Spectroscopic and saturation magnetization properties of the manganese- and cobalt-substituted Fur (ferric uptake regulation) protein from Escherichia coli
A. Adrait et al., Spectroscopic and saturation magnetization properties of the manganese- and cobalt-substituted Fur (ferric uptake regulation) protein from Escherichia coli, BIOCHEM, 38(19), 1999, pp. 6248-6260
The Fur apoprotein has been purified and reconstituted with Co2+ and Mn2+ i
ons. These samples have been analyzed by UV-visible, EPR, and H-1 NMR spect
roscopies, by XAS, and by magnetization measurements. The apo-Fur protein i
s able to bind one metal dication (Co2+ or Mn2+) per monomer, A saturation
magnetization study confirms the presence of a high-spin metal dication [Mn
(II) S = 5/2 and Co(II) S = 3/2]. The two metal ions per Fur dimer are not
in magnetic interaction (\J\ < 0.1 cm(-1)). The UV-visible spectrum of the
cobalt-substituted form (Co-Fur) presents two main bands at 660 nm and 540(
br) nm with epsilon(540 nm) = 65 M-1 cm(-1). The EPR spectrum gives the fol
lowing g values: g(x) = 5.0(5), g(y) = 4.0(2), and g(z) = 2.3(1), which are
in accordance with a nearly axial (E/D < 0.11) site. The value of 55 cm(-1
) for the splitting (Delta) between the ground and the first excited state
has been derived from an EPR saturation study and is in agreement with magn
etization data. The EXAFS data of Co-Fur indicate a metal environment compr
ising five nitrogen/oxygen atoms at 2.11 Angstrom, the absence of sulfur, a
nd the presence of histidines as ligands. H-1 NMR of Co-Fur in H2O and D2O
shows at least two exchangeable signals coming from histidine NH protons an
d shows the signature of carboxylate group(s). The combined spectroscopic d
ata allow us to propose that the main metal site of Fur in Co-Fur contains
at least two histidines, at least one aspartate or glutamate, and no cystei
ne as ligands and is in an axially distorted octahedral environment.