Protein-Based Determination of Model Redox Potentials for the High-Potential and the Low-Potential Redox Transition of [4Fe-4S]-Cys4 Clusters using Continuum Electrostatics

[4Fe-4S] clusters in protein exist commonly in three different redox states: the oxidized state [4Fe-4S]3+ , the semi-reduced state [4Fe-4S]2+ , and the reduced state [4Fe-4S]1+ . Applying the Virtual Model Compound Approach, we determine the model redox potentials for the respective transitions using high resolution protein crystal structures. A careful re-refinement 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 semi-reduced state, and -157 mV for the transition from the semi-reduced state to the reduced state. The reduced and the oxidized state can exist in several microstates, which are equivalent in an 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 which is based on chiral volumes. The principle of this labeling scheme can be applied also to other compounds that contain many formally equivalent atoms.