Native essential metals

Displacing the Essential Metal Ion in Biomolecules. It is estimated that approximately one third of all enzymes require metal as a cofactor or as a structural component. Those that involve metals as a structural component do so either for catalytic capability, for redox potential, or to confer steric arrangements necessary to protein function. Metals can cause toxicity via substitution reactions in which the native, essential metal is displaced/replaced by another metal. In some cases, the enzyme can still function after such a displacement reaction. More often, however, enzyme function is diminished or completely abolished. For example, Cd can substitute for Zn in the protein famesyl protein transferase, an important enzyme in adding famesyl groups to proteins such as Ras. In this case, Cd diminishes the activity of the protein by 50%. Pb can substitute for Zn in 8-aminolevulinic acid dehydratase (ALAD), and it causes inhibition in vivo and in vitro. ALAD contains eight subunits, each of which requires Zn. Another classic example of metal ions substituting for other metal ions is Pb substitution for Ca in bones. 

Fig. 2.20 Formal fitness landscape of various biocatalytic and inactive (i.e., the latter being located outside the window of essentiality ) metal ions for the carboxypeptidase A (Vallee and Williams 1968). The enzyme reconstituted by cobalt (treatment with EDTA, then addition of Co +) is considerably more active than the native Zn version , while Cd (and other ions) afford poorly to non-active metalloproteins in this case. (Relative) catalytic turnover rates are from Vallee and Williams 1968, x and c values this work and previous publications by this author. Here, sufficient abundance of the corresponding ions is taken for granted owing to in-vitro addition to apoprotein, so c k is not hmited by c, but merely represents the catalytic features pertinent to k, obtained from either biochemistry (several metal ions being present, e.g. in phosphatases) or experiments with reconstituted apoproteins...

The substitution of cobalt for the native zinc ions of alkaline phosphatase results in an active enzyme with distinctive optical properties, generated by the interaction of cobalt with the ligands of the protein. These properties may be employed to investigate the modes of binding of cobalt to the enzyme and also serve in a remarkable fashion to distinguish the catalytically essential metal atoms from those which play only a structural role. 

The reddish metal was already known in prehistoric times. It occasionally occurs as a native metal, but mostly in conspicuous green ores, from which it is extracted relatively easily. It is convenient to work, but not very hard. Not very optimal as a tool ("Otzi the Iceman" had a copper axe with him). Only through the addition of tin is the more useful bronze obtained. Its zinc alloy is the versatile and widely used brass. Copper is one of the coinage metals. Water pipes are commonly made of copper. Its very good thermal and electrical conductivity is commonly exploited (cable ), as well as its durability (roofs, gutters), as the verdigris (basic copper carbonate) protects the metal. Cu phthalocyanines are the most beautiful blue pigments. Seems to be essential to all life as a trace element. In some molluscs, Cu replaces Fe in the heme complex. A 70-kg human contains 72 mg. 

Among protein aromatic groups, histidyl residues are the most metal reactive, followed by tryptophan, tyrosine, and phenylalanine.1 Copper is the most reactive metal, followed in order by nickel, cobalt, and zinc. These interactions are typically strongest in the pH range of 7.5 to 8.5, coincident with the titration of histidine. Because histidine is essentially uncharged at alkaline pH, complex-ation makes affected proteins more electropositive. Because of the alkaline optima for these interactions, their effects are most often observed on anion exchangers, where complexed forms tend to be retained more weakly than native protein. The effect may be substantial or it may be small, but even small differences may erode resolution enough to limit the usefulness of an assay. 

In 1954, Perutz introduced the isomorphous replacement method for determining phases. In this procedure a heavy metal, such as mercury or platinum, is introduced at one or more locations in the protein molecule. A favorite procedure is to use mercury derivatives that combine with SH groups. The resulting heavy metal-containing crystals must be isomorphous with the native, i.e., the molecules must be packed the same and the dimensions of the crystal lattice must be the same. However, the presence of the heavy metal alters the intensities of the spots in the diffraction pattern and from these changes in intensity the phases can be determined. Besides the solution to the phase problem, another development that was absolutely essential was the construction of large and fast computers. It would have been impossible for Perutz to determine the structure of hemoglobin in 1937, even if he had already known how to use heavy metals to determine phases. 

More recently, isotopic labeling experiments have assumed a major role in establishing the detailed mechanism of enzymic action. It was shown that alkaline phosphatase possesses transferase activity whereby a phos-phoryl residue is transferred directly from a phosphate ester to an acceptor alcohol (18). Later it was found that the enzyme could be specifically labeled at a serine residue with 32P-Pi (19) and that 32P-phosphoserine could also be isolated after incubation with 32P-glucose 6-phosphate (20), providing strong evidence that a phosphoryl enzyme is an intermediate in the hydrolysis of phosphomonoesters. The metal-ion status of alkaline phosphatase is now reasonably well resolved (21-23). Like E. coli phosphatase it is a zinc metalloenzyme with 2-3 g-atom of Zn2+ per mole of enzyme. The metal is essential for catalytic activity and possibly also for maintenance of native enzyme structure. 

Vapor phase dissolution (VPD) is commonly used for surface and contamination analysis of semiconductor wafers [374-379]. HF vapor is used to remove a silicon oxide or native silicon layer. A drop of hydrofluoric acid or deionized water (with a volume of 50 to 200 jxL) is placed on the surface and rolled around the surface to dissolve the metals. The small drop is then analyzed by ICP-MS by using either a direct injection nebulizer, a micronebulizer, or ETV. The ability of ICP-MS to measure several elements rapidly in a small volume of solution is essential. 

Meteorites consisting mainly of metallic iron alloyed with nickel are termed siderites. Siderolites differ from the above in contaimng stony matter as well as native metal. Aerolites consist essentially of stony matter. 

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