Proton coupling to [4Fe-4S](2+/+) and [4Fe-4Se](2+/+) oxidation and reduction in a designed protein

The coupling of a single proton to 4Fe-4S]2+/+ oxidation/reduction in a de novo designed iron-sulfur protein maquette is presented. The reduced state pKared is 9.3, and the oxidized state pKaox is <6.5. The reduced state pKared shifts to 8.3 upon incorporation of a 4Fe-4Se]2+/+ cluster, implicating the cluster itself or its primary coordination sphere as the proton-coupling site.