, elements are classified as essential and non-essential elements. Recent trace element research showed many of the metals also play a major significant role in metabolisms but they required very trace quantities. The metal ions of iron (Fe), zinc (Zn), manganese (Mn), copper (Cu), cobalt (Co) molybdenum (Mo) and the non-metals selenium (Se) and iodine (I) bromine (Br), rubidium (Rb), aluminum (Al), nickel (Ni), titanium (Ti) and barium (Ba) etc. exceeded

certain limits, it become toxic to organisms. Predicting the role of metals in living organisms is difficult since its intrinsic characteristics, redox properties, charge, and, above all, size. Toxicity of the metals mainly through altering the cell metabolism and thereby blocking synthesis of fundamental enzymes. To mitigate this, organisms have developed different detoxifying mechanisms such as blocking of toxic metals through oligopeptides or proteins and chelating metals encoding by genes, which are called metallothioneins (MTs) which possess residues of cysteine (Cys) having thiol or sulfhydryl groups (R-SH) and thereby immobilize the metallic ions. The prime function of MTs is the storage, transportation and binding of metals. Margoshes and Vallee's discovery of Cd-binding, cystein-rich protein in the horse kidney marked a birth field for the research that focused to study a novel polypeptide super family, the metallothioneins (MTs). The MTs are family consisting of low molecular weight (6-7 kDa), cysteine (Cys)-rich, non-enzymatic proteins that are found ubiquitously in animals, in higher plants, in eukaryotic microorganisms, and many prokaryotes [1, 2]. The metal ions is sequestered through MTs by binding with Cys rich metal binding – motifs Cys-Cys, Cys-X-Cys, or Cys-X-X-Cys) that [3, 4]. Based on the Cys residues arrangement, MTs grouped into different classes [5, 6]. All the cysteines present in the reduced form are been coordinated to the metal ions that gives rise to the spectroscopic appearance of the metal-thiolate groups. Cysteines in the MTs are preserved across the species, it is suspected that, cysteines are essential for the function and the MTs is required for the life. MTs have some composition of unusual amino acid as they do not contain any aromatic amino acids and the most important is that, one third of its residues contacysteines. Moreover, MTs show spectroscopic features of metal thoilates (mercaptides) [7]. MTs can bind Cd, Ni, Zn, Cu, and Pb with the affinities that depend up on the bioavailability and concentration of heavy metals. Three major scientist groups have contributed in the field of MTs include:


MTs play a critical role in protecting the cell from harmful factors i.e. metals, free radicals, etc. through controlling cell growth, differentiation, proliferation, explicating their nuclear vs.cytoplasmic localization [8]. MTs is cytoplasmic protein, accumulated in lysosomes, trandpoted to the nucleus and intermediated space in mitochondria [9, 10]. MTs transported the metals from one cell to other cell by receptor- mediated mechanism and metal is transported to the cytosol but protein remains in an endocytotic compartment [11, 12]. According to Wang et al. [13], MTs have exhibits four main functions and it is illustrated in the **Figure 1**.

Nevertheless, although MTs were discovered over four decades back, their physiological functions are not fully understood still [14]. Based on the this prelude this review is being taken up to refresh the MTs structure, classification and role in metal detoxification.

*Metallothioneins: Diverse Protein Family to Bind Metallic Ions DOI: http://dx.doi.org/10.5772/intechopen.97658*

**Figure 1.**

*MTs - four main functions. (adopted from [13]).*
