title: Characterization of the metal clusters in the nitrogenase molybdenum-iron and vanadium-iron proteins of Azotobacter vinelandii using magnetic circular dichroism spectroscopy.
abstract: Low-temperature magnetic circular dichroism (MCD) spectroscopy has been used to investigate the metal clusters in the conventional nitrogenase MoFe protein and alternative VFe protein from Azotobacter vinelandii. In the dithionite-reduced state, the MCD spectrum of the MoFe protein is extremely similar to that previously observed for the S = 3/2 spin state of the M clusters in the MoFe protein of Klebsiella pneumoniae. A paramagnetic cluster with an S = 3/2 ground state is also responsible for the temperature-dependent MCD transitions of dithionite-reduced VFe protein. However, the electronic and magnetic properties of this cluster are quite distinct from those of M centers in conventional nitrogenase. When these proteins are oxidized with thionine, the MoFe protein exhibits MCD spectra and magnetization characteristics identical with those observed for the P clusters in K. pneumoniae, while those of the VFe protein are only similar. However, the paramagnetism in the thionine-oxidized VFe protein, like the conventional enzyme, probably arises from an S = 5/2 spin system with near-axial symmetry and a negative zero-field splitting. Novel clusters with electronic, magnetic, and redox properties similar to those of conventional P clusters are, therefore, present in the VFe protein.
authors.forename: J E
authors.initials: JE
authors.lastname: Morningstar
authors.forename: M K
authors.initials: MK
authors.lastname: Johnson
authors.forename: E E
authors.initials: EE
authors.lastname: Case
authors.forename: B J
authors.initials: BJ
authors.lastname: Hales
mesh_terms: D001395:Azotobacter / Q000201:enzymology*; D001426:Bacterial Proteins; D002942:Circular Dichroism; D004227:Dithionite / Q000494:pharmacology; D005288:Ferredoxins / Q000378:metabolism*; D007700:Kinetics; D008280:Magnetics; D008667:Metalloproteins / Q000378:metabolism*; D008983:Molybdoferredoxin / Q000378:metabolism*; D009591:Nitrogenase / Q000378:metabolism*; D010084:Oxidation-Reduction; D011487:Protein Conformation; D013816:Thermodynamics
chemical_list: D001426:Bacterial Proteins; D005288:Ferredoxins; D008667:Metalloproteins; D008983:Molybdoferredoxin; C052750:vanadium-iron protein, Azotobacter; D004227:Dithionite; D009591:Nitrogenase