Advantages of high-resolution phasing: MAD to atomic resolution

The structure of the endoglucanase A from Clostridium thermocellum CelA was re-solved by three-wavelength MAD. Experimental phases were obtained in the resolution range 25–1.0 Å. Various structure-solution approaches were tested in order to quantify the contribution of each wavelength. Two-wavelength MAD phasing was sufficient to obtain excellent experimental phases. SAD at the remote wavelength also resulted in interpretable maps. The three-wavelength MAD electron-density map was of excellent quality: for parts of the structure, atom types and bond types could be easily assigned. Double bonds in peptide links and side chains could be located owing to their increased electron density indicating their π character. Comparison with a previously determined structure of CelA at 1.65 Å showed that, apart from a few additional multiple conformers and differently oriented side chains, major differences occur at the protein–solvent interface. A complete additional solvent shell could be observed and the inner shells have been completed. The high accuracy of the structure allowed unambiguous assignment of the protonation state for the active-site catalytic carboxylates.