R. Kummerle et al., SITE-DIRECTED MUTAGENESIS OF RUBREDOXIN REVEALS THE MOLECULAR-BASIS OF ITS ELECTRON-TRANSFER PROPERTIES, Biochemistry, 36(50), 1997, pp. 15983-15991
Rubredoxins contain a single non-heme iron atom coordinated by four cy
steines. This iron is redox active and confers a role to these protein
s in electron transfer chains. The structural features responsible for
setting the values of the reduction potential and of the electron sel
f-exchange rate constant have been probed by site-directed mutagenesis
. Replacements of the highly conserved residues in positions 8, 10, an
d 11 (valine, glycine, and tyrosine, respectively) all lead to shifts
of the reduction potential, up to 75 mV. These cannot be explained by
simple considerations about the physicochemical properties of the subs
tituting side chains but rather indicate that the value of the reducti
on potential is finely tuned by a variety of interactions. In contrast
, the electron self-exchange rate constant measured by nuclear magneti
c resonance does not vary much, except when a charged residue is inclu
ded in position 8 or 10, at the surface of the protein closest to the
iron atom. Analysis of the data with a model for electrostatic interac
tions, including both monopolar and dipolar terms, indicates that the
presence of a charge in this region not only increases the repulsion b
etween molecules but also affects the electron transfer efficiency of
the bimolecular complexes formed. The studies presented constitute a f
irst step toward probing the structural elements modulating the reacti
vity of the FeS4 unit in a protein and defining the electron transfer
active site(s) of rubredoxin.