Roie of Chister-Ligated Aspartate in Gating Electron Transfer in the Four-Iron Ferredoxin from the Hyperthermophilic Archaeon Pyrococcus furiosus

摘要

The "bacterial"-type or cubane ferredoxins (Fds) are small electron-transfer proteins containing one or two [Fe_4S_4] and/ or [Fe_3S_4] clusters, with the overwhelming majority possessing complete Cys ligation via the consensus sequence Cys~I-X_2-Cys~(II)-X_2-Cys~(III)—Cys~(IV). The rate of electron transfer between proteins depends on the driving force (the difference in reduction potentials, E°, of the reactants), the reorganization energy, A, that results from structural changes accompanying loss or gain of an electron, and the "conductivity" of the intervening protein medium. E° values for Cys-only ligated cubane Fds range from +80 to -700 mV but are usually near -400 mV. E° is modulated by H-bonds to the cluster, polarity of the cluster environment, and water access to the cluster, among others, but precise mechanisms are incompletely understood.