|
Figure 4.
Figure 4. Protonation of His 25, not of the cluster, causes the
pH dependence of reduction potential of the
[[3Fe−4S](Cys[3])]^2-/3- center in F25H. a, CD spectra of
native FdI and F25H FdI reduced by sodium dithionite at
different pH values: pH 6.0 (thin line) and pH 8.0 (thick line).
All samples were in 50 mM TAPS, 50 mM PIPES and 2 mM sodium
dithionite. The observed spectral change for native FdI is due
to protonation of a  [2]-S
for the [[3Fe−4S](Cys[3])]^3- center^26. b, The thermodynamics
of the one-electron reduction of the [[3Fe−4S](Cys[3])]^2-
cluster as measured from the temperature dependence of E^0'. The
total free energy change (  G)
is shown along with its enthalpic ( H)
and entropic (-T S[red])
components. Data are for native FdI at pH 8.5 (no associated
proton transfer), F25H FdI at pH 9.5 (no associated proton
transfer), wild type FdI at pH 5.0 (associated protonation of
the cluster) and F25H FdI at pH 5.0 (associated protonation of
H25). Errors are  0.5
kJmol^-1 for G[red]
and 2
kJ mol for S[red]
and H[red].
|
