Phytochelatins PCs

Murasugi and his group in 1981 first discovered the peptides in Cd-binding complexes produced in fission yeast, Schizosaccharomyces pombe exposed to Cd2+ ions and named these as cadystins (Murasugi et al. 1981). Cadystins are of two type cadystin A and B. These cadystins were latter named as phytochelatins. Phytochelatins are capable of binding various metals including Cd, Cu, Zn, As, Cd or metalloid As. The structure of PCs is (y Glu-Cys)nX in which X is Gly, y-Ala, Ser or Glu and n = 2-11 depending on the organism. Many plants cope with the higher levels of heavy metals by binding them in complexes with PCs and sequestering the complexes inside their cells. Biosynthesis of PCs from their common precursors; glutamine (Glu, E), cysteine (Cys, C) and glycine (Gly, E) is presented in Fig. 2.3. The pathway is completely overlapped with that of GSH biosynthesis because PCs are synthesized from GSH as a direct substrate. The biosynthesis of GSH consists of two sequential reactions mediated by y-glutamyl-cyst synthatase (yECS) and both reactions require ATP as substrate. The yEC synthetase activity is induced by many metals, including Cd, Hg, Cu, Ni, Pb, As and Zn, however, Cd is by for the strongest inducer. Also, the activity of yEC synthase is inhibited by treatment with buthionine sulfoximine (BSO). PC synthase mediates the synthesis of PCs from GSH. This enzyme is constitutively expressed but requires metal inducers. Cd2+ ions are most efficient activators among the metal ions. There

Glutamate Glycine

Cysteine-i—H yGlu-Cys -1—i yGlu-Cys-Gly -1—► ("yGlu-Cys)n-Gly

■ i i i i i yEC synthetase GSH synthetase PC synthase n = 2-11

GSH1, CAD2 genes GSH2 gene CAD1, PCS1, PCS2 genes

Fig. 2.3 Biosynthesis of PCs in higher plants are genes encoding the key enzymes for PC biosynthesis (Fig. 2.3). More recently, PC synthase gene (PCS1, CAD1) has been isolated from A. thaliana (Ha et al. 1999). This gene may be more widespread and have more general functions. PCs are also reported to be involved in the homeostasis of Zn2+ and Cu+/Cu2+ by providing a transient storage form for the ions (Grill et al. 1988; Thumann et al. 1991). The induction of PCs by the anion arsenate has been observed in a survey for peptide-inducing metal ions (Grill et al. 1987) and suggests a unique mode of PC synthase activation. However, Maitani et al. (1996) failed to demonstrate an As-PC complex. This result indicates that PCs do not fulfill a detoxifying function during As poisoning. Raab et al. (2004) have developed a method to ascertain the nature of As-PC complexes in extracts of the As-tolerant grass Holcus lonatus and the As hyperaccumulator Pteris cretica using parallel metal (loid)-specific (inductively coupled plasma-mass spectrometry) and organic-specfic (electrospray ionization-mass spectrometry) detection systems. In H. lanatus, the As(III)-PC3 complex was the dominant complex, although GSH, PC2, and PC3 were found in the extract. P. cretica only synthesizes PC2 and forms dominantly the GSH-AS(III)-PC2 complex. In both plant species, As is dominantly in non-bound inorganic forms, with 13% being present in PC complexes for H. lanatus and 1% in P. cretica (Raab et al. 2004).

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