Water molecules in the antibody-antigen interface of the structure of the Fab HyHEL-5-lysozyme complex at 1.7 A resolution: comparison with results from isothermal titration calorimetry.

The structure of the complex between hen egg-white lysozyme and the Fab HyHEL-5 at 2.7 A resolution has previously been reported [Cohen et al. (1996), Acta Cryst. D52, 315–326]. With the availability of recombinant Fab, the X-­ray structure of the complex has been re-evaluated at 1.7 A resolution. The refined structure has yielded a detailed picture of the Fab–lysozyme interface, showing the high complementarity of the protein surfaces as well as several water molecules within the interface that complete the good fit. The model of the full complex has improved significantly, yielding an Rwork of 19.5%. With this model, the structural results can be compared with the results of isothermal titration calorimetry. An attempt has been made to estimate the changes in bound waters that accompany complex formation and the difficulties inherent in using the crystal structures to provide the information necessary to make this calculation are discussed.