Abstract
Phytochelatins (PCs) are short Cys-rich peptides with repeating γ-Glu-Cys motifs found in plants, algae, certain fungi, and worms. Their biosynthesis has been found to be induced by heavy metals—both biogenic and toxic. Among all metal inducers, Cd(II) has been the most explored from a biological and chemical point of view. Although Cd(II)-induced PC biosynthesis has been widely examined, still little is known about the structure of Cd(II) complexes and their thermodynamic stability. Here, we systematically investigated glutathione (GSH) and PC2–PC6 systems, with regard to their complex stoichiometries and spectroscopic and thermodynamic properties. We paid special attention to the determination of stability constants using several complementary techniques. All peptides form CdL complexes, but CdL2 was found for GSH, PC2, and partially for PC3. Moreover, binuclear species CdxLy were identified for the series PC3–PC6 in an excess of Cd(II). Potentiometric and competition spectroscopic studies showed that the affinity of Cd(II) complexes increases from GSH to PC4 almost linearly from micromolar (log K7.4GSH = 5.93) to the femtomolar range (log K7.4PC4 = 13.39) and additional chain elongation does not increase the stability significantly. Data show that PCs form an efficient system which buffers free Cd(II) ions in the pico- to femtomolar range under cellular conditions, avoiding significant interference with Zn(II) complexes. Our study confirms that the favorable entropy change is the factor governing the elevation of phytochelatins’ stability and illuminates the importance of the chelate effect in shifting the free Gibbs energy.
Highlights
Polycysteine peptides and short proteins play a fundamental role in the metabolism and detoxification of essential and toxic heavy-metal ions.[1,2] In addition to metallothioneins (MTs), which are small cluster-forming proteins encoded in many genomes from bacteria to humans, phytochelatins (PCs) play similar functions.[2−5] They are produced by plants, algae, and certain fungi or worms to handle and detoxify heavy-metal ions.[2,6−11] The major difference from MTs is their polydisperse character
A spectroscopic titration of PC5 reveals the formation of two complexes with stoichiometries CdPC5 and Cd2PC5 with an visible red shift being characteristic for the second species (Figure 1E)
The presence of so many stoichiometries derives from the various Cys residues present in the particular PCs, their high flexibility, and lack of tendencies for secondary structure formation
Summary
Polycysteine peptides and short proteins play a fundamental role in the metabolism and detoxification of essential and toxic heavy-metal ions.[1,2] In addition to metallothioneins (MTs), which are small cluster-forming proteins encoded in many genomes from bacteria to humans, phytochelatins (PCs) play similar functions.[2−5] They are produced by plants, algae, and certain fungi or worms to handle and detoxify heavy-metal ions.[2,6−11] The major difference from MTs is their polydisperse character. The biosynthesis of PCs is initiated by administering a wide range of heavy-metal ions and several anionic species.[2,7] For example, Cd(II), Pb(II), Zn(II), Sb(III), Ag(I), Ni(II), Hg(II), Cu(II), Sn(II), Au(I), Bi(III), AsO43−, and SeO32− induce formation in Rauvolfia serpentina cell suspension cultures.[14] Several studies show the participation of PGEs (platinum-group elements, such as Pt(II), Rh(III), and Pd(II)) in the synthesis of phytochelatins in some plant organs.[15,16] Interestingly, inorganic ions induce PC synthesis to different extents; Cd(II) demonstrated in many examples induces
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