Characterization of zinc-depleted alanyl-tRNA synthetase from Escherichia coli: role of zinc

Transition Metal Homeostasis

This chapter focuses on transition metals. All transition metal cations are toxic-those that are essential for Escherichia coli and belong to the first transition period of the periodic system of the element and also the "toxic-only" metals with higher atomic numbers. Common themes are visible in the metabolism of these ions. First, there is transport. High-rate but low-affinity uptake systems provide a variety of cations and anions to the cells. Control of the respective systems seems to be mainly through regulation of transport activity (flux control), with control of gene expression playing only a minor role. If these systems do not provide sufficient amounts of a needed ion to the cell, genes for ATP-hydrolyzing high-affinity but low-rate uptake systems are induced, e.g., ABC transport systems or P-type ATPases. On the other hand, if the amount of an ion is in surplus, genes for efflux systems are induced. By combining different kinds of uptake and efflux systems with regulation at the levels of gene expression and transport activity, the concentration of a single ion in the cytoplasm and the composition of the cellular ion "bouquet" can be rapidly adjusted and carefully controlled. The toxicity threshold of an ion is defined by its ability to produce radicals (copper, iron, chromate), to bind to sulfide and thiol groups (copper, zinc, all cations of the second and third transition period), or to interfere with the metabolism of other ions. Iron poses an exceptional metabolic problem due its metabolic importance and the low solubility of Fe(III) compounds, combined with the ability to cause dangerous Fenton reactions. This dilemma for the cells led to the evolution of sophisticated multi-channel iron uptake and storage pathways to prevent the occurrence of unbound iron in the cytoplasm. Toxic metals like Cd2+ bind to thiols and sulfide, preventing assembly of iron complexes and releasing the metal from iron-sulfur clusters. In the unique case of mercury, the cation can be reduced to the volatile metallic form. Interference of nickel and cobalt with iron is prevented by the low abundance of these metals in the cytoplasm and their sequestration by metal chaperones, in the case of nickel, or by B12 and its derivatives, in the case of cobalt. The most dangerous metal, copper, catalyzes Fenton-like reactions, binds to thiol groups, and interferes with iron metabolism. E. coli solves this problem probably by preventing copper uptake, combined with rapid efflux if the metal happens to enter the cytoplasm.

Critical role of zinc ion on E. coli glutamyl-queuosine-tRNA(Asp) synthetase (Glu-Q-RS) structure and function

Glutamyl-queuosine-tRNA(Asp) synthetase (Glu-Q-RS) and glutamyl-tRNA synthetase (GluRS), differ widely by their function although they share close structural resemblance within their catalytic core of GluRS. In particular both Escherichia coli GluRS and Glu-Q-RS contain a single zinc-binding site in their putative tRNA acceptor stem-binding domain. It has been shown that the zinc is crucial for correct positioning of the tRNA(Glu) acceptor-end in the active site of E. coli GluRS. To address the role of zinc ion in Glu-Q-RS, the C101S/C103S Glu-Q-RS variant is constructed. Energy dispersive X-ray fluorescence show that the zinc ion still remained coordinated but the variant became structurally labile and acquired aggregation capacity. The extent of aggregation of the protein is significantly decreased in presence of the small substrates and more particularly by adenosine triphosphate. Addition of zinc increased significantly the solubility of the variant. The aminoacylation assay reveals a decrease in activity of the variant even after addition of zinc as compared to the wild-type, although the secondary structure of the protein is not altered as shown by the Fourier transform infrared spectroscopy study.

Zinc and the immune system

Zn is an essential trace element for all organisms. In human subjects body growth and development is strictly dependent on Zn. The nervous, reproductive and immune systems are particularly influenced by Zn deficiency, as well as by increased levels of Zn. The relationship between Zn and the immune system is complex, since there are four different types of influence associated with Zn. (1) The dietary intake and the resorption of Zn depends on the composition of the diet and also on age and disease status. (2) Zn is a cofactor in more than 300 enzymes influencing various organ functions having a secondary effect on the immune system. (3) Direct effects of Zn on the production, maturation and function of leucocytes. (4) Zn influences the function of immunostimulants used in the experimental systems. Here we summarize all four types of influence on the immune function. Nutritional aspects of Zn, the physiology of Zn, the influence of Zn on enzymes and cellular functions, direct effects of Zn on leucocytes at the cellular and molecular level, Zn-altered function of immunostimulants and the therapeutic use of Zn will be discussed in detail.