Fig. 1.

Schematic of design strategies for first, second and third rounds of design. In the first set of designs (Phase 1), tetrahedral zinc binding sites were designed with three coordinating residues and one open coordination site with at least two of the ligands on stable secondary-structure elements. In Phase 2, we designed two second-shell hydrogen bonding residues in addition to three coordinating histidines. To increase the diversity of designed sites, Phase 3 allowed one of the coordinating residues to be an aspartate or glutamate and removed the secondary-structure requirement for ligand residues, instead requiring all ligand residues to have C_α B factor values of 30 or greater.