The Importance of Stereochemically Active Lone Pairs For Influencing Pb^II and As^III Protein Binding

Abstract

Heavy metal toxicity is a worldwide problem which is associated with the metal’s high affinity for thiolate rich proteins. Despite the tremendous toxicity concern, the mode of binding of As(III) and Pb(II) to proteins is poorly understood. To clarify the requirements for toxic metal binding to metalloregulatory sensor proteins such as As(III) in ArsR/ArsD and Pb(II) in PbrR or replacing Zn(II) in δ-aminolevulinc acid dehydratase (ALAD), we have employed computational and experimental methods examining these heavy metals binding to designed peptide models. The computational results show that the mode of coordination of As(III) and Pb(II) is greatly influenced by the steric bulk within the second coordination environment of the metal. The proposed basis of this selectivity is the large size of the ion and, most important, the influence of the stereochemically active lone pair in hemi-directed complexes of the metal ion as being crucial. The experimental data show that switching a bulky leucine layer above the metal binding site by a smaller alanine residue enhances the Pb(II) binding affinity by a factor of five supporting experimentally this lone pair steric hindrance hypothesis. These complementary approaches demonstrate the potential importance of a stereochemically active lone pair as a metal recognition mode in proteins and, specifically, how the second coordination sphere environment affects the affinity and selectivity of protein targets by certain toxic ions.

Protein Design: Toward Functional Metalloenzymes

“…The model corresponding to the lowest computed energy was described with two endo Cys residues and one exo Cys residue, whereas the most energetically unfavorable conformer had two exo Cys and one endo . 712 Similar DFT results were also reported for Pb(II) complexation by these systems. EXAFS studies showed that both TRI L16C and TRI L12C bind As(III) with an As–S bond length of 2.25 Å, which is consistent with the reported values for ArsR (2.25 Å) and small molecule As(III)S 3 ligation site distances (2.20–2.33 Å).…”

Spectroscopic, structural, and thermodynamic aspects of the stereochemically active lone pair on lead(II): Structure of the lead(II) dota complex

“…The steric consequences of the lone pair (Lp) of electrons on Pb(II) has been of enduring interest [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18], and there has been some doubt expressed even as to the existence of a stereochemically active Lp on Pb(II) [14,15,17]. Of interest here is that a study of the Tl(I) complex of 18-crown-6 (see Figure 1 for ligand abbreviations) has been used to suggest that there is no 6sp hybridization involved in producing what appears to be a stereochemically active Lp on the Tl(I) [17].…”

Sculpting Metal‐binding Environments in De Novo Designed Three‐helix Bundles

“…[7,14,15] Much of our effort and success in the de novo design field has been carried out using the 3SCC scaffold of TRI, [16–31] Grand (Gr), [24,26,32,33] and BABY [18,33,34] peptides, as well as in CoilSer (CS), [22,27,28,33,35–37] which serves as a crystallographic analogue for TRI (Table 1). A metal-binding site is generated by incorporating a cysteine or penicillamine residue at the “a” (Figure 2b) or “d” (Figure 2c) position in a 3SCC scaffold.…”

“…[37] We discovered that the subtle difference between the “a” and “d” positions can produce distinctive outcomes in heavy metal binding affinity and geometry, which can be attributed to the pre-organization of the sulfur ligands prior to metal binding. [14,17] Overall, we have gained a deeper understanding in the metallobiochemistry of heavy metals, such as As(III), [19,28,35] Cd(II), [14,19–22,24–26,32] Hg(II), [16–19,22,23,25,27,34] and Pb(II), [21,28,33,38] in a tristhiolate site. We have demonstrated how to control the coordination number and geometry of Cd(II) [14,24,26,32] and Hg(II), [18,19,23,25] determined the affinity for Cd(II) [21] and Pb(II), [21,33] based on site preferences for “a” or “d” sites of the Cys residues, and elucidated the effects of the core aliphatic groups in the second coordination sphere on the molecular recognition of Cd(II) [14,24,26] and Pb(II).…”