Unusual Trigonal-Planar Copper Configuration Revealed in the Atomic Structure of Yeast Copper−Zinc Superoxide Dismutase

The three-dimensional structure of yeast copper−zinc superoxide dismutase (CuZnSOD) has been determined in a new crystal form in space group R32 and refined against X-ray diffraction data using difference Fourier and restrained crystallographic refinement techniques. The unexpected result is that the copper ion has moved approximately 1 Å from its position in previously reported CuZnSOD models, the copper−imidazolate bridge is broken, and a roughly trigonal planar ligand geometry characteristic of CuI rather than CuII is revealed. Final R values for the two nearly identical room temperature structures are 18.6% for all 19 149 reflections in the 10.0−1.7 Å resolution range and 18.2% for 17 682 reflections (F > 2σ) in the 10.0−1.73 Å resolution range. A third structure has been determined using X-ray data collected at −180 °C. The final R value for this structure is 19.0% (R free = 22.9%) for all 24 356 reflections in the 10.0−1.55 Å resolution range. Virtually no change in the positions of the ligands to the zinc center is observed in these models. The origin of the broken bridge and altered Cu−ligand geometry is discussed.