d-Cysteine Ligands Control Metal Geometries within De Novo Designed Three-Stranded Coiled Coils

4 years ago

d-Cysteine Ligands Control Metal Geometries within De Novo Designed Three-Stranded Coiled Coils

Jeanne A. Stuckey, Leela Ruckthong, Anna F. A. Peacock, Vincent L. Pecoraro, Lars Hemmingsen, Cherilyn E. Pascoe

Although metal ion binding to naturally occurring l-amino acid proteins is well documented, understanding the impact of the opposite chirality (d-)amino acids on the structure and stereochemistry of metals is in its infancy. We examine the effect of a d-configuration cysteine within a designed l-amino acid three-stranded coiled coil in order to enforce a precise coordination number on a metal center. The d chirality does not alter the native fold, but the side-chain re-orientation modifies the sterics of the metal binding pocket. l-Cys side chains within the coiled-coil structure have previously been shown to rotate substantially from their preferred positions in the apo structure to create a binding site for a tetra-coordinate metal ion. However, here we show by X-ray crystallography that d-Cys side chains are preorganized within a suitable geometry to bind such a ligand. This is confirmed by comparison of the structure of ZnIICl(CSL16DC)32− to the published structure of ZnII(H2O)(GRAND-CSL12AL16LC)3−. Moreover, spectroscopic analysis indicates that the CdII geometry observed by using l-Cys ligands (a mixture of three- and four-coordinate CdII) is altered to a single four-coordinate species when d-Cys is present. This work opens a new avenue for the control of the metal site environment in man-made proteins, by simply altering the binding ligand with its mirror-imaged d configuration. Thus, the use of non-coded amino acids in the coordination sphere of a metal promises to be a powerful tool for controlling the properties of future metalloproteins. Cysteine in the mirror: The effect of changing the configuration of an amino acid from l to d within metal-binding peptide sequences is investigated. By substituting l-Cys with d-Cys residues a new binding situation for metal ions was created (see figure). This observation opens new possibilities for protein designers to control the properties of future metalloproteins.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1002/chem.201700660