Binuclear Cu A Formation in Biosynthetic Models of Cu A in Azurin Proceeds via a Novel Cu(Cys) 2 His Mononuclear Copper Intermediate

Chakraborty, Saumen; Polen, Michael J.; Chacón, Kelly N.; Wilson, Tiffany D.; Yu, Yang; Reed, Julian; Nilges, Mark J.; Blackburn, Ninian J.; Lu, Yi

doi:10.1021/acs.biochem.5b00659

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 CuA center, an electron transfer cupredoxin domain of photosynthetic and respiratory complexes and, recently, a protein co-regulated with particulate methane and ammonia monooxygenases. The redox potential, Cu–Cu spectroscopic features, and a valence delocalized state of CuA are difficult to reproduce in synthetic models, and every artificial protein CuA center to-date has used a modified cupredoxin. Here we present a fully functional CuA center designed in a structurally non-homologous protein, cytochrome c peroxidase (CcP), by only two mutations (CuACcP). We demonstrate with UV-visible absorption, resonance Raman, and MCD spectroscopy that CuACcP is valence delocalized. CW and pulsed (HYSCORE) X-band EPR show it has a highly compact gz area and small Az hyperfine principal value with g and A tensors that resemble axially perturbed CuA. Stopped-flow kinetics found that CuA formation proceeds through a single T2Cu intermediate. The reduction potential of CuACcP is comparable to native CuA and can transfer electrons to a physiological redox partner. We built a structural model of the designed Cu binding site from EXAFS 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 atom 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 to a metal redox center.

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