Different mechanismare there to tolerate and overcome Cd toxicity. A major mechanism forheavy-metal resistance involves alterations in the membrane transport system ofan organism, resulting in the reduction or denial of entry of Cd into the organism. Alternatively, the intracellular orextracellular sequestration of heavy metals by adsorption to cell walls or bybinding to a specific biopolymer results in tolerance to heavy-metal toxicity. Another mechanism involves the utilization of energy-dependention efflux pumps, which capture and remove the cytoplasmic metals through thecell membrane Metallothionein: a stress protein inprotection against metal toxicityMetallothionein is a groupof cysteine-rich, metal binding, non enzymatic protein of molecular weight 6-7kDa. It lacks aromatic amino acid, and cysteine occupies one-third of itsresidues. MTs are found in higher plants, animals, some prokaryotes andeukaryotic microorganisms. Metallothionein are metal seizing biomolecules utilizedby bacterial cells to disable the toxic metals and restrain active binding site.
At transcription level, synthesis of MTs are regulated, stimulated by a numberof factors like hormones and cytotoxic residues. Different toxic metal ions likecadmium, copper, zinc and nickel are sequestered by MTs.Metallothioneins are partitioned into threeclasses on the premise of cysteine structure.
These are (a) Metallothionein-I,(b) Metallothionein-II and (c) Metallothionein- III. They are cys-cys,cys-X-cys, cys-X-X-cys ,where as mammalian metallothioneins comes under thecategory of class I metallothioneins . Class I and II metallothioneins arestress inducible and are expressed ubiquitously. Mammalian MTs amino acidscontain approximately sixty one numbers of amino acids of same organization. Theyconsist of 20 cysteine buildups that stay same throughout the sequence of aminoacid.
Every cysteine residues of a metallothionein coordinates with 7 mol of Znor Cd, resulting in strong affinity for Cd and Zn