Tuning the geometries of a de novo blue copper protein by axial interactions

The axial interactions of Cu(2+) in type 1 copper proteins control the physical characteristics of the proteins. We tuned the geometries of a de novo designed blue copper protein with a four-helical bundle structure. The designed protein axially bound various ligands, such as chloride, phosphate, sulfate, acetate, azide, and imidazole, to Cu(2+), exhibiting a blue or green color. The UV-vis spectral bands were observed at approximately 600 nm and approximately 450 nm, with the A (~450)/A (~600) ratios between 0.14 and 1.58. The stronger axial interaction shifted the geometry of the type 1 copper site from trigonal planar geometry (blue copper) toward a tetrahedral-like geometry (green copper). Resonance Raman spectral analyses showed that the phosphate-bound type had the highest-strength Cu-S bond, similar to that of plastocyanin. The chloride-bound type exhibited features similar to those of stellacyanin and nitrite reductase, and the imidazole-bound type exhibited features similar to those of azurin M121E mutant.