Analytical Sciences

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

Mun'delanji VESTERGAARD,*¹ Sachiko MATSUMOTO,*¹ Shingo NISHIKORI,*² Kentaro SHIRAKI,*³ Kazumasa HIRATA,*⁴ and Masahiro TAKAGI*¹

*1 School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan
*2 Division of Molecular Cell Biology, Institute of Molecular Embryology and Genetics, Kumamoto University, 2-2-1 Honjo, Kumamoto 862-0976, Japan
*3 Institute of Applied Physics, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573, Japan
*4 Department of Environmental Bioengineering Laboratory, Graduate School of Pharmaceutical Sciences, Osaka University, 1-6 Yamada-oka, Suita, Osaka 565-0871, Japan

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.

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