Abstact

The structure of DLH (C123S) with PMS bound was solved to 2.5 A resolution (R factor = 15.1%). PMSF in 2-propanol was delivered directly to crystals in drops and unexpectedly caused the crystals to dissolve. New crystals displaying a different morphology emerged within 2 h in situ, a phenomenon that appears to be described for the first time. The changed crystal form reflected altered crystal-packing arrangements elicited by structural changes to the DLH (C123S) molecule on binding inhibitor. The new unit cell remained in the P2(1)2(1)2(1) space group but possessed different dimensions. The structure showed that PMS binding in DLH (C123S) caused conformational changes in the active site and in four regions of the polypeptide chain that contain reverse turns. In the active site, residues with aromatic side chains were repositioned in an edge-to-face cluster around the PMS phenyl ring. Their redistribution prevented restabilization of the triad His202 side chain, which was disordered in electron-density maps. Movements of other residues in the active site were shown to be related to the four displaced regions of the polypeptide chain. Their implied synergy suggests that DLH may be able to accommodate and catalyse a range of compounds unrelated to the natural substrate owing to an inherent coordinated flexibility in its overall structure. Implications for mechanism and further engineering studies are discussed.