Chelation of Cadmium Ions by Phytochelatin Synthase: Role of the Cystein-rich C-Terminal

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The interactions between Cd²⁺ and the C-terminal region of phytochelatin (PC) synthase using recombinant wild-type and mutant PC synthase were studied. We show that site-directed mutagenesis of Cys residues at C³⁵⁸C³⁵⁹XXXC³⁶³XXC³⁶⁶ motif decreases the number of Cd²⁺ and other heavy metal ions interacting with the enzyme, and that the motif binds the metals discriminatingly. The optimum binding ratio of PC synthase to Cd²⁺ was also determined. The findings indicate that Cys exists as a free SH residue and that it is involved in the regulation of PC enzyme activity by transferring the metals into closer proximity with the catalytic domain. These results are important in understanding heavy metal detoxification mechanisms in higher plants, a step towards phytoremediated-applications.