Experiment 5: Identification of a ligand bound in the active site of lysozyme

In this experiment, tetragonal crystals of hen egg white lysozyme grown from HEPES buffer in the presence of MPD are examined in order to identify weakly bound HEPES molecules in the active site of the enzyme. The HEPES molecule can be identified in a regular difference Fourier electron density map after refining the rest of the protein structure. Additional support for the ligand being a HEPES molecule can be obtained from the anomalous difference electron density map. Since the diffraction data in this experiment were collected at a wavelength of λ = 1.77 Å the S-atom in the sulfonate group of HEPES appears as a peak in the anomalous difference Fourier electron density map.