Environmental Chemistry Letters
Phytochelatins or (γ–glutamyl-cysteine)n-glycine are specialized peptides produced by plants and algae to mitigate toxic metal exposure. Previous experimental studies have reported biological production of these peptides specifically in response to high levels of heavy metals including Cu, Cd, and Zn. Stability constants and structural characterization of metal-phytochelatin complexes are largely lacking. This information is required to gain mechanistic insights on the metal selectivity of phytochelatins. Here we elucidate structural coordination in concert with thermodynamic stability predictions by performing molecular dynamics simulations of a fully hydrated phytochelatin molecule complexed with Ca, Mg, Fe(II), Zn(II), and Cu(II). Our molecular dynamics results predicted the following order for the thermodynamic stability of the different complexes: Zn(II) ≥ Cu(II) ≥ Fe(II) > Mg > Ca. Shorter binding distances and greater coordination from carboxylate and carbonyl O atoms explained the favorable binding energies with Zn(II) and Cu(II) over the other metal cations. Conformational re-arrangement of phytochelatin following metal chelation was captured by monitoring changes in the solvent-accessible volume. Accessibility of solvent molecules to the phytochelatin structure was inversely proportional to the distance between the coordinated ligands and the chelated metal. These new findings demonstrate the influence of the metal-phytochelatin structure on the metal binding thermodynamics and the phytochelatin conformation, both of which are important to evaluating the intracellular role of phytochelatin in mediating algal response to toxic heavy metal exposure.