The single Fe(II) in reduced rubredoxin from Clostridium pasteurianum was f
ound to be quantitatively displaced by either Cd2+ or Zn2+ when a modest mo
lar excess of the substituting metal salt was anaerobically incubated with
the reduced rubredoxin under mild conditions, namely, room temperature, pH
5.4-8.4, and no protein denaturants. Under the same conditions, cadmium-for
-zinc substitution was also achieved upon aerobic incubation of the zinc-su
bstituted rubredoxin with a modest molar excess of Cd2+. Displacements of F
e(II) from the reduced rubredoxin were not observed upon anaerobic incubati
on with Ni2+, Co2+, or VO2+ salts, and no reaction with any of the divalent
metal ions was observed for the oxidized [Fe(III)] rubredoxin. Fe(II) coul
d not be re-inserted into the Zn- or Cd-substituted rubredoxins without res
orting to protein denaturation. H-1 and Cd-113 NMR experiments showed that
the cadmium-substituted rubredoxin prepared by the non-denaturing substitut
ion method retained the pseudotetrahedral M(SCys)(4) coordination geometry
and secondary structural elements characteristic of the native rubredoxin,
and that "unzipping" of the beta-sheet did not occur during metal substitut
ion. Rates of Fe(II) displacement by M2+ (M = Cd or Zn) increased with incr
easing M2+/rubredoxin ratio, decreasing pH, and lower ionic strength. The s
ubstitution rates were faster for M = Cd than for M = Zn. Rates of Cd2+ sub
stitution into a V8A-mutated rubredoxin were significantly faster than for
the wild-type protein. The side-chain of Vg is on the protein surface and c
lose to the metal-ligating Cys42S gamma at the M(SCys)(4) site. Therefore,
the rate-limiting step in the substitution process is suggested to involve
direct attack of the [M(SCys)(4)](2-) site by the incoming M2+, without glo
bal unfolding of the protein. Implications of these results for metal ion i
ncorporation into rubredoxins in vivo are discussed.