Protein-Based Determination of Model Redox Potentials for the High-Potential and Low-Potential Redox Transition of...

[4Fe-4S] clusters in proteins exist commonly in three different redox states: the oxidized state [4Fe-4S]³⁺, the semireduced state [4Fe-4S]²⁺, and the reduced state [4Fe-4S]¹⁺. Applying the Virtual Model Compound Approach, we determined the model redox potentials for the respective transitions using high-resolution protein crystal structures. A careful rerefinement of the structures improves the reliability of the calculated model redox potentials. We find a value of +653 mV for the transition from the oxidized to the semireduced state and -157 mV for the transition from the semireduced state to the reduced state. The reduced and oxidized states can exist in several microstates, which are equivalent in a homogeneous environment. Furthermore, in ferredoxins and in the iron protein of nitrogenase, solvent accessibility causes one microstate to be preferred over the others. In high-potential iron-sulfur proteins, the cluster is buried, and all microstates have approximately the same population. To assign the charges unambiguously to the atoms of the cluster, we introduce a consistent labeling scheme that is based on chiral volumes. The principle of this labeling scheme can also be applied to other compounds that contain many formally equivalent atoms.