THE CATALYTIC SITE OF SERINE PROTEINASES AS A SPECIFIC BINDING CAVITYFOR XENON

Background: Under moderate pressure, xenon can bind to proteins and fo rm weak but specific interactions. Such protein-xenon complexes can be used as isomorphous derivatives for phase determination in X-ray crys tallography. Results: Investigation of the serine proteinase class of enzymes shows that the catalytic triad, the common hydrolytic motif of these enzymes, is a specific binding site for one xenon atom and show s high occupancy at pressures below 12 bar. Complexes of xenon with tw o different serine proteinases, elastase and collagenase, were analyze d and refined to 2.2 Angstrom and 2.5 Angstrom resolution, respectivel y, In both cases, a single xenon atom with a low temperature factor is located in the active site at identical positions. Weak interactions exist with several side chains of conserved amino acids at the active sice. Xenon binding does not induce any major changes in the protein s tructure and, as a consequence, crystals of the xenon complexes are hi ghly isomorphous with the native protein structures. Xenon is also fou nd to bind to the active site of subtilisin Carlsberg, a bacterial ser ine proteinase, that also has a catalytic triad motif. Conclusions: As the region around the active site shows conserved structural homology in all serine proteinases, it is anticipated that xenon binding will prove to be a general feature of this class of proteins.