The effect of the side chain length of asp and glu on coordination structure of Cu2⁺ in a de novo designed protein

Metal ions in proteins are important not only for the formation of the proper structures but also for various biological activities. For biological functions such as hydrolysis and oxidation, metal ions often adopt unusual coordination structures. We constructed a stable scaffold for metal binding to create distorted metal coordination structures. A stable four stranded a-helical coiled-coil structure was used as the scaffold, and the metal binding site was in the cavity created at the center of the structure. Two His residues and one Asp or Glu residue were used to coordinate the metal ions, AM2D and AM2E, respectively. Cu²⁺ bound to AM2D with an equatorial planar coordination structure with two His, one Asp, and H₂O as detected by electron spin resonance and UV spectral analyzes. On the other hand, Cu²⁺ had a slightly distorted square planar structure when it bound two His and Glu in AM2E, due to the longer side-chain of the Glu residue as compared to the Asp residue. Computational analysis also supported the distorted coordination structure of Cu²⁺ in AM2E. This construct should be useful to create various coordinations of metal ions for catalytic functions.