METALLOTHIONEINS (MTs)

We have seen that heavy metals can replace essential metals in enzymes and destroy the enzymatic activity. In addition, by coordinating to sulfur-bearing amino acids in the protein chain they might cause an enzyme to be “bent out of shape” and lose its activity. Protection of enzymes from toxic metals is thus requisite for their proper function. Serving this purpose is a group of proteins that have the following characteristics: (I) The molecular weights are about 6000 with 31 to 62 amino acids. (2) One-third of these amino acids are cysteine [HSCH₂CH(NH₂)COOH] residues, grouped in Cys — Cys and Cys—X—Cys groups (X = a separating amino acid). (3) None of the cysteines are linked by S—S bridges (cystine). (4) There are few or no histidines or amino acids with aromatic side chains. (5) With sucha high percentage of amino acids bearing thiol groups and “dumped” along the protein chain, the thioneins are able to bind several metal ions per molecule, preferentially the softer metals such as Zn²⁺, Cu²⁺, Cd²⁺, Hg²⁺, Ag⁺ etc. Metallothioneins containing Zn²⁺ and Cu²⁺ might possibly be important in the transport of these essential elements, but the evidence is mostly negative. On the other hand, the binding of heavy metals such as cadmium and mercury suggests a protective function against these toxic metals. Indeed, increased amounts of thineins are found in the liver, kidney, and spleen after exposure of them. Furthermore, it can be demonstrated that cell lines that fail to produce thioneins are extremely sensitive to cadmium poisoning while “over-producers” have enhanced protection. It has been suggested that the binding of thioneins to cadmium and other heavy metals, with extremely high stability constants, is one of protection alone; perhaps the reduced binding of copper, an essential metal but one toxic in high concentrations, severs a “buffering function” of providing copper for enzymes but not at levels sufficiently high to be toxic. The question of whether the weaker binding of the less toxic zinc serves a similar function, or “just happens.” is moot.

For +2 cations such as zinc(II) and cadmium (II) such metallothionein molecule contains up to seven metals atoms. X-ray studies indicate that the metals atoms are in approximately tetrahedral sitesbound to the cysteins sulfur atoms. The soft mercury (II) ion has a higher affinity for sulfur and will disppace cadmium from metallothionein. At first the mercury ions occupy tetrahedral sites but as the number increases, the geometries of the metal sites and protein changes until about nine Hg(II) atoms are bound in a linear (S—Hg—S) fashion. Up to twelve +1 cations such as copper(I) and silver (I) can bind per molecule, indicating a coordination number lower than four, probably three.

An intriguing problem about which we know very little is the mechanism of metal indentification by the body that triggers its response, as in the case of the build-up of metallathioneins upon exposure to toxic metals. Perhaps the best understood of the metalloregulatory proteins in MerR that protects bacteria from mercurial toxicity. It is extremely sensitive to Hg²⁺, and distinguishes it from its congeners Zn²⁺ and Cd²⁺. There is good evidence that the mercury receptor forms threecoordinate mercury(II) complexes, making possible this specificity.