X-RAY CRYSTAL-STRUCTURE OF GAMMA-CHYMOTRYPSIN IN HEXANE

Crystals of gamma-chymotrypsin grown in aqueous solution were soaked i n n-hexane, and the structures of both the soaked and the native cryst als were determined to 2.2-Angstrom resolution. Seven hexane molecules and 130 water molecules were found in the hexane-soaked crystals. Two of the seven hexane molecules are found near the active site, and the rest are close to hydrophobic regions on or near the surface of the e nzyme. In the hexane structure, water molecules that were not observed in the native structure form a clathrate around one of the hexane mol ecules. Only 97 water molecules were found in the native structure. Th e temperature factors for atoms in the hexane environment are lower th an those in the aqueous environment. There are significant changes bet ween the two structures in the side chains of both polar and neutral r esidues, particularly in the vicinity of the hexane molecules. These c hanges have perturbed the hydrogen-bonding patterns. The electron dens ity for the peptide bound in the active site has been dramatically alt ered in hexane and appears to be tetrahedral at the carbon that is cov alently bound to Ser 195. The crystalline enzyme retains its active co nformation in the nonpolar medium and can catalyze both hydrolysis and synthesis reactions in hexane.