Structure of the product complex of acetyl-Ala-Pro-Ala with porcine pancreatic elastase at 1.65 Å resolution

A single crystal of porcine pancreatic elastase was mounted in a thin-walled capillary and allowed to react with acetyl-Ala-Pro-Ala-paranitroanalide. Diffraction data to 1.65 Å resolution were measured and the isomorphous structure was solved from the difference Fourier map. The structure contains two surprises. 1. (1) Two molecules of the product: acetyl-Ala-Pro-Ala molecule are bound in the extended binding site. 2. (2) Both molecules are bound backwards with respect to the established mode of peptide binding.