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TDPAC-Untersuchungen zur Bindung von Molybdaen im Eisen-Molybdaen-Cofaktor der Nitrogenase

By C. Pohlmann and Forschungszentrum Karlsruhe GmbH Technik und Umwelt (Germany). Inst. fuer Toxikologie


The nitrogenase belongs to the molybdenzymes and catalyses the reduction of molecular nitrogen to ammonia. The enzyme system of the conventional nitrogenase consists of two components, the iron protein and the molybdenum-iron protein (MoFe-protein). The MoFe-protein contains two kinds of metal clusters. One of them, the iron-molybdenum cofactor (FeMo-cofactor), is postulated to be the catalytic center of the biological nitrogen fixation. The FeMo-cofactor consists of [Fe_4S_3]- and [Fe_3MoS_3]-cuban fragments bridged by three sulfur atoms and homocitrate. In its complexity and size it is unique in nature. The binding of molybdenum in the FeMo-cofactor has been investigated using the time differential perturbed #gamma#-#gamma#-angular correlations (TDPAC) spectroscopy. By observing the hyperfine interaction of the nuclear quadrupole moment of molybdenum-99 with the electric field gradient at its site information about the charge density distribution and thus about the ligand field configuration can be obtained. The measured quadrupole parameters describe size and symmetry of the electric field gradient as well as the homogenity of the binding state and relaxation effects. Because of its high sensitivity and temperature independence the TDPAC method is suitable for the investigation of biological probes under physiological conditions. The TDPAC-measurements of the bacteria species Rhodobacter capsulatus, Azotobacter vinelandii and Klebsiella oxitoca yield two different Mo-binding states in the FeMo-cofactor of nitrogenase. Comparison with results of other methods has shown that the two binding states could be different redox states of the FeMo-cofactor. In further studies the binding of molybdenum at the Mo-storage protein of A. vinelandii has been investigated. The results of the TDPAC-measurements consist with the assumption that the storage protein contains a polymolybdate type nucleus. In addition to the MO-Fe-protein further Mo-binding proteins were found in R. capsulatus. They are part of a recetly detected Mo-binding and -transport system. (orig.)Available from FIZ Karlsruhe / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Year: 1997
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