Incorporation of second coordination sphere d-amino acids alters Cd(II) geometries in designed thiolate-rich proteins

We use a de Novo protein design strategy to demonstrate that the second coordination sphere of a metal site plays a key role in controlling coordination geometries of Cd(II)-tris-thiolate complexes. Specifically, we show that alteration of chirality within the core hydrophobic packing region of a three-stranded coiled coil (3SCC) can control the coordination number of Cd(II) by limiting steric encumbrance to the metal center. Within a specific class of 3SCCs [Ac-G-(LKALEEK) (n) -G-NH2], where n = 4 is TRI and n = 5 is GRAND, one l-Leu may be substituted by l-Cys to generate a planar tris-thiolate array capable of metal binding. In the native peptide containing only the l-configuration of leucine, the three-Cys ligand site leads to a mixture of 3- and 4-coordinate Cd(II). When the l-Leu above (toward the N-terminus) the tris-Cys site is substituted with d-Leu, solely a 3-coordinate structure [Cd(II)S-3] was obtained. When d-Leu is located below (toward the C-terminus), a mixture of two coordination geometries, presumably Cd(II)S3O and Cd(II)S3O2, is observed, while substitution with d-Leu both above and below the tris-Cys plane yields a higher percentage of 4-coordinate Cd(II)S3O species. Thus, the use of d-amino acids around a metal's coordination sphere provides a powerful tool for controlling the properties of future designed metalloproteins.