Soft metal ions, Cd(II) and Hg(II), induce triple-stranded alpha-helical assembly and folding of a de novo designed peptide in their trigonal geometries

We previously reported the de novo design of an amphiphilic peptide [YGG(IE KKIEA)(4)] that forms a native-like, parallel triple-stranded coiled coil. Starting from this peptide, we sought to regulate the: assembly of the pept ide by a metal ion. The replacement of the lie 18 and Ile22 residues with A la and Cys residues, respectively, in the hydrophobic positions disrupted o f the triple-stranded alpha-helix structure. The addition of Cd(II), howeve r, resulted in the reconstitution of the triple-stranded alpha-helix bundle , as revealed by circular dichroism (CD) spectroscopy and sedimentation equ ilibrium analysis. By titration with metal ions and monitoring the change i n the intensity of the CD spectra at 222 nm, the dissociation constant K-d was determined to be 1.5 +/- 0.8 mu M fur Cd(II). The triple-stranded compl ex formed by the Cd-113(II) ion showed a single Cd-113 NMR resonance at 572 ppm whose chemical shift was not affected by the presence of Cl- ions. The Cd-113 NMR resonance was connected with the beta H protons of the cysteine residue by H-1-Cd-113 heteronuclear multiple quantum correlation spectrosc opy. These NMR results indicate that the three cysteine residues are coordi nated to the cadmium ion in a trigonal-planar complex. Hg(II) also induced the assembly of the peptide into a triple-stranded alpha-helical bundle bel ow the Hg(II)/peptide ratio of 1/3. With excess HE(II), however, the alpha- helicity of the peptide was decreased, with the change of the Hg(II) coordi nation state from three to two. Combining this construct with other functio nal domains should facilitate the production of artificial proteins with fu nctions controlled by metal ions.