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Low temperature magnetic circular dichroism spectroscopy of the iron molybdenum cofactor and the complementary cofactor less molybdenum iron protein of klebsiella pneumoniae nitrogenase



Low temperature magnetic circular dichroism spectroscopy of the iron molybdenum cofactor and the complementary cofactor less molybdenum iron protein of klebsiella pneumoniae nitrogenase



Biochemical Journal 219(2): 495-504



The major metal clusters of the MoFe protein, Kp1, of K. pneumoniae nitrogenase were characterized separately by low-temperature magnetic-circular-dichroism spectroscopy. The spectra and magnetization curves of the extracted Fe-Mo cofactor (FeMoco) and of P clusters in NifB- Kp1, the inactive, FeMoco-less, MoFo protein from an nifB mutant, were measured and compared with those of the holoprotein. (When FeMoco and NifB- Kp1 are combined, active Kp1 is formed.) Reduced NifB- Kp1 had a spectrum with a weak, paramagnetic, component superimposed on a diamagnetic background. The paramagnetic component was assigned to a contaminating EPR-active species. Thionine-oxidized NifB- Kp1 had a spectrum and magnetization properties very similar to those of thionine-oxidized Kp1, demonstrating that the P clusters are not significantly affected by the absence of the FeMoco clusters. The spectra of reduced isolated FeMoco had similar magnetization curves but sharper features and higher intensities than those of this center in dithionite-reduced Kp1. A shoulder near 580 nm in the Kp1 spectrum was absent from that of FeMoco. This may be due to the loss of a lignad or to a change in symmetry of the FeMoco cluster on extraction.

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Accession: 005834814

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