Cadmium cytoplasmic protein

Thioneins are apoproteins that are exceptionally sulfur-rich (composed of greater 30 mol% cysteine). These proteins are found in high abundance in liver and kidney cytoplasm where they form metallothioneins (the holo-protein forms) upon complexation with metal ions. Thi-onein synthesis is induced by the presence of metals, especially zinc, copper, mercury, and cadmium. 

Cadmium and zinc are related transition metals with contrasting biological roles. Zinc is an essential ion (functioning catalytically and structurally in proteins) and probably has a specific transport mechanism for entering all cells, whereas cadmium is a toxic ion with no known biological functions. Cd " needs to be excluded, if possible, or to be extruded when found inside the cell. Because of the chemical similarity between Zn and Cd, it is difficult to exclude cadmium specifically from zinc uptake systems. The cell uses an efflux transporter as the strategy for keeping a low cytoplasmic concentration of cadmium. 

Proteins do not need to be metalloproteins to interact with cadmium, and the proteins with reactive cysteines described in Section 4.2.1 are examples of non-metalloproteins interacting with cadmium. In addition, some proteins that are not recognized as metalloproteins bind zinc extremely tightly. For example, the cytoplasmic domain of receptor protein tyrosine phosphatase p binds zinc with a value of 27 pM [109]. Cadmium is expected to bind tightly to these sites. Significantly, the widespread use of CdCl2 in crystallization studies, to prepare heavy metal derivatives and to phase the diffraction data, provides hundreds of protein structures in which Cd " interacts with amino-acid residues, often in positions that are not known to bind cations. 

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