The toxicity threshold of an ion is defined by its ability to produce radicals copper, iron, chromateto bind to sulfide and thiol groups copper, zinc, all cations of the second and third transition periodor to interfere with the metabolism of other ions.
Control of the respective systems seems to be mainly through regulation of transport activity flux controlwith control of gene expression playing only a minor role. 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.
Common themes are visible in the metabolism of these ions. On the other hand, if the amount of an ion is in surplus, genes for efflux systems are induced. In the unique case of mercury, the cation can be reduced to the volatile metallic form. 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.
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.
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. High-rate but low-affinity uptake systems provide a variety of cations and anions to the cells.
This chapter focuses on transition metals.
First, there is transport. The most dangerous metal, copper, catalyzes Fenton-like reactions, binds to thiol groups, and interferes with iron metabolism.
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.
Pathways to Central Metabolism: 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.酸素が遺伝暗号表を書き換えたのか？（） 多くの生物は 20 種類のアミノ酸を指定する共通の遺伝暗号を使用してい.
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