Metal binding to porcine pancreatic elastase: Calcium or not calcium

Porcine pancreatic elastase has been crystallized at slightly acidic pH under two similar but slightly different conditions. Diffraction data were collected at a wavelength of 1.5 Å to a maximum resolution of 1.7 Å. Both difference electron-density maps and anomalous difference electron-density maps suggest that in crystals grown from a sodium sulfate solution PPE binds Na⁺ in its metal-binding site. In contrast, PPE binds Ca²⁺ in crystals grown from a solution containing sodium citrate and calcium chloride. This observation is in contradiction to most PPE structures reported in the PDB. In addition to the metal-binding site, up to three other binding sites, which appear to be anion-binding sites, could be identified based on the observed anomalous intensity differences.