Metallothionein: metal ions like cadmium, copper, zinc and

a stress protein in protection against metal toxicity

Metallothionein is a group
of cysteine-rich, metal binding, non enzymatic protein of molecular weight 6-7
kDa. It lacks aromatic amino acid, and cysteine occupies one-third of its
residues. MTs are found in higher plants, animals, some prokaryotes and
eukaryotic microorganisms. Metallothionein are metal seizing biomolecules utilized
by bacterial cells to disable the toxic metals and restrain active binding site
.At transcription level, synthesis of MTs are regulated, stimulated by a number
of factors like hormones and cytotoxic residues. Different toxic metal ions like
cadmium, copper, zinc and nickel are sequestered by MTs.

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Metallothioneins are partitioned
into three classes 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 the category of class I metallothioneins . Class I and II
metallothioneins are stress inducible and are expressed ubiquitously. Mammalian
MTs amino acids contain approximately sixty one numbers of amino acids of same
organization. They consist of 20 cysteine buildups that stay same throughout
the sequence of amino acid. Every cysteine residues of a metallothionein
coordinates with 7 mol of Zn or Cd, resulting in strong affinity for Cd and Zn

function of metallothionein

MTs role in metal homeostasis

is a physiologically important metal and the most abundant metal bound to
constitutive MT. Zn provides essential structural and catalytic functions to a
wide variety of proteins. More than 300 different enzymes depend on Zn for
proper protein folding and biological function. Zn is also crucial in the
regulation of gene expression because numerous transcription factors have

finger motifs” that are maintained by Zn. Apo-MT (metallothionein with no
metals bound) is a Zn acceptor because of the abundance of free sulfhydryl
groups and their high affinity for Zn. However the sulfhydryl groups are highly
reactive, and Zn, although bound with high

can undergo exchange reactions, which allows Zn to be transferred from MT to
other proteins.The affinity of sulfhydryl groups for Zn can also make MT an
efficient metal ion scavenger. This implies a possible regulatory role of MT in
the activation or inactivation of various molecular effectors. Such a
possibility was demonstrated by showing that apo-MT can chelate Zn out of the
transcription factor IIIA (TFIIIA), a process that inactivates TFIIIA .
Therefore, it is tempting to speculate that MT might be essential for Zn
homeostasis by regulating Zn absorption, or as a donor of Zn to various enzymes
and transcription factors during development or protein synthesis