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Abstract
Crystals of gamma-chymotrypsin grown in aqueous solution were soaked in
n-hexane, and the structures of both the soaked and the native crystals were
determined to 2.2-A 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 enzyme. In the hexane structure, water molecules that
were not observed in the native structure form a clathrate around one of the
hexane molecules. Only 97 water molecules were found in the native structure.
The temperature factors for atoms in the hexane environment are lower than those
in the aqueous environment. There are significant changes between the two
structures in the side chains of both polar and neutral residues, particularly
in the vicinity of the hexane molecules. These changes have perturbed the
hydrogen-bonding patterns. The electron density for the peptide bound in the
active site has been dramatically altered in hexane and appears to be
tetrahedral at the carbon that is covalently bound to Ser 195. The crystalline
enzyme retains its active conformation in the nonpolar medium and can catalyze
both hydrolysis and synthesis reactions in hexane.
