Model Peptides Based on the Binding Loop of the Copper Metallochaperone Atx1:  Selectivity of the Consensus Sequence MxCxxC for Metal Ions Hg(II), Cu(I), Cd(II), Pb(II), and Zn(II)

Abstract

The amino acid sequence MxCxxC is conserved in many soft-metal transporters that are involved in the control of the intracellular concentration of ions such as Cu(I), Hg(II), Zn(II), Cd(II), and Pb(II). A relevant task is thus the selectivity of the motif MxCxxC for these different metal ions. To analyze the coordination properties and the selectivity of this consensus sequence, we have designed two model peptides that mimic the binding loop of the copper chaperone Atx1: the cyclic peptide P(C) c(GMTCSGCSRP) and its linear analogue P(L) (Ac-MTCSGCSRPG-NH2). By using complementary analytical and spectroscopic methods, we have demonstrated that 1:1 complexes are obtained with Cu(I) and Hg(II), whereas 1:1 and 1:2 (M:P) species are successively formed with Zn(II), Cd(II), and Pb(II). The complexation properties of the cyclic and linear peptides are very close, but the cyclic compound provides systematically higher affinity constants than its unstructured analogue. The introduction of a xPGx motif that forms a type II beta turn in P(C) induces a preorganization of the binding loop of the peptide that enhances the stabilities of the complexes (up to 2 orders of magnitude difference for the Hg complexes). The affinity constants were measured in the absence of any reducing agent that would compete with the peptides and range in the order Hg(II) > Cu(I) >> Cd(II) > Pb(II) > Zn(II). This sequence is thus highly selective for Cu(I) compared to the essential ion Zn(II) that could compete in vivo or compared to the toxic ions Cd(II) and Pb(II). Only Hg(II) may be an efficient competitor of Cu(I) for binding to the MxCxxC motif in metalloproteins.

A Series of Tripodal Cysteine Derivatives as Water‐Soluble Chelators that are Highly Selective for Copper(I)

“…[14,42] As expected the back-titrations show endpoints at one metal equivalent. The conditional stability constant at physiological pH (K pH 7.4 ) and the stability constant (K) calculated from the conditional stability constants and the ligands pK a given in Table 1 are listed in Table 6.…”

“…The objective of this work was the design of efficient chelators of the essential metal ion copper in the + I oxidation [14,15] In particular, the selectivity with respect to Zn II , which is a bioavailable ion present in cells and thus a potential competitor, is a key parameter to obtain efficient chelators in vivo. [12] We have shown here that the introduction of three cysteine residues in a tripodal architecture provides even more efficient Cu I chelators with large selectivities with respect to Zn II .…”

“…[13] In a previous report, the model cyclodecapeptide P C , c(MTCGSCSRP), incorporating the binding sequence (MTCSGC) of the yeast copper chaperone Atx1 was found highly selective for Cu I and Hg II with respect to Zn II , another essential metal ion found in cells. [14,15] Therefore, a glycopeptide also containing two cysteines and targeted at the hepatocytes was recently designed and demonstrated to be able to chelate intracellular copper. This compound may be a good candidate to fight copper overload in the liver.…”

“…To design efficient and selective soft metal ion chelators, we take advantage of the high affinity of cysteine sulfur donors for Cu I , evidenced in proteins trafficking or sequestering this metal in cells. [12,14,15,19] As trithiolato coordination environments afford very stable Cu I complexes in metallothioneins, ligands with three cysteine residues that promote a MS 3 geometry are very attractive. Therefore, three cysteine moieties can be attached with peptide bonds to nonbiological scaffolds to get pseudopeptide ligands.…”

Peptide-based biopolymers in biomedicine and biotechnology

“…Third, many peptide-based biomaterials are easily degraded by the body, thus making them desirable as drug delivery vehicles and tissue engineering scaffolds. Fourth, biology is replete with peptide sequences that exhibit structural transitions in response to the binding of metal ions and other biological ligands [13][14][15][16][17][18][19][20][21][22][23][24][25][26]; these sequences offer enormous possibilities in the design of biologically responsive materials.…”