Design of a Peptidic Turn with High Affinity for Hg^II

Abstract

A four amino acid peptide containing the β-turn template dPro-Pro in the middle and two cysteines (Cys) in the terminal positions (CdPPC) has been synthesized and its mercury(II) coordination properties studied using different spectroscopic methods. The UV-vis, CD, (199m)Hg PAC, and Raman spectroscopic studies indicate the binding of Hg(II) to the two Cys, forming the dithiolatemercury(II) complex Hg(CdPPC). Electrospray ionization mass spectrometry corroborates the 1:1 complex formation. A log K = 40.0 was determined for the formation constant of the Hg(CdPPC) complex using competition potentiometric studies. Replacement of the dPro-Pro motif by a Pro-Pro unit generated a peptide (CPPC) capable of forming a similar species [Hg(CPPC)] but showing a lower affinity for Hg(II) (at least 3-3.5 orders of magnitude lower). The introduction of the dPro-Pro motif is crucial to induce the folding of the CdPPC peptide into a β-turn, preorganizing the two Cys for mercury(II) coordination. While the simple dPro-Pro unit mimics the overall preorganization achieved by the protein scaffold in metalloproteins containing the conserved metal ion chelation unit CXXC, the high thiophilicity of this metal stabilizes the final complex in a wide pH range (1.1-10). Using computational modeling, the structures of two conformers for Hg(CdPPC) have been optimized that differ mainly in the orientation of the plane containing S-Hg-S with respect to the anchoring C atoms.

Metal ion mediated transition from random coil to β-sheet and aggregation of Bri2-23, a natural inhibitor of Aβ aggregation

“…55 Constants determined for cysteine can be assigned to amine (10.21), thiolate (8.24) and carboxylate (2.12) protonations ( Table 1). As in previous studies 57 five complex species have been found within the measured pH range all engaging two ligands in Hg(II) sequestration (Table 2). Subsequently, potentiometric titration at a 1 : 1 : 1 Hg(II)/Cys/Bri2-23 ratio was carried out.…”

“…Therefore, complex formation constants could not be determined directly from potentiometric titration and competitive potentiometric experiments were carried out with cysteine as a competitor ligand under our experimental conditions. 57 First, the protonation constants for cysteine were determined from potentiometric titrations followed by the determination of stability constants for its mercuric complexes where the formation constant reported by Stricks and Kolthoff for Hg(Cys) 2 species was used as the fixed value. 55 Constants determined for cysteine can be assigned to amine (10.21), thiolate (8.24) and carboxylate (2.12) protonations ( Table 1).…”

Protein Design: Toward Functional Metalloenzymes

“…746 More recently, a tetrapeptide Ac-C D -PPC-NH 2 (CdPPC) was designed by Iranzo and co-workers. 749 The D-Pro-Pro motif forms a preorganized β -turn that directs the orientation of the two Cys, shown to bind to Hg(II) with a high affinity (the stability constant log β = 40.0), forming a linear mononuclear dithiolate–Hg(II) complex. However, the peptide with stereochemically reversed configuration CPPC (Ac-CPPC-NH 2 ) showed a much lower stability constant, demonstrating the efficacy of the D-Pro-Pro template in stabilizing the linear Hg(II) coordination.…”

Mercury Complexes with Tripodal Pseudopeptides Derived from D‐Penicillamine Favour a HgS3 Coordination

“…The digonal HgS 2 environment has for instance been favoured in complexes with highly constrained short peptide sequences that pre-orient the cysteine binding groups thanks to a peptidic turn. [19] More flexible peptide structures are mimics of metal transporter binding loops. [20] The uncommon trigonal environment HgS 3 found in Mer-R is more difficult to achieve.…”