Linear Free-Energy Analysis of Mercury(II) and Cadmium(II) Binding to Three-Stranded Coiled Coils

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Abstract
Investigators have studied how proteins enforce nonstandard geometries on metal centers to assess the question of how protein structures can define the coordination geometry and binding affinity of an active-site metal cofactor. We have shown that cysteine-substituted versions of the TRI peptide series [AcG-(LKALEEK)4G-NH2] bind HgII and CdII in geometries that are different from what is normally found with thiol ligands in aqueous solution. A fundamental question has been whether this structural perturbation is due to protein influence or a change in the metal geometry preference. To address this question, we have completed linear free-energy analyses that correlate the association of three-stranded coiled coils in the absence of a metal with the binding affinity of the peptides to the heavy metals, HgII and CdII. In this paper, six new members of this family have been synthesized, replacing core leucine residues with smaller and less hydrophobic residues, consequently leading to varying degrees of self-association affinities. At the same time, studies with some smaller and longer sequenced peptides have also been examined. All of these peptides are seen to sequester HgII and CdII in an uncommon trigonal environment. For both metals, the binding is strong with micromolar dissociation constants. For binding of HgII to the peptides, the dissociation constants range from 2.4 × 10-5 M for Baby L12C to 2.5 × 10-9 M for Grand L9C for binding of the third thiolate to a linear HgII(pep)2 species. The binding of HgII to the peptide Grand L9C is similar in energetics for metal binding in the metalloregulatory protein, mercury responsive (merR), displaying ∼50% trigonal HgII formation at nanomolar metal concentrations. Approximately, 11 kcal/mol of the HgII(Grand L9C)3- stability is due to peptide interactions, whereas only 1−4 kcal/mol stabilization results from HgII(RS)2 binding the third thiolate ligand. This further validates the hypothesis that the favorable tertiary interactions in protein systems such as merR go a long way in stabilizing nonnatural coordination environments in biological systems. Similarly, for the binding of CdII to the TRI family, the dissociation constants range from 1.3 × 10-6 M for Baby L9C to 8.3 × 10-9 M for TRI L9C, showing a similar nature of stable aggregate formation.

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