Superposition of the BMP-liganded structures of wild type MtOMPDC and the Q185A, R203A, and Q185A/R203A mutants of residues that generate the IBE of the 5’-phosphate group. Panel A, the polypeptide dimers. Panel B, active site of wild type. Panel C, active site of the Q185A mutant. Panel D, Active site of the R203A mutant. Panel E, active site of the Q185A/R203A mutant. The red spheres represent water molecules.