title: Identification of iron-sulfur centers in the iron-molybdenum proteins of nitrogenase.
abstract: The core extrusion method has been applied to the determination of the type ([2Fe-2S], [4Fe-4S]) and number of iron-sulfur centers in the FeMo proteins of the nitrogenases from Clostridium pasteurianum and Azotobacter vinelandii. The method involves extrusion with o-xylyl-alpha, alpha'-dithiol, ligand exchange of the extrusion products with p-CF3C6H4SH (RFSH), and identification and quantitation of the resultant [FenSn(SRF)4]2- complexes (n = 2,4) by 19F NMR spectroscopy. In hexamethylphosphoramide/water, 4:1 (vol/vol), 49-56% of the Fe content was extruded as [Fe4S4(SRF)4]2-, corresponding to 3,4-4.0 Fe4S4 cores per alpha 2 beta 2 subunit complex. The extruded iron does not arise from the FeMo cofactor, separate examination of which detected no extrusion products, and corresponds to 90-103% of noncofactor iron. No significant quantity of Fe2S2 cores was extruded. These results indicate the presence of four [4Fe-4S] centers per alpha 2 beta 2 subunit complex in preparations undepleted in iron. There are two main structural populations of iron atoms in these proteins, those in the cubane-type Fe4S4 cores and those in the FeMo cofactor.
mesh_terms: D001395:Azotobacter / Q000201:enzymology; D001665:Binding Sites; D003013:Clostridium / Q000201:enzymology; D007501:Iron / Q000032:analysis; D007506:Iron-Sulfur Proteins; D009682:Magnetic Resonance Spectroscopy; D008667:Metalloproteins; D008982:Molybdenum / Q000032:analysis*; D009591:Nitrogenase; D011485:Protein Binding; D013045:Species Specificity
chemical_list: D007506:Iron-Sulfur Proteins; D008667:Metalloproteins; D008982:Molybdenum; D007501:Iron; D009591:Nitrogenase