Crystallographic evidence for noncoplanar catalytic aspartic acids in plasmepsin II resides in the Protein Data Bank

The carboxylate atoms of the two catalytic aspartic acid residues in aspartic proteases are nearly coplanar and in the uncomplexed form share an in-plane nucleophilic water molecule that is central to the mechanism of these enzymes. This note reports that while reviewing the electron-density maps derived from the deposited data for uncomplexed plasmepsin II from Plasmodium falciparum [Asojo et al. (2003), J. Mol. Biol. 327, 173-181; PDB code 1lf4 ], it was discovered that the aspartic acid residues in this structure should in fact be distinctly noncoplanar. The crystallographic model from the deposited coordinates has been re-refined against the 1.9 Å resolution published diffraction data to an R_cryst of 21.2% and an R_free of 22.2%. The catalytic water molecule is present, but the plane of the carboxylate group of Asp214 is rotated by 66° from its original position.