MR-SAD: phasing employing the NMR structure without model bias

To help resolve long-standing questions regarding the catalytic activity of the serine proteases the structure of porcine pancreatic elastase has been analyzed by high-resolution neutron and X-ray crystallography.In order to mimic the tetrahedral transition intermediate a peptidic inhibitor was used.Neutron and X-ray diffraction data have, for the first time, visualized a SIHB formed between catalytic residues His57 and Asp102, as well as the oxyanion located at the oxyanion hole, and has identified strong hydrogen bonds as contributing to inhibitor recognition in PPE.These are fundamentally important structural data for the catalytic reaction and molecular recognition of an enzyme.