title: d-Cysteine Ligands Control Metal Geometries within De Novo Designed Three-Stranded Coiled Coils.
abstract: 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 CdIIgeometry 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.
authors.affiliation: Department of Chemistry, University of Michigan, Ann Arbor, Michigan, 48109, USA.|Present address: Department Chemistry, Faculty of Science, King Mongkut's University of Technology Thonburi (KMUTT), Bang Mod, ThungKhru, Bangkok, 10140, Thailand.
authors.forename: Leela
authors.initials: L
authors.lastname: Ruckthong
authors.affiliation: Department of Chemistry, University of Michigan, Ann Arbor, Michigan, 48109, USA.|Present address: School of Chemistry, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK.
authors.forename: Anna F A
authors.initials: AFA
authors.lastname: Peacock
authors.affiliation: Department of Chemistry, University of Michigan, Ann Arbor, Michigan, 48109, USA.
authors.forename: Cherilyn E
authors.initials: CE
authors.lastname: Pascoe
authors.affiliation: Department of Chemistry, University of Copenhagen, Universitetsparken 5, 2100, København Ø, Denmark.
authors.forename: Lars
authors.initials: L
authors.lastname: Hemmingsen
authors.affiliation: Life Sciences Institute, University of Michigan, Ann Arbor, Michigan, 48109, USA.
authors.forename: Jeanne A
authors.initials: JA
authors.lastname: Stuckey
authors.affiliation: Department of Chemistry, University of Michigan, Ann Arbor, Michigan, 48109, USA.
authors.forename: Vincent L
authors.initials: VL
authors.lastname: Pecoraro
keywords: X-ray crystallography; cadmium; d-amino acids; protein design; zinc