Zn^IIand Hg^IIbinding to a designed peptide that accommodates different coordination geometries

Abstract

Designed metal ion binding peptides offer a variety of applications in both basic science as model systems of more complex metalloproteins, and in biotechnology, e.g. in bioremediation of toxic metal ions, biomining or as artificial enzymes. In this work a peptide (HS: Ac-SCHGDQGSDCSI-NH2) has been specifically designed for binding of both Zn(II) and Hg(II), i.e. metal ions with different preferences in terms of coordination number, coordination geometry, and to some extent ligand composition. It is demonstrated that HS accommodates both metal ions, and the first coordination sphere, metal ion exchange between peptides, and speciation are characterized as a function of pH using UV-absorption-, synchrotron radiation CD-, (1)H-NMR-, and PAC-spectroscopy as well as potentiometry. Hg(II) binds to the peptide with very high affinity in a {HgS2} coordination geometry, bringing together the two cysteinates close to each end of the peptide in a loop structure. Despite the high affinity, Hg(II) is kinetically labile, exchanging between peptides on the subsecond timescale, as indicated by line broadening in (1)H-NMR. The Zn(II)-HS system displays more complex speciation, involving monomeric species with coordinating cysteinates, histidine, and a solvent water molecule, as well as HS-Zn(II)-HS complexes. In summary, the HS peptide displays conformational flexibility, contains many typical metal ion binding groups, and is able to accommodate metal ions with different structural and ligand preferences with high affinity. As such, the HS peptide may be a scaffold offering binding of a variety of metal ions, and potentially serve for metal ion sequestration in biotechnological applications.

Zinc(ii) and cadmium(ii) complexes of N-terminally free peptides containing two separate cysteinyl binding sites

“…In the presence of zinc(II) or cadmium(II) ions there is no any measurable change in the positions of the signals of the terminal Ala residue, although the extent of complexation is almost 100% by this pH. The pK values characterized these deprotonation processes are different in the case of ACSSACS-NH 2 from those of CSSACS-NH 2 or other cysteine containing peptides with free carboxylate group on C-termini 14,15 or terminally protected multicysteine peptides with Ac-Xaa-Cys-……..-NH 2 sequences 32,33,34 . Further increase of pH results in the upfield shift of the methyl protons but this occurs in parallel with the free ligand suggesting that the terminal amino group does not have a significant contribution to metal binding.…”

“…This observation unambiguously proves that the amino group of terminal Ala residue remains protonated in the species [MH 2 L 2 ] and [MHL] + , while the two thiolate residues are the major metal binding sites in the form of an 18-membered macrochelate. For example the pK(ZnL/ZnH -1 L)=7.95 for ACSSACS-NH 2 , while 9.05 for CSSACS-NH 2 and 8.65 for Ac-SCHGDQGSDCSI-NH 2 34 . The difference in the preferred binding modes of the hexa-and heptapeptides is reflected in the possibility of the formation of bis(ligand) complexes, too.…”

C‐terminal Cysteines of CueR Act as Auxiliary Metal Site Ligands upon HgII Binding—A Mechanism To Prevent Transcriptional Activation by Divalent Metal Ions?

“…[2] Ourp revious studies on model peptides of the metal bindingl oop of CueR also showedt hat these fragments bind Cu I with ah igh affinity. [11] However, accordingt om odel peptide studies [12,13] and QM/MM calculations, [14] Hg II ions may be coordinatede ven more efficiently. Moreover,H g II is also able to bind to aC Cs equence, [15] and therefore coordinationo fH g II ion by the CCHH motif is also highly probable.…”

Specificity of the Metalloregulator CueR for Monovalent Metal Ions: Possible Functional Role of a Coordinated Thiol?

“…For the Zn II -MBL-VC complexes an additional increase in absorbance is observed at high pH (>10), presumably reflecting deprotonation of a coordinating water molecule. [8] In the presence of Ag I the absorbance at 230 nm of the two peptides is comparable to that of the Hg II -MBL-VC complexes at low pH, and considerably higher than for the free peptide, implying coordination of both Cys sulphurs. However, a surprising change in absorbance is observed at pH ~6.5 both for the Ag I -MBL-VC and Ag I -MBL-EC systems, apparently reflecting deprotonation of an ionizable group.…”