scholarly journals Electron-paramagnetic-resonance and magnetic-circular-dichroism studies of the binding of cyanide and thiols to the iron-molybdenum cofactor from Klebsiella pneumoniae nitrogenase

1994 ◽  
Vol 297 (2) ◽  
pp. 373-378 ◽  
Author(s):  
A J M Richards ◽  
D J Lowe ◽  
R L Richards ◽  
A J Thomson ◽  
B E Smith
Keyword(s):  
Circular Dichroism ◽  
Low Temperature ◽  
Klebsiella Pneumoniae ◽  
Magnetic Circular Dichroism ◽  
Metal Cluster ◽  
Cluster Structure ◽  
Paramagnetic Resonance ◽  
Iron Molybdenum Cofactor ◽  
Electron Paramagnetic ◽  
Circular Dichroïsm

FeMoco, a low-M(r) metal cluster of probable composition Fe7MoS9 complexed with homocitrate, has been extracted with N-methylformamide from the MoFe protein of the nitrogenase enzyme from Klebsiella pneumoniae. The binding of cyanide and thiols to the FeMoco cluster in its paramagnetic S = 3/2 oxidation level has been studied by low-temperature e.p.r. and magnetic-circular-dichroism (m.c.d.) spectroscopies. Cyanide binds to isolated FeMoco at more than one site, and causes changes in the g values form g = 4.6, 3.2, 2.0 to g = 4.29, 3.82, 2.02 E.p.r. competition studies indicate that one cyanide can be displaced by thiolate from one type of site. The form of the low-temperature m.c.d. spectrum is little changed by ligand binding, thus the basic cluster structure remains intact. However, when benzenethiol is bound, a new intense band (lambda 387 nm) is observed, indicating the generation of an increased ligand-to-cluster charge-transfer interaction.

scholarly journals Low-temperature magnetic-circular-dichroism spectroscopy of the iron-molybdenum cofactor and the complementary cofactor-less MoFe protein of Klebsiella pneumoniae nitrogenase

1984 ◽  
Vol 219 (2) ◽  
pp. 495-503 ◽  
Author(s):  
A E Robinson ◽  
A J M Richards ◽  
A J Thomson ◽  
T R Hawkes ◽  
B E Smith
Keyword(s):  
Low Temperature ◽  
Klebsiella Pneumoniae ◽  
Magnetic Circular Dichroism ◽  
Molybdenum Cofactor ◽  
Magnetization Curves ◽  
Mofe Protein ◽  
Paramagnetic Component ◽  
Magnetic Circular Dichroism Spectroscopy ◽  
Iron Molybdenum Cofactor ◽  
Circular Dichroïsm

The major metal clusters of the MoFe protein, Kpl, of Klebsiella pneumoniae nitrogenase were characterized separately by low-temperature magnetic-circular-dichroism spectroscopy. The spectra and magnetization curves of the extracted iron-molybdenum cofactor, FeMoco, and of ‘P’ clusters in NifB - Kpl, the inactive, FeMoco -less, MoFo protein from an nifB mutant, were measured and compared with those of the holoprotein. (When FeMoco and NifB - Kpl are combined, active Kpl is formed.) Reduced NifB - Kpl had a spectrum with a weak, paramagnetic, component superimposed on a diamagnetic background. The paramagnetic component was assigned to a contaminating, e.p.r.-active, species. Thionine-oxidized NifB - Kpl had a spectrum and magnetization properties very similar to those of thionine-oxidized Kpl, 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 centre in dithionite-reduced Kpl . Furthermore, a shoulder near 580 nm in the Kpl spectrum was absent from that of FeMoco . This may be due to the loss of a ligand or to a change in symmetry of the FeMoco cluster on extraction.

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