A Binuclear Cu(A) Center Designed in an All α-Helical Protein Scaffold.

The primary and secondary coordination spheres of metal binding sites in metalloproteins have been investigated extensively, leading to the creation of high-performing functional metalloproteins; however, the impact of the overall structure of the protein scaffold on the unique properties of metalloproteins has rarely been studied. A primary example is the binuclear Cu(A) center, an electron transfer cupredoxin domain of photosynthetic and respiratory complexes and, recently, a protein coregulated with particulate methane and ammonia monooxygenases. The redox potential, Cu-Cu spectroscopic features, and a valence delocalized state of Cu(A) are difficult to reproduce in synthetic models, and every artificial protein Cu(A) center to-date has used a modified cupredoxin. Here, we present a fully functional Cu(A) center designed in a structurally nonhomologous protein, cytochrome c peroxidase (CcP), by only two mutations (Cu(A)CcP). We demonstrate with UV-visible absorption, resonance Raman, and magnetic circular dichroism spectroscopy that Cu(A)CcP is valence delocalized. Continuous wave and pulsed (HYSCORE) X-band EPR show it has a highly compact g(z) area and small A(z) hyperfine principal value with g and A tensors that resemble axially perturbed Cu(A). Stopped-flow kinetics found that Cu(A) formation proceeds through a single T2Cu intermediate. The reduction potential of Cu(A)CcP is comparable to native Cu(A) and can transfer electrons to a physiological redox partner. We built a structural model of the designed Cu binding site from extended X-ray absorption fine structure spectroscopy and validated it by mutation of coordinating Cys and His residues, revealing that a triad of residues (R48C, W51C, and His52) rigidly arranged on one α-helix is responsible for chelating the first Cu(II) and that His175 stabilizes the binuclear complex by rearrangement of the CcP heme-coordinating helix. This design is a demonstration that a highly conserved protein fold is not uniquely necessary to induce certain characteristic physical and chemical properties in a metal redox center.