Protein stabilization by engineered metal chelation.

A ligand can shift a protein's folding/unfolding equilibrium by binding with higher affinity to the native state. A metal-chelating site consisting of two histidines separated by three residues (His-X3-His) engineered into an alpha-helix provides a general and easily-implemented means for protein stabilization by this mechanism. We have tested this approach with the iso-1-cytochrome c of Saccharomyces cerevisiae substituted with histidine at positions 4 and 8 in its N-terminal alpha-helix. One mM Cu(II) complexed to iminodiacetate stabilizes the cytochrome c variant by ca. 1 kcal/mol, as determined by guanidinium chloride-induced unfolding. The protein's folding/unfolding equilibrium is shifted by a free energy equal to that calculated from the metal ion's preferential binding to the native protein. Given the ubiquity of surface alpha-helices and the additional possibility of inserting di-histidine chelating sites into turns and beta-structures, we conclude that this is a useful method for protein stabilization.