Cadmium phytochelatins

Lee, S., Moon, J. S., Ko, T.-S., Petros, D., Goldsbrough, P. B., and Korhan, S. S., 2003b, Overexpression of Arabidopsis phytochelatin synthase paradoxically leads to hypersensitivity to cadmium stress, Plant Physiol. 131 656-663. 

Li, Y., Dhankher, O.P., Carreira, L., Lee, D., Chen, A., Schroeder, J.I., Balish, R.S., Meagher, R.B. (2004). Overexpression of phytochelatin synthase in Arabidopsis leads to enhanced arsenic tolerance and cadmium hypersensitivity. Plant Cell Physiol. 45 1787-97. 

Figure 7 Phytochelatin concentrations in the marine diatom Thalassiosira weissflogii as a function of log Cd ) (unchelated cadmium concentration). Chain lengths for each phytochelatin dimer (circles) trimer (triangles), and tetramer (squares) (after Ahner et al, 1994).

In plants, two kinds of metal-binding peptides or proteins are synthesized. Plant metallothioneins are inducible cysteine-rich entities very like those found in animals. Differential expression (induction) of metallothionein genes can be due to both variation of external heavy metal concentrations and the influence of various environmental factors. The principle role of plant metallothioneins seems to be in homeostasis rather than in metal detoxification. Plants are also known to have so-called phytochelatins, which are non-protein thiols specifically induced upon exposure to heavy metals. A close positive relationship between the concentrations of cadmium and phytochelatins in the plant shoot material has been observed and linked to the degree of growth inhibition (Keltjens and Van Beu-sichem, 1998). These observations make the use of phytochelatins promising for the assessment of heavy metal effect on plants. 

Keltjens, W.G. and Van Beusichem, M.L. (1998) Phytochelatins as biomarkers for heavy metal stress in maize (Zea mays L) and wheat (Triticum aestivum L) combined effects of copper and cadmium. Plant and Soil, 203, 119-126. 

He ZY, Li JC, Zhang HY, Ma M. Different effects of calcium and lanthanum on the expression of phytochelatin synthase gene and cadmium absorption in Lactuca sativa. Plant Sci 2005 168 309-18. 

Tukendore a and Rauser WE (1990) Changes in f utathione and phytochelatins in roots of maize seedling exposed to cadmium. Plant Sd 70 155-166. 

Existing data on phytochelatins and related polypeptides show that they differ from MT not only in biosynthesis but also in structure. Whereas no 3D structure has yet been reported, electronic spectroscopy (c.d. and UV-Vis) and EXAFS data on Cd-7-EC peptides led to different proposals as to the nature of the cadmium binding in such peptides, polynuclear cadmium thiolate aggregates versus isolated Cd(SCys)4 units, respectively. However, contrary to the conclusion of the previous work, a recent X-ray absorption spectroscopy study on Cd-7-EC peptides and model systems provides evidence for the formation of a polynuclear cadmium cluster. " ... 

III MTs known as phytochelatins (PCs). Originally named cadistins for their clear biochemical connection to cadmium, PCs have the general formula (y-glu-cys)n-gly, with n varying from 2 to perhaps 11. The gamma peptide bond links the carboxyl from the side chain of glutamate to the amine of cysteine (Fig. 9). The terminal glycine is sometimes replaced by another amino acid or simply absent. 

A similar cadmium-binding complex of peptides is produced by the alga Chlorella fusca as well as other Phyto-phyta (Gekeler et al., 1988). Phytochelatins have been identified in the roots of heavy-metal-sensitive Acer pseudoplatanus and resistant Silene cucubalus plants grown in zinc-rich soil, whereas plants grown in the absence of this metal lacked these peptides. Metal-binding phytochelatins appear to be specifically induced in plants in heavy-metal-enriched ecosystems (Grill et al., 1988). 

Phytochelatin a plant peptide produced in response to lieavy metals, e.g. cadmium, copper, mercury, lead and zinc. The structure is (Y-Glu-Cys) -Gly (n = 3-7). Like Metallothionein (see), P. form metal-thiolate bonds and thus sequester toxic metal ions. They are probably derived from glutathione rather than RNA-directed protein synthesis. 

In this paper, we describe the construction and diaracterization of recombinant E. coli sbains that anchor and displty fimctional tynthetic phytochelatins ranging fiom 7 to 20 cysteines (EC7, ECS, ECU, and EC20) onto the cell surface of E. coli. We demonstrate that these synthetic phytochelatins confer cadmium and mercury binding capability on the host cells and the resulting novel bioadsorbents accumulate a substantially higjier amount of cadmium than ffie wild-type cells. 

There are at least three major types of natural proteins and peptides which are rich in cysteinyl residues metallothioneins, zinc finger proteins, and phytochelatins. The biological role and structural characteristics of metallothioneins are dealt with in Chapter 11 of this book. Zinc finger peptides are possible targets for cadmium(ll) toxicity, while phytochelatins play an important role in the defense mechanisms of plants (Chapter 13). The multicysteine peptides are frequently used to mimic the cadmium(ll) binding ability of all these natural substances. 

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