Protonation/deprotonation-driven switch for the redox stability of low-potential ferredoxin

Ferredoxin is a small iron-sulfur protein and acts as an electron carrier. Low-potential ferredoxins harbor [4Fe-4S] cluster(s), which play(s) a crucial role as the redox center. Low-potential ferredoxins are able to cover a wide range of redox potentials (-700 to -200 mV); however, the mechanisms underlying the factors which control the redox potential are still enigmatic. Here, we determined the neutron structure of ferredoxin from Bacillus thermoproteolyticus, and experimentally revealed the exact hydrogen-bonding network involving the [4Fe-4S] cluster. The density functional theory calculations based on the hydrogen-bonding network revealed that protonation states of the sidechain of Asp64 close to the [4Fe-4S] cluster critically affected the stability of the reduced state in the cluster. These findings provide the first identification of the intrinsic control factor of redox potential for the [4Fe-4S] cluster in low-potential ferredoxins.