Investigation of the unusual electronic structure of Pyrococcus furiosus 4Fe ferredoxin by EPR spectroscopy of protein reduced at ambient and cryogenic temperatures
J. Telser et al., Investigation of the unusual electronic structure of Pyrococcus furiosus 4Fe ferredoxin by EPR spectroscopy of protein reduced at ambient and cryogenic temperatures, INORG CHEM, 38(15), 1999, pp. 3550-3553
The hyperthermophilic archaeon Pyrococcus furiosus contains a novel ferredo
xin (Pf-Fd) in which, in the native 4Fe form, three of the Fe ions are coor
dinated to the protein by cysteinyl thiolato ligands, but the fourth Fe is
coordinated by an aspartyl carboxylato ligand ([Fe4S4(cys)(3)(asp)](2-,3-))
. Chemical reduction at ambient temperature of the oxidized 4Fe form (Pf-Fd
4Fe-ox, S = 0 ground state, with the cluster core indicated by [Fe4S4](2+)
(ox)) produces a reduced 4Fe form (Pf-Fd 4Fe-red, with the cluster core ind
icated by [Fe4S4](+)(red)) Pf-Fd 4Fe-red, [Fe4S4](+)(red) core, in frozen s
olution exhibits S = 1/2 and 3/2 electronic states that are not in thermal
equilibrium. The two spin states thus represent alternate ground states of
the reduced cluster (cluster cores indicated by [Fe4S4](+)(red1) and [Fe4S4
](+)(red2), respectively), rather than a ground and excited spin state. Low
-temperature (77 K) reduction of 4Fe-ox in frozen solution by gamma-irradia
tion produces in high yield the reduced state of the cluster that is trappe
d in the structure of the oxidized parent cluster, and thus has a cluster c
ore denoted by [Fe4S4](+)(ox). The [Fe4S4](+)(ox) form also exhibits non th
ermally converting S = 3/2 and 1/2 components in the same proportion as see
n for [Fe4S4](+)(red). The EPR signal of the S = 3/2 component that results
from cryoreduction ([Fe4S4](+)(ox2)) is indistinguishable, within experime
ntal variability, from that seen in the ambient-temperature, chemically red
uced protein ([Fe4S4](+)(red2)), and the signals of the two S = 1/2 compone
nts ([Fe4S4](+)(ox1) and [Fe4S4](+)(red1), respectively) closely resemble e
ach other, although they are not identical. Previous NMR studies at ambient
temperature showed evidence for only one species in fluid solution for bot
h Pf-Fd 4Fe-ox and 4Fe-red. Taken together, the NMR and EPR results indicat
e that fluid solutions of either oxidized or reduced Pf-Fd contain only one
conformer, but that frozen solutions of each contain two distinct conforme
rs, with each one of the pair of oxidized protein forms having a correspond
ing reduced form. A shift in the coordination mode of the aspartyl carboxyl
ato ligand is proposed to account for this conformational flexibility.