Nonessential metal ions

Transport of a nonessential metal ion across a biological membrane kml = nr4s) 104... 

Proteins involved in metal ion homeostasis can be divided into two broad families, metal trafficking proteins and metalloregulatory proteins. The metal trafficking proteins include membrane transporters that translocate metal ions across cell membranes and soluble metallochaper-one proteins that deliver metal ions to specific target proteins. The metalloregulatory proteins utilize metal ions to regulate gene expression." Proteins in both families maintain concentrations of both essential and nonessential metal ions. The coordination chemistry of metal homeostatic... 

For many metal ions, particularly those within the same column of the periodic table, the need for essential metal ions runs counter to the need to minimize the uptake of toxic, nonessential metal ions. For example, the epithelial layers of the intestine have to remain... 

This cascade in research is reflected in the increasing amount of literature published each year on the biochemical behavior of the lithium ion and the aim of this article is to bring together much of this information, emphasizing the ubiquity of this small, apparently nonessential, metal cation in biology. 

Dixon and Webb present an extensive consideration of activation mechanisms involving the reversible binding of an activator (less often termed an agonist ) to the enzyme. Nonessential activation refers to enzyme-de-pendent processes that can convert substrate(s) to prod-uct(s) in the absence of the activator, albeit at a slower rate. Essential activators are molecular entities that are required by the enzyme in the catalysis of a reaction. In a sense, essential activators are similar to second (or third) substrates, albeit they are not converted to products. An example of an essential activator might be an enzyme that requires the binding of a metal ion for catalysis to proceed. Below are a few cases of essential activation. 

Heavy metal ions such as Cu, Zn, Mn, Fe ", NP", and Co are essential micronu-tritients for plant metabolism. When these ions are not available to the roots, plants develop specific deficiency symptoms, though when present in excess, these - as well as nonessential metals such as Cd ", Hg " and Pb " " - can become extremely toxic. At high concentrations, all these metals may cause symptoms such as chlorosis and necrosis, stunting, leaf discoloration and inhibition of root growth (Marschner... 

All enzymatic reactions involving ATP require ion as an activator. These types of reactions are very common in nature, especially with kinases. In such cases, the true substrate is Mg ATP complex, that is, a substrate-activator complex, and free ATP molecules are not the active substrates of enzymes. In addition to forming an active complex with substrate, metal ions may also combine with the enzyme at an additional specific activation site, this additional binding site may be essential or nonessential. Thus, the metal ions may be treated as true substrates of enzymes. 

Metallothioneins (MTs) are small (6000-7000 Da), intracellular, cysteine-rich ( 33% of the amino acids are cysteines) metal binding proteins that were first discovered in 1957 in equine kidney cortex. The subsequent purification of this protein identified it as the only known native cadmium-containing protein. Further studies showed this protein to bind both essential (e.g., Cu and Zn) and nonessential (e.g., Cd and Hg) metal ions and to be truly ubiquitously distributed in nature [215-218]. Additionally, MT biosynthesis is induced at the transcriptional level by a wide range of factors, which includes heavy metal ions that were subsequently found bound to the protein. All of the above factors suggest a role for this protein in heavy metal homeostasis, transport and detoxification [215-218]. Despite 55 years of extensive studies on MTs, which has included the high resolution characterization of the 3D stmcture and metal binding properties [134,136,138,151,154,218-222], the essential physiological functional role(s) of MT remains elusive. Of particular note in these studies was the determination of the NMR solution structure of a mammalian MT in a tour deforce effort by the Wiithrich lab which resulted in the reinterpretation and correction of the mily mammalian MT crystal structure currently available [223,224]. 

In polluted seawater or freshwater, the concentration of nonessential transition metals in the water can be so high that their gill uptake can be fatal to fish. Given that the gill in a freshwater fish is designed to reacquire lost ions, the freshwater fish in a polluted environment is fighting two contradictory needs the need to acquire... 

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