Extended X-ray absorption fine structure studies of the anion complexes of FeZn uteroferrin

Molybdate and tungstate are strong inhibitors of the purple acid phosphatases. The binding modes of these anions to the FeZn derivative of uteroferrin, the purple acid phosphatase from porcine uterus (FeZnUf), have been characterized by X-ray absorption spectroscopy at both the iron and zinc K-edges. Pre-edge data show that both FeZnUf·MoO₄ and FeZnUf·WO₄ have six-coordinate iron sites. Analysis of the EXAFS regions shows that the iron sites of both molybdate and tungstate complexes are best simulated by a shell of three O or N atoms at 2.08-2.09 Å and a shell of two O or N atoms at 1.93-1.95 Å. On the other hand, the zinc sites have shells of five O or N atoms at ~2.1 Å and one O or N atom at ~2.5 Å. Because of the higher resolution of the FeZnUf·MoO4 data, the main shell at ~2.1 Å can be further split into shells of four O or N at 2.04 Å and one O or N at 2.22 Å, the latter being associated with a molybdate oxygen. Outer-sphere EXAFS analysis indicates an Fe-Zn separation of ~3.4 Å for both FeZnUf·MoO₄ and FeZnUf·WO₄, Fe-Mo/W distances of 3.2 Å, and Zn-Mo/W distances of 3.6-3.7 Å. Thus, molybdate and tungstate bridge the FeZn active site like phosphate, but do so unsymmetrically. The asymmetric bidentate bridging mode of molybdate and tungstate helps explain the effect of these anions on the redox properties of the diiron uteroferrin.