Essential transition metals

Intracellular distribution of essential transition metals is mediated by specific metallochaperones and transporters localized in endomembranes. In other words, the major processes involved in hyperaccumulation of trace metals from the contaminated medium to the shoots by hyperaccumulators as proposed by Yang et al. (2005) include bioactivation of metals in the rhizosphere through root-microbial interaction enhanced uptake by metal transporters in the plasma membranes detoxification of metals by distributing metals to the apoplasts such as binding to cell walls and chelation of metals in the cytoplasm with various ligands (such as PCs, metallothioneins, metal-binding proteins) and sequestration of metals into the vacuole by tonoplast-located transporters. 

Specificity Essential transition metals are specific for their own physiological functions, that is, they cannot be directly replaced by chemically similar elements. 

A critical factor in metal homeostasis is the bioavaUabifity of the specific metal. The bioavailability of a metal can be distinct from the overall abundance of the metal in the environment. For example, iron is the fourth-most abundant element in the Earth s crust. However, in the current-day oxygen-rich atmosphere of Earth, iron is largely present in the ferric (Fe +) form. In aqueous, aerobic environments at neutral or basic pH, ferric iron forms nearly insoluble iron hydroxides. Because of the insolubility of Ee +, iron is one of the least bioavailable of the essential transition metals. The concentration of iron in seawater (3 X 10 ppm) is nine orders of magnitude lower than the crustal concentration (5 x 10 ppm). In contrast, zinc is... 

Some of the most intriguing differences between the met-allomes of different cell types occur in subcellular organelles or vesicles. Eukaryotic cells in particular have carefully compartmentalized essential transition metals for specific biological purposes. The mitochondria and the chloroplast both contain high levels of metalloproteins relative to the cytoplasm and may have distinct metal quotas. Mammahan cerebrocortical neurons possess zinc-filled vesicles with labile zinc pools that approach... 

Pilon, M., Cohu, C. M., Ravet, K., Abdel-Ghany, S. E., Gaymard, F. (2009). Essential transition metal homeostasis in plants. Current... 

At this point, we must recall what a supported heterogeneous catalyst is. It consists of a porous oxide like alumina or silica, with a very large specific area (200 m /g typically), at the surface of which are dispersed the clusters of the active metal. In oil reforming, the metals used are essentially transition metals of group VIII, the main one being platinum, with a loading of the order of 1 wt.-%. 

The inertness snggests the reason that Mo is the only essential transition metal fonnd in biological systems that is not a first-row transition metal. Most second- and third-row transition metals appear to be too strongly bound and inert to engage in metabolic processes (Hoeschele et al. 1991). However, the inertness of some second- and third-row elements, such as Pt(II), can be useful for the chemotherapeutic activity of these complexes. The inertness probably plays a role also in the mutagenicity and mild carcinogenicity observed for cisplatin and the mutagenicity of other inert complexes, especially Cr(III) complexes. 

With respect to essential transitional metals specifically, it is important to recall that animals run the risk of both micronutrient deficiency and toxicity (see chapter 1). For terrestrial animals, the diet is the only source of essential micronutrients, and the amount of these micronutrients in food (regardless of whether the animal is an herbivore or carnivore) is quite low—which is, after all, why they are... 

Membrane proteins involved in the active transport of abundant ions such as calcium and potassium do not seem to be capable of carrying Cd " across membranes. Convincing data exist only in the case of some transporters of essential transition metal cations. 

The fact that metal cation importers mediate cadmium entry into cells whereas Cd " does not appear to be a substrate for exporters of essential transition metals points at an important issue of cadmium speciation in biological systems. Upon exposure of cells, particularly those protecting the body from its envirmunent, such as epithelial cells, Cd " is presented in relatively weak complexes allowing Cd " to easily dissociate and bind to cadmium transporters, and it may cross membranes as a free ion. In contrast, once inside cells, strong Ugands of Cd " are generally... 

Available evidence more convincingly indicates that homeostasis of essential transition metals is sensitive to cadmium poisoning by indirect mechanisms. All demonstrated and suggested pathways cannot be presented here, but the... 

Aldolases are a class of lyases that catalyze the reversible, stereoselective addition of an aldol donor component (nucleophile) to an acceptor component (electrophile) [9,75-81]. Typical products are 3-hydroxy ketones, a structural element that is frequendy incorporated in the framework of complex natural products. Mechanistically, aldolases promote the abstraction of the aldol donor a-proton, thereby generating a carbon nucleophile bound at the active site. In type I aldolases, an enamine is produced by covalent binding to a conserved lysine residue while in the type 11 aldolases, an enediol is formed by chelating coordination to an essential transition metal cation (mostly Zn +), which is acting as a Lewis acid promotor. Aldolases are often highly selective for the donor substrate, tolerating... 

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