EXPLORING HYDROPHOBIC SITES IN PROTEINS WITH XENON OR KRYPTON

X-ray diffraction is used to study the binding of xenon and krypton to a variety of crystallised proteins: porcine pancreatic elastase; subt ilisin Carlsberg from Bacillus licheniformis; cutinase from Fusarium s olani; collagenase from Hypoderma lineatum; hen egg lysozyme, the lipo amide dehydrogenase domain from the outer membrane protein P64k from N eisseria meningitidis; urate-oxidase from Aspergillus flavus, mosquito cidal delta-endotoxin CytB from Bacillus thuringiensis and the ligand- binding domain of the human nuclear retinoid-X receptor RXR-alpha. Und er gas pressures ranging from 8 to 20 bar, xenon is able to bind to di screte sites in hydrophobic cavities, ligand and substrate binding poc kets, and into the pore of channel-like structures, These xenon comple xes can be used to map hydrophobic sites in proteins, or as heavy-atom derivatives in the isomorphous replacement method of structure determ ination. (C) 1998 Wiley-Liss, Inc.