Heterometallic [AgFe₃S₄] ferredoxin variants

Abstract

Heterometallic [AgFe₃S₄] iron-sulfur clusters assembled in wild-type Pyrococcus furiosus ferredoxin and two variants, D14C and D14H, are characterized. The crystal structure of the [AgFe₃S ₄] D14C variant shows that the silver(I) ion is indeed part of the cluster and is coordinated to the thiolate group of residue 14. Cyclic voltammetry shows one redox pair with a reduction potential of +220 mV versus the standard hydrogen electrode which is assigned to the [AgFe₃S ₄]^2+/+ couple. The oxidized form of the [AgFe ₃S₄] D14C variant is stable in the presence of dioxygen, whereas the oxidized forms of the [AgFe₃S₄] wild type and D14H variants convert to the [Fe₃S₄] ferredoxin form. The monovalent d ¹⁰ silver(I) ion stabilizes the [Fe₃S ₄]^+/0 cluster fragment, as opposed to divalent d ¹⁰ metal ions, resulting in more than 0.4 V difference in reduction potentials between the silver(I) and, e.g., zinc(II) heterometallic [MFe ₃S₄] ferredoxins. The trend in reduction potentials for the variants containing the [AgFe₃S₄] cluster is wild type ≤ D14C < D14H and shows the same trend as reported for the variants containing the [Fe₃S₄] cluster, but is different from the D14C < D14H < wild type trend reported for the [Fe₄S ₄] ferredoxin. The similarity in the reduction potential trend for the variants containing the heterometallic [AgFe₃S₄] cluster and the [Fe₃S₄] cluster can be rationalized in terms of the electrostatic influence of the residue 14 side chains, rather than the dissociation constant of this residue, as is the case for [Fe ₄S₄] ferredoxins. The trends in reduction potentials are in line with there being no electronic coupling between the silver(I) ion and the Fe₃S₄ fragment.