Metal-binding properties of phytochelatin-related peptides

Phytochelatins (PCs, (gamma Glu-Cys)(n)-Gly, n=2-11) are produced by higher plants, algae and some fungi in order to detoxify Cd2+ by sequestration to form Cd-PCs complexes. In order to investigate what chemical structures of PCs are responsible for their metal-binding ability, various cysteine-rich peptides ((X-Cys)(7)-Gly, X=Glu, Asp, Lys, Gly, Ser and Gln) were chemical ly synthesized. Water-solubility, metal-binding property, and detoxificatio n effect toward Cd2+ were analyzed and compared with those of (gamma EC)(7) G. (SC)(7)G and (QC)(7)G were insoluble at pH below 10, and (GC)(7)G was no t soluble at any pH between I and 12, indicating that charged side chains w ere at least required for the molecules to be solubilized in aqueous soluti on. By spectroscopic analyses using DTNB method and UV method, we found tha t (EC)(7)G and (DC)(7)G had almost equivalent abilities of Cd2+-binding as PC ((gamma EC)(7)G), indicating that the distance between each thiol group was not a major factor for the binding to Cd2+. (beta DC)(7)G and (KC)(7)G interacted to Cd2+ with fourth coordination as in the case of other soluble PC-related peptides. However, compared to (gamma EC)(7)G, (beta DC)(7)G di splayed a slightly weaker binding to Cd2+, and (KC)(7)G showed a drastic de crease in binding ability. The affinities of PC-related peptides toward Cd2 + were evaluated as below; (gamma EC)(7)G=(EC)(7)G=(DC)(7)G >(beta DC)(7)G > > (KC)(7)G=weak binding. The results of Cd2+ -detoxification assays were consistent with the affinity between Cd2+ and the peptides. We concluded th at the structure consisting of thiol and carboxyl groups were essential for the formation of a tight Cd-peptides complex such as Cd-PCs. (C) 2001 Else vier Science B.V. All rights reserved.