Abstract
Higher plants, algae and some yeasts respond to potentially toxic heavy metals such as cadmium by synthesizing phytochelatins and related cysteine-rich polypeptides. We have used X-ray absorption spectroscopy to study the nature of cadmium binding in such peptides isolated from maize (Zea mays) exposed to low levels of cadmium, and in two synthetic cadmium-peptide complexes, Cd-(gamma-Glu-Cys)(3)Gly and Cd-(a-Glu-Cys)3Gly. We have used the synthetic ions [Cd(SPh)(4)](2-), [Cd-4(SPh)(10)](2-) and [S4Cd10(SPh)(16)](4-) as crystallographically defined models for the cadmium site. The Cd K-edge extended X-ray absorption fine structure (EXAFS) data, together with the Cd K, L-I, L-II and L-III near-edge spectra, reveal a predominantly tetrahedral coordination of cadmium by sulfur in both the phytochelatin and synthetic peptide complexes. In particular, the Cd Lm-edge lacks a peak at 3534.9 eV which was found to be prominent for oxygen- or nitrogen-coordinated species. The Cd-S distance in the phytochelatin complex is 2.54 Angstrom. The Cd K-edge EXAFS does not show any isolated, well-defined Cd-Cd interactions; however, contrary to the conclusion of previous work, their absence is not necessarily indicative of isolated cadmium-thiolate ligation. Evidence from other studies suggests that high static disorder, combined with a large vibrational component, serve to effectively wash out this contribution to the EXAFS. The sulfur K-edge, moreover, shows a low-energy feature both in the phytochelatin and in the synthetic cadmium-peptide complexes which is consistent with sulfide bound in a cluster with cadmium as found for [S4Cd10(SPh)(16)](4-) This feature strongly suggests the presence of a polynuclear cadmium cluster in maize phytochelatin. (C) 1999 Published by Elsevier Science B.V, All rights reserved.
Original language | English |
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Pages (from-to) | 351-364 |
Number of pages | 14 |
Journal | Biochimica et Biophysica Acta (BBA) - Protein Structure and Molecular Enzymology |
Volume | 1429 |
Issue number | 2 |
DOIs | |
Publication status | Published - 11 Jan 1999 |
Keywords
- cadmium
- phytochelatin
- Cd EXAFS
- X-ray absorption spectroscopy
- Cd L-edge
- SK-edge
- metal-binding peptides
- K-edge
- fine-structure
- sulfide crystallites
- thiolate clusters
- Indian mustard
- EXAFS
- metallothioneins
- complexes
- spectra