The activity and selectivity of fission yeast Pop2p are affected by a high affinity for Zn²⁺ and Mn²⁺ in the active site

Structural identification of the active site ions. (A) The active site of the refined structure of Pop2p crystallized at physiological concentrations of Mg²⁺, Mn²⁺, and Zn²⁺ (see the main text for details). The anomalous difference densities calculated from data collected at the high (1.271 Å, top, green density) and low (1.297 Å, middle, red density) side of the Zn anomalous edge. The bottom panel shows the two densities superimposed. The intensities of the peaks are 12.7 σ (A site, 1.271 Å, top), 8.0 σ (B site, 1.271 Å, top), 4.6 σ (A site, 1.297 Å, middle), and 6.2 σ (B site, 1.297 Å, middle). (B) Overall structure of S. pombe Pop2p shown as cartoon with the active site residues as sticks and the anomalous densities from A as overlay. (C) The active site of the refined structure of Pop2p determined from crystals soaked in physiological concentrations of Mg²⁺ and Mn²⁺, but in the absence of Zn²⁺, with anomalous difference maps at the two wavelengths superimposed on the structure as in A (top and middle panels), and bottom, superimposed on the 2F_o-F_c refined map from the data collected at 1.271 Å (gray). The intensities of the peaks are 5.7 σ (B site, 1.271 Å, top), 4.4 σ (B site, 1.297 Å, middle), 2.6 σ (A site, 2F_o-F_c, bottom), and 4.6 σ (B site, 2F_o-F_c, bottom).