title: Insights from soft X-rays: the chlorine and sulfur sub-structures of a CK2alpha/DRB complex.
abstract: The diffraction pattern of a protein crystal is normally a product of the interference of electromagnetic waves scattered by electrons of the crystalline sample. The diffraction pattern undergoes systematic changes in case additionally X-ray absorption occurs, meaning if the wavelength of the primary X-ray beam is relatively close to the absorption edge of selected elements of the sample. The resulting effects are summarized as "anomalous dispersion" and can be always observed with "soft" X-rays (wavelength around 2 A) since they match the absorption edges of sulfur and chlorine. A particularly useful application of this phenomenon is the experimental detection of the sub-structures of the anomalous scatterers in protein crystals. We demonstrate this here with a crystal of a C-terminally truncated variant of human CK2alpha to which two molecules of the inhibitor 5,6-dichloro-1-beta-D-ribo-furanosyl-benzimidazole (DRB) are bound. The structure of this co-crystal has been solved recently. For this study we measured an additional diffraction data set at a wavelength of 2 A which showed strong anomalous dispersion effects. On the basis of these effects we detected all sulfur atoms of the protein, the two liganded DRB molecules and a total of 16 additional chloride ions some of them emerging at positions filled with water molecules in previous structure determinations. A number of chloride ions are bound to structural and functional important locations fitting to the constitutive activity and the acidophilic substrate specificity of the enzyme.
authors.affiliation: Universität zu Köln, Institut für Biochemie, Zülpicher Strasse 47, 50674, Köln, Germany.
authors.forename: Jennifer
authors.initials: J
authors.lastname: Raaf
authors.forename: Olaf-Georg
authors.initials: OG
authors.lastname: Issinger
authors.forename: Karsten
authors.initials: K
authors.lastname: Niefind
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D001665:Binding Sites; D047390:Casein Kinase II / Q000037:antagonists & inhibitors / Q000737:chemistry*; D002713:Chlorine / Q000737:chemistry*; D004004:Dichlororibofuranosylbenzimidazole / Q000737:chemistry*; D006801:Humans; D008958:Models, Molecular; D047428:Protein Kinase Inhibitors / Q000737:chemistry; D017433:Protein Structure, Secondary; D055672:Static Electricity; D013379:Substrate Specificity; D013455:Sulfur / Q000737:chemistry*; D014961:X-Ray Diffraction
chemical_list: D047428:Protein Kinase Inhibitors; D002713:Chlorine; D004004:Dichlororibofuranosylbenzimidazole; D013455:Sulfur; D000255:Adenosine Triphosphate; D047390:Casein Kinase II